[Federal Register Volume 59, Number 158 (Wednesday, August 17, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-18941]

[[Page Unknown]]

[Federal Register: August 17, 1994]




Fuels and Fuel Additives; Waiver Decision/Circuit Court Remand

AGENCY: Environmental Protection Agency (EPA).

ACTION: Notice


SUMMARY: On July 12, 1991, under section 211(f)(4) of the Clean Air Act 
(Act), the Ethyl Corporation (Ethyl) requested a waiver to permit the 
sale of its gasoline additive, methylcyclopentadienyl manganese 
tricarbonyl (MMT), an octane enhancer commercially labeled by Ethyl as 
HiTEC 3000, for use in unleaded gasoline. The Administrator of EPA 
denied Ethyl's application for a waiver on January 8, 1992, based 
primarily on concerns regarding the potential for increases in 
hydrocarbon emissions resulting from MMT use. Ethyl subsequently sought 
judicial review of that decision in the U.S. Court of Appeals for the 
District of Columbia Circuit. Based on new emissions data developed and 
submitted to EPA by Ethyl, EPA requested that the Court of Appeals 
remand Ethyl's application to the Agency for further action.
    On November 30, 1993, the Administrator of EPA found that Ethyl had 
met its burden to demonstrate under section 211(f)(4) that approval of 
its remanded application would not cause or contribute to a failure to 
meet emission standards. Ethyl agreed to resubmit its application at 
that time, thereby affording further time for the Agency to consider 
the issue of potential health effects associated with use of MMT in 
unleaded gasoline. Ethyl and EPA later agreed to further extend the 
deadline for final action on Ethyl's application to July 13, 1994. The 
Agency is today denying Ethyl's request for a waiver for HiTEC 3000 
based on unresolved concerns regarding the potential impact of 
manganese emissions resulting from MMT use on public health.

ADDRESSES: Copies of the information relative to this application are 
available for inspection in public docket A-93-26, A-91-46 and A-90-16 
at the Air Docket (LE-131) of the EPA, Room M-1500, 401 M Street, S.W., 
Washington, D.C. 20460, (202) 260-7548, between the hours of 8:30 a.m. 
to noon and 1:30 p.m. to 3:30 p.m. weekdays. As provided in 40 CFR Part 
2, a reasonable fee may be charged for copying services.

FOR FURTHER INFORMATION CONTACT: Joseph R. Sopata, Chemist, or James W. 
Caldwell, Chief, Fuels Section, Field Operations Support Division 
(6406J), U.S. Environmental Protection Agency, 401 M Street, S.W., 
Washington, D.C. 20460, (202) 260-2635.



I. Background
II. Statutory Framework
    A. History of Statute
    B. Two Stage Process
    C. Consideration of Potential Health Effects
III. Method of Review
    A. ``Causes or Contributes'' to Emission Standard Failure
    B. Discretionary Review
IV. Analysis of Emissions Data
    A. Description of Previous Test Programs
    B. Comments on Vehicle Emissions Issues
    C. Available Data Meet Previously Utilized Criteria
    D. Data on Newer-Technology Vehicles Meet More Stringent 
    E. Finding
V. The Onboard Diagnostics Issue
VI. Manganese Health Assessment
    A. Introduction
    B. Health Effects Assessment
    1. Background
    2. Earlier Assessments
    3. 1993 Revised RfC
    4. Alternative Approaches to Deriving RfCs
    a. Conventional NOAEL- or LOAEL-Based Approach
    b. NOSTASOT Approach
    c. Benchmark Analyses
    d. Bayesian Analyses
    e. Summary of RfC Estimates
    C. Exposure Assessment
    1. Background
    2. Additional Canadian Studies
    3. The PTEAM Study
    4. Estimated Mn Exposure Levels Associated with MMT
    D. Risk Characterization
    E. References
    F. Comments on Health Assessment and EPA Response
VII. Fuel and Fuel Additive Registration and Research Needs
VIII. Other Issues
IX. Decision

I. Background

    Section 211(f)(1)(A) of the Act makes it unlawful, effective March 
31, 1977, for any manufacturer of a fuel or fuel additive to first 
introduce into commerce, or to increase the concentration in use of, 
any fuel or fuel additive for use in light-duty motor vehicles 
manufactured after model year 1974 which is not substantially similar 
to any fuel or fuel additive utilized in the certification of any model 
year 1975, or subsequent model year, vehicle or engine under section 
206 of the Act. An interpretive rule defining the term ``substantially 
similar'' under section 211(f)(1)(A) was promulgated for unleaded 
gasoline at 46 FR 38582 (July 28, 1981), and revised at 56 FR 5352 
(February 11, 1991). Section 211(f)(1)(B) of the Act makes it unlawful, 
effective November 15, 1990, for any manufacturer of a fuel or fuel 
additive to first introduce into commerce, or to increase the 
concentration in use of, any fuel or fuel additive for use by any 
person in motor vehicles manufactured after model-year 1974 which is 
not substantially similar to any fuel or fuel additive utilized in the 
certification of any model year 1975, or subsequent model year, vehicle 
or engine under section 206 of the Act. Thus, section 211(f)(1)(B) 
expands the prohibitions of 211(f)(1)(A), which apply only to light-
duty vehicles.
    Section 211(f)(4) of the Act provides that upon application by any 
fuel or fuel additive manufacturer, the Administrator of EPA may waive 
the prohibitions of section 211(f)(1) if the Administrator determines 
that the applicant has established that such fuel or fuel additive will 
not cause or contribute to a failure of any emission control device or 
system (over the useful life of any vehicle in which such device or 
system is used) to achieve compliance by the vehicle with the emissions 
standards to which it has been certified pursuant to section 206 of the 
Act. If the Administrator does not act to grant or deny a waiver within 
180 days of receipt of the application, the statute provides that the 
waiver shall be treated as granted. The subject of this notice is an 
application by Ethyl under section 211(f)(4) of the Act for a waiver 
for the fuel additive methylcyclopentadienyl manganese tricarbonyl 
(MMT), commercially labeled by Ethyl as HiTEC 3000, to be blended in 
unleaded gasoline resulting in a level of 0.03125 (1/32) gram per 
gallon manganese (gpg Mn).
    This Agency action is a reconsideration of Ethyl's fourth 
application for a waiver for MMT. Ethyl's first application was 
submitted on March 17, 1978 for concentrations of MMT resulting in 1/16 
and 1/32 gpg Mn in unleaded gasoline. Ethyl's second application was 
submitted on May 26, 1981 for concentrations of MMT resulting in 1/64 
gpg Mn in unleaded gasoline. The Administrator denied these requests 
for waivers due to concerns regarding increases in exhaust hydrocarbon 
emissions resulting from MMT use. The decisions and justifications 
thereof may be found in the September 18, 1978 Federal Register, 43 FR 
41424, and the December 1, 1981 Federal Register, 46 FR 58630. Ethyl's 
third application was submitted on May 9, 1990, for concentrations of 
MMT resulting in a level of 0.3125 (1/32) gpg Mn in unleaded gasoline 
(the same levels which are requested in the application which is the 
subject of today's notice). Ethyl withdrew its third application on 
November 1, 1990, before the deadline for the Administrator to make a 
determination on the application. Because no determination had been 
made at the time Ethyl withdrew that application, EPA accepted the 
withdrawal and immediately terminated the proceeding without action on 
the application.
    Ethyl's fourth application was submitted on July 12, 1991. This 
application was, from a practical standpoint, an extension of the third 
application, the entire record of which was incorporated by Ethyl into 
the current proceeding. On January 8, 1992, the Administrator of EPA 
denied Ethyl's fourth application for a waiver (57 FR 2535, January 22, 
1992). The application was denied based in part upon data submitted by 
Ford Motor Company (Ford) which indicated that, for the model groups 
tested by Ford and, for the conditions under which Ford tested its 
vehicles, the increases in hydrocarbon exhaust emissions as a result of 
the use of MMT were substantially greater than those observed in the 
Ethyl test program. The Agency stated in its decision that a likely 
factor which might account for the differences observed between the 
Ethyl and Ford test programs was the severity of the driving cycle. 
However, the Agency also concluded that other factors might be 
responsible for the observed differences. In the denial decision, the 
Agency stated that it had always accepted data from test programs which 
``model'' the fleet in support of waiver applications, but that if an 
interested party were to present data indicating that a potentially 
significant subset of the fleet, not tested by the applicant, was 
especially susceptible to the negative effects of the additive, the 
Agency could reasonably require specific testing on representative 
models of that sub-fleet.
    In its decision, the Agency also stated that it believes it is 
reasonable to consider the effect of a fuel on vehicles' ability to 
meet future emissions standards. (The ``Tier I'' tailpipe standards 
prescribed by section 202(g) of the Act began to take effect in model 
year 1994, which began approximately in September 1993.\1\) Therefore, 
regarding the Ford data mentioned above, the Agency stated in its 
decision that the concerns raised by that data related to both current 
and future standards.

    \1\56 FR 25724-25790 (June 5, 1991).

    Although not the basis of the 1992 denial, another important issue 
arose during the consideration of Ethyl's third and fourth 
applications. The Agency, as well as several commentors, expressed 
concerns regarding the possible adverse health effects of an increase 
in airborne manganese resulting from MMT use. These concerns were 
centered around: (1) The known severe neurotoxic effects of high-level 
exposure to manganese through inhalation, (2) the lack of data 
regarding the chronic effects of low-level inhalation exposure to 
manganese in humans, and (3) the lack of knowledge regarding potential 
exposures due to MMT use. It was repeatedly pointed out by commenters 
that neurotoxic damage could occur prior to the onset of overt 
    In those proceedings, Ethyl also submitted comments regarding 
manganese emissions. Ethyl indicated that the manganese emissions 
resulting from the use of MMT in unleaded gasoline would be so small as 
to not materially affect human exposure to airborne manganese. In 
support of its view, Ethyl submitted analyses and data on exposure 
modeling and monitoring in both its 1990 and 1991 applications (and in 
subsequent submissions associated with the remand discussed below). 
(The issue of manganese emissions and public health is discussed in 
more detail in Section VI of this document.)
    During EPA's consideration of the 1990 Ethyl submission, EPA's 
Office of Research and Development (ORD) conducted a manganese 
inhalation risk assessment based on the available data which found that 
because of ``the considerable uncertainties and data gaps in the 
available information * * * it is not possible * * * to conclude 
definitively that the increased use of MMT as a fuel additive will (or 
will not) increase public health risk.''\2\ (EPA also investigated 
potential hazards associated with water contamination resulting from 
accidental spills or leakages of pure MMT and concluded that spills or 
leaks, if they occurred, are likely to be contained and therefore would 
not pose a human health risk due to groundwater contamination. However, 
data available to EPA are insufficient to determine whether spills and 
leaks could affect exposure to benthic organisms.)

    \2\See ``Comments on the Use of Methylcyclopentadienyl Manganese 
Tricarbonyl in Unleaded Gasoline'', Docket A-90-16.

    Additionally, in order to obtain assistance in describing 
information needed to improve its manganese health risk assessment (and 
also to improve its environmental hazard identification of issues 
associated with MMT itself), EPA, in conjunction with the National 
Institute of Environmental Health Sciences, conducted a Manganese/MMT 
Symposium and Workshop on March 12-15, 1991. The conference allowed the 
Agency to solicit scientific information from invited extramural 
scientists reflecting a wide range of scientific disciplines. Invited 
participants included representatives of Ethyl Corporation, the 
Environmental Defense Fund, the Centers for Disease Control, the U.S. 
Food and Drug Administration and Environment Canada. A summary of the 
workshop discussions was provided to each participant and the 
information obtained from this meeting was also used by EPA to prepare 
a report on prioritized research needed for improving its manganese 
inhalation risk assessment.\3\

    \3\Preuss, P.W. (1991) ORD Document on Information Needed to 
Improve the Risk Characterization of Manganese Tetraoxide 
(Mn3O4) and Methylcyclopentadienyl Manganese Tricarbonyl, 
December 12, 1991 (memorandum to Richard Wilson). Washington, DC: 
U.S. Environmental Protection Agency, Office of Research and 
Development; December 16, 1991. For further information the reader 
is referred to Air Docket A-93-26, II-A-16.

    EPA raised the issue of potential health effects associated with 
manganese exposure as a concern in its January 1992 denial, but did not 
base its decision on this concern because the Agency concluded that the 
uncertainties regarding hydrocarbon emissions increases prevented EPA 
from making the requisite ``cause or contribute'' determination 
concerning effects on regulated emissions.
    On February 13, 1992, Ethyl filed a petition for review of the 
January 8, 1992 waiver denial decision in the United States Court of 
Appeals for the District of Columbia Circuit. EPA and Ethyl 
subsequently entered discussions concerning a possible settlement of 
the court case. In the context of those discussions, Ethyl submitted to 
the Agency new emissions test data developed by Ethyl since the denial 
    Based on its inspection and analysis of the new Ethyl data, EPA 
tentatively concluded that the data indicated that driving cycle did 
not contribute significantly to MMT-induced increases in HC emissions. 
(EPA's preliminary analysis was placed in docket A-92-41.) However, in 
addition to addressing the issue of driving cycle, the Ethyl data 
appeared to confirm the finding by Ford that 1991 Escorts experienced a 
much higher MMT-induced HC increase than that observed in other models 
tested (either in Ethyl's new program or in the original Ethyl test 
program). The Agency remained concerned that these data might indicate 
that certain engine and emissions control system configurations are 
more vulnerable to a MMT-induced emissions increase irrespective of 
driving cycle.
    To facilitate further settlement discussions with Ethyl, EPA 
decided to attempt to formulate an emission testing program intended to 
address in a timely manner specific unresolved issues concerning the 
effect of MMT on emissions: (1) whether other vehicles utilizing fuels 
containing MMT are likely to experience increases in hydrocarbon 
emissions similar to those observed in 1991 Ford Escorts; and (2) 
whether fuels containing MMT have significant adverse effects on 
emissions from vehicles utilizing the technologies most likely to be 
employed to meet future standards. On October 28, 1992, EPA held a 
public workshop to assist the Agency in its attempt to formulate such 
an emission testing program (57 FR 44740, September 29, 1992). In 
particular, EPA hoped to obtain information and assistance from 
technical experts outside of the Agency concerning the test program 
and, in view of the significance of any future waiver decision 
concerning MMT for the auto industry and the general public, EPA was 
interested in obtaining comments concerning a decisional framework 
designed to address and resolve these issues. A proposed emission test 
program developed by the Agency and presented at the public workshop, 
was effectively adopted by Ethyl as its most recent vehicle emissions 
test program involving the 1993 model fleet.
    Although further settlement discussions between Ethyl and EPA were 
held subsequent to the public workshop, the parties were not successful 
in reaching a settlement. However, despite the failure of the parties 
to reach agreement, EPA concluded that the Administrator's denial 
decision should be reconsidered in light of the new emissions data 
generated by Ethyl subsequent to the decision. Accordingly, EPA 
requested that the United States Court of Appeals for the District of 
Columbia remand the denial decision to EPA for reconsideration.
    On April 6, 1993, the Court of Appeals issued a decision granting 
the Agency's motion and remanding the case to the Agency to redetermine 
within 180 days whether to grant or deny Ethyl's application. The 
mandate implementing this judgement was transmitted to the Agency on 
June 3, 1993. Pursuant to the court's remand decision, the Agency 
published a notice indicating the commencement of a comment period (58 
FR 35950, July 2, 1993). The Administrator's final decision on remand 
was due within 180 days after the transmittal of the court's mandate, 
or by November 30, 1993.
    After the Court of Appeals granted the Agency's motion to remand 
the denial decision concerning Ethyl's July 12, 1991 application, Ethyl 
submitted to EPA a substantial amount of additional data on emission 
testing with fuels containing MMT. (Specific aspects of these data are 
discussed below in Section IV of this document).
    During the course of the remand of Ethyl's waiver application, the 
EPA Office of Research and Development (ORD) reviewed the available 
data concerning the health effects associated with inhalation of 
manganese as part of a process to revise the reference concentration 
(RfC) for inhaled manganese.\4\ An inhalation reference concentration 
is defined as an estimate (with uncertainty spanning perhaps an order 
of magnitude) of a continuous inhalation exposure to the human 
population (including sensitive subgroups) that is likely to be without 
appreciable risk of deleterious non-cancer health effects during a 
lifetime. The methodology for establishing an RfC accounts for 
uncertainties and gaps in the health data base through the assignment 
of uncertainty factors. In November, 1993, ORD completed preparation 
and review of, and EPA released to Ethyl, a document identifying and 
describing the rationale for a new inhalation RfC of 0.05 ug/m\3\ for 
manganese and manganese compounds.

    \4\In 1990, an inhalation reference concentration (RfC) for 
manganese of 0.4 ug/m\3\ was verified and placed on IRIS. The 
original RfC for manganese figured into a 1990 risk assessment of 
MMT prepared by the EPA Office of Research and Development (ORD). 
Subsequently, in light of new information submitted by Ethyl and new 
results from more recently published studies concerning manganese 
inhalation health effects in workers, EPA reexamined the RfC for 
manganese and revised it to a value of 0.05 ug/m\3\ in 1993. This 
revised RfC for manganese was made available to Ethyl and placed on 
IRIS in November 1993.

    Ethyl subsequently provided to EPA a detailed critique of the 
approach utilized to derive the revised manganese RfC. Among other 
things, Ethyl argued that EPA used an inappropriate procedure to derive 
the RfC from a study of occupational manganese exposures by Roels, et 
al. (1992). Ethyl also argued that use of MMT would not result in 
significant changes in background manganese exposures, and that the 
favorable effects on public health resulting from changes in the 
composition of gasoline when MMT is utilized would outweigh any 
potential for adverse health effects. (Copies of documents describing 
the revised RfC and of the Ethyl comments are available in the public 
    As the deadline of November 30, 1993, for final action by EPA on 
Ethyl's waiver application approached, EPA concluded that the extensive 
data base on the emission effects of MMT assembled by Ethyl and others 
during the consideration of the application was sufficient to permit a 
decision concerning whether Ethyl had satisfied the statutory 
requirement to show that use of MMT will not cause or contribute to 
exceedence of emission standards. However, there had been insufficient 
opportunity for public comment concerning the use of a revised 
manganese inhalation RfC in assessing any risks that might be posed by 
granting Ethyl's application. Ethyl argued that it had not been 
afforded an adequate opportunity to study the derivation of the RfC and 
to comment on its implications for Ethyl's application. While EPA 
scientists did not necessarily agree with the specific technical 
arguments concerning the revised RfC and other issues pertaining to 
health effects made by Ethyl, EPA concluded that it might be useful to 
review the revised RfC in light of further analyses of the available 
data as well as the underlying data from occupational studies of 
inhaled manganese if such data could be readily obtained. EPA also 
concluded that it would be desirable in any case to have further 
dialogue with Ethyl and other interested parties on issues related to 
the health effects of manganese before EPA was to make a final decision 
concerning Ethyl's waiver application.
    As a result of these factors, Ethyl and EPA entered into 
discussions concerning a possible extension of the deadline for a 
decision. Ultimately, an agreement between Ethyl and EPA concerning 
such an extension was implemented on November 30, 1993, and notice of 
the agreement was published in the Federal Register on December 9, 1993 
(58 FR 64761). The agreement provided for an extension of 180 days in 
the deadline for final action by EPA on Ethyl's waiver application for 
HiTEC 3000.\5\ EPA was thus required to take final action either 
granting or denying Ethyl's resubmitted application by May 29, 1994. 
For purposes of the resubmitted application, the EPA Administrator 
determined that Ethyl had demonstrated, as required by section 
211(f)(4), that use of HiTEC 3000 at the specified concentration will 
not cause or contribute to a failure of any emission control device or 
system (over the useful life of any vehicle in which such device or 
system is used) to achieve compliance by the vehicle with the emission 
standards with respect to which it has been certified.\6\ The Agency 
stated clearly in the December 9, 1993 Federal Register notice that 
this determination would not preclude any subsequent regulatory action 
based on emission effects under Clean Air Act section 211(c) or any 
other provision of the Clean Air Act in the event that the resubmitted 
Ethyl waiver application were to be granted in the future. The Agency 
also made it clear that this determination would not apply in the 
context of any other new waiver application concerning HiTEC 3000 or 
MMT which might be submitted in the future if EPA were to deny Ethyl's 
resubmitted waiver application on other grounds. Further review of 
Ethyl's application during this 180 day period focused in particular on 
the issues relating to the potential health effects on public health if 
EPA were to permit use of MMT as a fuel additive.

    \5\To implement this agreement, Ethyl withdrew its July 12, 1991 
waiver application, as remanded by the Court of Appeals, and 
immediately resubmitted the application.
    \6\As is explained in section IV of this document, this decision 
was based primarily upon application of the previously used 
statistical tests to the submitted emissions data. As is also 
explained in section IV, the Agency believes that these tests may be 
outdated and is considering a formal change in its method of 
analysis of such data.

    Ethyl and EPA both desired and intended to assure continuity 
between the proceedings concerning the July 12, 1991 waiver 
application, as remanded to EPA by the Court of Appeals, and Ethyl's 
resubmitted waiver application. The entire administrative record 
compiled by EPA in support of the original denial decision, as well as 
all submissions to the public docket concerning the remanded 
application, was incorporated in the record this final decision on the 
resubmitted application. The docket number for the resubmitted waiver 
application also remained the same.
    The additional 180 days that were provided by Ethyl's agreement to 
resubmit the waiver application were utilized by EPA to evaluate 
remaining issues that may have been relevant to today's decision. In 
particular, EPA continued to examine the effects on public health that 
might be associated with approval of Ethyl's application. EPA 
considered any additional underlying data concerning studies of 
occupational manganese exposure that were obtained by or submitted to 
EPA, as well as any additional data or information pertaining to the 
health effects of manganese submitted by Ethyl or other interested 
persons during the comment period. Any additional information that was 
submitted was also considered in exploring alternative candidate RfC 
estimates and their relationship to the verified revised RfC. EPA also 
used the additional time provided by the extension to make a decision 
on how the RfC should be utilized in assessing health effects that may 
be associated with MMT use, evaluate potential exposure to manganese 
compounds associated with MMT use, complete a risk assessment 
concerning Ethyl's application, and decide what additional data, if 
any, should be provided by Ethyl either before or after MMT is 
introduced into the market.
    On April 28, 1994, EPA provided Ethyl Corporation with a draft of 
the revised risk assessment, which updated the 1991 ORD assessment and 
incorporated further analyses performed during the 180-day extension 
period. Subsequent to providing Ethyl with this draft, Ethyl provided 
EPA with comments on the draft and some additional new data on ambient 
manganese concentrations in several Canadian cities. In order to allow 
the Agency time to consider this new data, Ethyl requested, and the 
Agency agreed to, an extension of the decision deadline until July 13, 
1994. An agreement implementing this extension was executed by EPA and 
Ethyl counsel on May 24, 1994.

II. Statutory Framework

A. History of Statute

    Congress first added section 211(f) to the Clean Air Act in 1977 
based primarily on concerns that fuels or additives might damage 
vehicle emission control devices. Thus, the original statute focused on 
vehicles designed to use unleaded gasoline, prohibiting the general use 
in fuels of materials not ``substantially similar'' to fuels used to 
certify vehicles to emissions standards. Section 211(f) also provided 
that the Administrator of EPA ``may waive the prohibitions * * * if he 
determines that the applicant has established that such fuel or fuel 
additive * * * will not cause or contribute to a failure of any 
emission control device or system * * * to achieve compliance by the 
vehicle with the emission standards with respect to which it has been 
certified pursuant to section 206.''\7\ Additionally, the statute 
provides that if the Administrator does not act to grant or deny the 
waiver request within 180 days of receipt of the application, the 
waiver request shall be treated as granted.

    \7\Section 206 of the Act sets forth the certification 
requirements with which vehicle manufacturers must comply in order 
to introduce into commerce new model year motor vehicles. Under 
Sec. 202 of the Act, standards for hydrocarbon (HC), carbon monoxide 
(CO), and oxides of nitrogen (NOx) emissions for gasoline, gaseous 
fuel, diesel and methanol-powered motor vehicles have been 
established. For gasoline, gaseous fuel and diesel-powered motor 
vehicles, standards have also been established for particulate 

    Section 211(f) was initially interpreted by the Agency as applying 
only to unleaded gasoline. In the 1990 Amendments, section 211(f)(1) 
was broadly expanded to cover all other fuels and fuel additives, 
including leaded gasoline, diesel fuel, and consumer additives.\8\ The 
1990 Amendments also apply the provisions of this subsection to 
vehicles other than lightduty vehicles. Section 211(f)(1)(B) of the Act 
makes it unlawful, effective November 15, 1990, for any manufacturer of 
a fuel or fuel additive to first introduce into commerce, or to 
increase the concentration in use of, any fuel or fuel additive for use 
by any person in motor vehicles manufactured after model year 1974 
which is not substantially similar to any fuel or fuel additive 
utilized in the certification of any model year 1975, or subsequent 
model year, vehicle or engine under section 206 of the Act. Thus, 
section 211(f)(1)(B) expands to all motor vehicles the fuel 
prohibitions of the original section 211(f)(1) (now redesignated as 
section 211(f)(1)(A)), which apply only to light-duty vehicles.\9\

    \8\H.R. Rep. No. 490, Part 1, 101st Cong., 2d Sess. 313 (1990).
    \9\An interpretive rule defining the term ``substantially 
similar'' under section 211(f)(1)(A) was promulgated for unleaded 
gasoline at 46 FR 38582 (July 28, 1981), and revised at 56 FR 5352 
(February 11, 1991). An advance notice of proposed rulemaking 
(ANPRM) has been published to begin the proces of promulgating an 
interpretive rule to define the term ``substantially similar'' under 
Sec. 211(f)(1)(B) for diesel fuel and diesel fuel additives. See 56 
FR 24362 (May 30, 1991).

    In adding section 211, the first focus of Congress was to prevent 
the introduction of new additives which may prove harmful to emission 
control devices but to allow for the introduction of such additives if 
it could be demonstrated that they would not harm emission control 
devices. Furthermore, in framing the statute such that the 
Administrator was not required to grant a waiver, Congress provided 
authority to the Administrator to take into account other 
considerations associated with introduction of the new material into 

B. Two Stage Process

    Section 211(f)(4) of the Act provides the legal authority for this 
waiver decision.\10\ The Agency interprets section 211(f)(4) of the Act 
as establishing a two stage process for the decision to grant or deny a 
waiver application. The first stage of the process focuses solely on 
whether a waiver applicant has met its burden to demonstrate that a 
fuel does not cause or contribute to a failure to meet emission 
standards. The second stage of the process reflects the discretionary 
authority provided to the Agency by the statute.

    \10\Section 211(f)(4) states that ``The Administrator, upon 
application of any manufacturer of any fuel or fuel additive, may 
waive the prohibitions established under paragraph (1) or (3) of 
this subsection, or the limitation specified in paragraph (2) of 
this subsection, if he determines that the applicant has established 
that such fuel or fuel additive or a specified concentration 
thereof, and the emission products of such fuel or additive or 
specified concentration thereof, will not cause or contribute to a 
failure of any emission control device or system (over the useful 
life of any vehicle in which such device or system is used) to 
achieve compliance by the vehicle with the emission standards with 
respect to which it has been certified pursuant to section 206. If 
the Administrator has not acted to grant or deny an application 
under this paragraph within one hundred and eighty days of receipt 
of such application, the waiver authorized by this paragraph shall 
be treated as granted.''

    In the first stage of the waiver process, the sole issue is whether 
a fuel ``causes or contributes'' to an emission standard failure. The 
waiver applicant bears the burden of demonstrating that a fuel will 
neither cause nor contribute to an emission standard failure for any 
regulated pollutant. Balancing of the emission effects of a fuel for 
one pollutant against those for other pollutant(s) is not permissible 
under the statutory language. For example, an applicant would not meet 
its burden of proof if its testing of a fuel shows that it causes or 
contributes to an emission standard failure for CO, even though testing 
shows decreases in emissions of HC and NOx. If an applicant does not 
meet its burden of demonstrating that the ``cause or contribute'' test 
is met, the Agency cannot grant a waiver. If an applicant does meet its 
burden, the Agency may then exercise its discretion to grant or to deny 
a waiver in the second stage of the process.\11\

    \11\Under the statute, if the Agency does not take action to 
grant or deny a waiver application within 180 days of submittal, the 
waiver is deemed granted.

    The statute provides that the Agency ``may'' grant a waiver 
application if the ``cause or contribute'' test is met, but does not 
require such an action. The Agency may therefore choose not to grant a 
waiver based on other issues (e.g., public health effects) that 
indicate that it would not be in the public interest to do so. In this 
second stage of the process, the Agency has a great deal of discretion 
to determine which issues should be examined and to balance the 
potential positive and negative impacts of a waiver. Such discretionary 
authority is grounded not only in Congress' use of the term ``may'' 
rather than the term ``shall'' in section 211(f)(4), but also in the 
goals and purposes of section 211 when read as a whole. The Agency does 
not believe that Congress intended to require EPA to grant a waiver 
under section 211(f)(4) when available information indicates that the 
fuel would be potentially subject to regulatory control under section 
211(c)(1) immediately upon issuance of the waiver. Similarly, EPA 
believes that Congress did not intend to preclude a determination of 
whether issuance of a waiver is consistent with other important goals 
of the Act once it has been demonstrated that the mandatory ``cause or 
contribute'' test has been met.
    This does not mean that the Administrator has unfettered discretion 
to deny a waiver application for any reason. The grounds for any denial 
must not be arbitrary or capricious or constitute an abuse of 
discretion. Thus, in using discretion to deny an application, the 
Administrator must identify and explain the factors on which a 
discretionary denial decision is based and must assure that the policy 
adopted is consistent for all similarly situated waiver applicants.

C. Consideration of Potential Health Effects

    Although the basis for a discretionary denial must be rational and 
non-arbitrary, nothing in the statute limits the type of factors which 
the Administrator may consider in deciding whether to deny an 
application. Section 101(b)(1) states that one of the purposes of the 
Act is to ``protect and enhance the quality of the Nation's air 
resources so as to promote the public health and welfare and the 
productive capacity of its population.'' Given this general goal of the 
Act, certainly the potential effects on public health of vehicle 
emissions would be a factor which the Administrator may reasonably 
consider when utilizing the discretion which section 211(f)(4) 
    Furthermore, under sections 211(b)(2) and 211(e), the Administrator 
must require the manufacturer of a fuel or additive to produce data 
concerning potential health effects as a condition of, or a 
prerequisite to, registration of the fuel or additive.\12\ Under 
section 211(c)(1), the Administrator may, based on data collected under 
sections 211(b) and 211(e) or otherwise available, issue regulations 
controlling manufacture or sale of any fuel or fuel additive which the 
Administrator finds ``may reasonably be anticipated to endanger the 
public health or welfare.'' These provisions indicate that Congress 
intended that the Administrator be concerned about the potential health 
effects of fuels and fuel additives.

    \12\Sections 211(a) and 211(b)(1) require the registration of 
fuels and additives designated by the Administrator as a 
precondition to introduction into commerce.

    The fact that the Administrator may control fuels or fuel additives 
which pose potential health effects under section 211(c)(1) does not 
mean that the Administrator may not consider health effects as a factor 
in deciding whether to grant a waiver under section 211(f)(4). Such a 
construction of the statute would lead to absurd results, precluding 
the Administrator from denying a waiver application and leading to 
potential introduction of a fuel or additive into commerce, even in the 
specific circumstances where the Administrator has concluded that there 
are grounds for issuance of a proposed regulation prohibiting the fuel 
or additive under section 211(c). However, although this reasoning 
indicates that Congress could not have reasonably intended to 
completely preclude the consideration of health effects under section 
211(f)(4), this does not mean that section 211(c) limits the 
circumstances in which the Administrator may consider potential health 
effects as part of a waiver decision. Clearly, it was the intention of 
Congress to treat fuels and fuel additives already registered and being 
sold for a particular purpose differently than those which have not 
already been introduced into commerce.

III. Method of Review

A. ``Causes or Contributes'' to Emission Standard Failure

    Under section 211(f)(4) of the Act, twenty-three applications for 
waivers of the section 211(f)(1) prohibitions have been received. Of 
these, twenty-two applications have sought a waiver for additives for 
unleaded gasoline. One, the most recent, sought a waiver of the section 
211(f)(1)(B) prohibitions for an additive to diesel fuel.\13\ Of these 
twenty-three applications, ten applications have been granted (some 
with conditions attached), ten have been denied, and three were 
withdrawn by the applicant prior to the Agency's decision.

    \13\57 FR 45790 (October 5, 1992).

    Section 211(f)(4) clearly places upon the waiver applicant the 
burden of establishing that its fuel will not cause or contribute to 
the failure of any vehicle to meet emission standards. Absent a 
sufficient showing, the Administrator cannot make the required 
determination and cannot grant the waiver. If interpreted literally, 
however, this burden of proof imposed by the Act would be virtually 
impossible for an applicant to meet, as it requires the proof of a 
negative proposition: that no vehicle will fail to meet emission 
standards to which it has been certified. Such a literal interpretation 
could be construed as requiring the testing of every vehicle. 
Recognizing that Congress contemplated a workable waiver provision, EPA 
has previously indicated that reliable statistical sampling and fleet 
testing protocols may be used to demonstrate that a fuel under 
consideration would not cause or contribute to a significant failure to 
meet emission standards by vehicles in the national fleet.\14\

    \14\See Waiver Decision on Tertiary Butyl Alcohol (``TBA''), 44 
FR 10530 (February 2, 1979).

    To determine whether a waiver applicant has established that the 
proposed fuel will not cause or contribute to vehicles failing emission 
standards, EPA reviews all the material in the public docket, including 
the data submitted with the application and public comments on the 
application, and analyzes the data to ascertain the fuel's emission 
effects. The analysis concentrates on four major areas of concern--
exhaust emissions, evaporative emissions, materials' compatibility, and 
driveability--and evaluates the data under statistical methods 
appropriate to the various types of emission effects. Emission data are 
analyzed according to the effects that a fuel is predicted to have on 
emissions over time. If the fuel is predicted to have only an 
instantaneous effect on emissions (that is, the emission effects of the 
fuel are immediate and remain constant throughout the life of the 
vehicle when operating on the waiver fuel), then ``back-to-back'' 
emissions testing will suffice.\15\

    \15\Back-to-back emission testing involves testing a vehicle on 
a base fuel (i.e., a gasoline which meets specifications for 
certification fuel or is representative of a typically available 
commercial gasoline), then testing that same vehicle on the fuel for 
which the waiver is requested. The difference in emission levels is 
attributed to the waiver fuel.

    Unlike materials traditionally allowed in unleaded gasoline, 
metallics, such as MMT, produce non-gaseous combustion products, some 
of which may be deposited in the parts of the vehicle that come in 
contact with the combustion products of the burned fuel. These areas of 
the vehicle include the combustion chamber, the catalyst, the oxygen 
sensor, and all parts of the exhaust system.\16\ Since these materials 
build up over time,\17\ it has been traditionally accepted that the 
emissions effects of such additives occur over time as miles are 
accumulated, and that the method of deposition suggests that the 
effects are permanent. If the fuel is predicted to have such a long-
term deteriorative effect, durability testing over the useful life of 
the vehicle,\18\ in addition to back-to-back testing, is 

    \16\Automakers and catalyst manufacturers point out that, since 
catalysts are designed with a honeycomb structure in order to 
maximize contact between engine combustion gases and catalyst 
materials, if channels within the honeycomb become blocked, the 
catalyst is less able to break down the exhaust gases. Furthermore, 
although the mechanisms associated with manganese deposits have not 
been completely described, catalyst manufacturers suggest that the 
mere disposition of manganese (without blockage of channels) would 
hinder the catalytic activity of the catalyst. Ethyl, however, 
believes that the manganese deposition on the catalyst does not 
hinder its activity.
    \17\Reply Comments of Ethyl Corporation in Support of the HiTEC 
3000 Waiver Application, August 10, 1990, 28.
    \18\The ``useful life'' of a 1993 or earlier model year light-
duty vehicle (LDV) (i.e., the amount of time or mileage accumulation 
through which the LDV must meet the standards to which it has been 
certified) is 50,000 miles or five years, whichever occurs first 
(Sec. 202(d)). The 1990 Amendments extended the useful life of LDVs 
to 100,000 miles or ten years, beginning with 1994 model year 
vehicles. The amendments also tightened emissions standards for 40 
percent of a vehicle manufacturer's LDV and light-duty truck (LDT) 
sales in model year 1994, 80 percent in model year 1995 and for all 
vehicles after model year 1995 (Sec. 202(g)). The useful life for 
heavy-duty vehicles and engines is generally 120,000 miles or eleven 
    \19\Durability testing over the useful life of the vehicle has 
involved testing two identical sets of vehicles for 50,000 miles (in 
the case of pre-1994 standards for LDVs), one set using the base 
fuel and the other using the waiver fuel. Each vehicle is tested for 
emissions at 5,000 mile intervals. This is essentially the same 
testing pattern which has been required for certification of a new 
motor vehicle under Sec. 206 of the Act. As noted above, under the 
1990 Amendments, the useful life of LDVs has been extended to 
100,000 miles beginning with the 1994 model year when more stringent 
emissions standards took effect (see Sec. 202 (d) and (g)).

    In addition to emissions data, EPA also reviews data on fuel 
composition and specifications, both to fully characterize a proposed 
fuel, and to determine whether that fuel would cause or contribute to a 
failure of vehicles to comply with their emission standards. Such a 
failure often can be predicted from characterization data. For example, 
volatility specifications of the fuel could demonstrate a tendency for 
high evaporative emissions. Similarly, data on materials compatibility 
could show potential failure of fuel systems, emission related parts, 
and/or emission control parts from use of the fuel. Such failures could 
result in greater emissions. Likewise, fuel characteristics that could 
cause significant driveability problems could result in tampering with 
emission controls and, thus, increased emissions.
    One issue raised previously in the context of Ethyl's present 
application was whether Ethyl was required to show that MMT will not 
cause or contribute to noncompliance with emission standards by 
vehicles certified to ``future'' emission standards (i.e., 1994 model 
year standards, which were not in effect at the time of the waiver 
application), as well as vehicles certified to ``current'' standards 
(i.e., standards in effect at the time of the waiver application). 
Ethyl believes that the statute only requires it to establish that MMT 
will not cause or contribute to the failure of vehicles to meet current 
emission standards. For the reasons outlined in the Agency's January 
1992 waiver decision, EPA disagrees with this reading of the statute 
and continues to believe that it is appropriate to consider the effects 
of an additive on vehicles' ability to meet more stringent future 
standards under circumstances similar to these.\20\ (See 57 FR 2537-8 
January 22, 1992.)

    \20\EPA also considered effects on compliance with future 
standards in a previous MMT decision. See 43 FR 41424 (September 18, 
1978), In Re Application for MMT Waiver.

    In the past, EPA has analyzed both instantaneous emission effects 
and durability effects using statistical tests to determine if the fuel 
additive will cause a ``significant'' number of vehicles to fail 
emissions tests.\21\ Generally speaking, these tests have focused on 
the portion of the fleet that will actually fail emission standards as 
a result of using the fuel or additive.\22\ Thus, the tests used to 
date by the Agency primarily consider only the ``cause'' language in 
the statute and do not consider the portion of the statute which 
requires that the applicant must also show that the fuel or additive 
will not ``contribute'' to the non-compliance of vehicles with emission 

    \21\For a detailed description of the statistical tests which 
have been used in the past for instantaneous effects see ``Decision 
Document'', Texas Methanol Waiver Decision, U.S. EPA Air Docket 
Number EN-87-06, and for those used for durability effects, see 43 
FR 41426.
    \22\In fact the primary criteria allows for the failure of some 
portion of the fleet as a result of use of the fuel or additive.

    The Agency believes that its present statistical tests and criteria 
do not give adequate weight to the requirement in Section 211(f)(4) 
that an applicant demonstrate that a fuel will not ``contribute'' to an 
emission standard failure.\23\ This is of particular significance in 
light of the Clean Air Amendments of 1990, which evidence a strong 
Congressional concern that more needs to be done to ensure that people 
are not exposed to unhealthy levels of airborne pollution. EPA is 
presently reviewing alternative criteria and statistical methodologies 
for determining whether use of a fuel or fuel additive will ``cause or 
contribute'' to emission exceedances. The Agency expects to initiate a 
rulemaking in the near future which will propose more appropriate 
criteria and statistical methodologies for reviewing waiver 
applications and will afford formal notice to future applicants of the 
Agency's intention to adopt revised criteria and methodologies.

    \23\In fact, the Agency raised questions about the 
appropriateness of these previously used approaches in its original 
decision on Ethyl's 1991 MMT waiver application. See 57 FR 2535, 
2537 and 2538 (January 22, 1992).

    As explained below, the Agency has concluded that it would not be 
appropriate to utilize new criteria and statistical tests concerning 
which Ethyl received no prior notice in evaluating Ethyl's application. 
However, in the event that Ethyl reapplies in the future for a section 
211(f)(4) waiver to allow the use of MMT or any other additive, that 
application will be evaluated in accordance with any new fuel waiver 
criteria in effect at that time.

B. Discretionary Review

    As discussed in part II of this decision, above, the Agency 
believes that the use of the term ``may'' in Section 211(f)(4) of the 
Act affords the Administrator broad discretion to consider other 
factors in deciding whether to grant a waiver, once a waiver applicant 
has demonstrated that a fuel or fuel additive will not cause or 
contribute to an emission standard failure. This construction of 
section 211(f)(4) is also consistent with the other provisions of and 
the general purposes underlying the Clean Air Act. Although the 
Administrator has not relied on this discretionary authority to deny a 
waiver in the past, certain general principles should guide the 
Administrator's exercise of such authority.
    Although the discretion of the Administrator to consider other 
factors in making a waiver decision is broad, it is not unfettered. To 
assure that any decision based on factors other than emission standard 
failures is not arbitrary and is based on a proper record, the 
applicant and other interested persons should be afforded proper notice 
of any additional factors to be considered by the Administrator and an 
opportunity to comment or submit information concerning those factors. 
Any decision based on the discretionary authority of the Administrator 
to consider other factors should include an explanation of the factors 
which were considered and the relation of those factors to the 
decision. Moreover, any policy adopted as part of a decision to deny a 
waiver on a discretionary basis should be applied consistently to all 
similarly situated applicants.
    Protection of the public health is a major goal of both the Clean 
Air Act in general and the section 211 fuels provisions in particular. 
Accordingly, the Agency believes that when a waiver is sought for a 
fuel or fuel additive and there are unresolved concerns regarding the 
potential impact of that fuel or fuel additive on public health, 
potential health effects can and should be examined as part of the 
waiver process. As part of this examination of the potential health 
effects of a fuel or additive, the Agency should review any relevant 
studies or analyses of which it is aware or which are brought to its 
attention by the waiver applicant or by commenters on the waiver 
    In addition to potential health effects, the Agency may consider 
other factors as appropriate in deciding whether it would be in the 
public interest to grant a waiver. In particular, the Agency may 
consider whether a waiver would be consistent with the objectives of 
the Clean Air Act. In each instance, the factors considered and relied 
upon should be clearly identified.

IV. Analysis of Emissions Data

A. Description of Previous Test Programs

    In support of its request, Ethyl conducted an extensive test 
program to determine the effect of MMT on the ability of vehicles to 
comply with current and future emission standards. It also considered 
the impact of MMT on nonregulated vehicle emissions, urban smog or 
ozone, refinery emissions, and crude oil use. Ethyl claimed that its 
test results established that MMT would not cause or contribute to 
exceedences of current or future emission standards. It also claimed 
that MMT use would result in other benefits consistent with Clean Air 
Act goals.
    In 1988, Ethyl assembled a test fleet of 48 light-duty vehicles, 
composed of eight different model types (six Buick Centurys (2.5 
liter), six Buick Centurys (2.8 liter), six Buick Centurys (3.8 liter), 
six Chevrolet Cavaliers (2.0 liter), six Ford Escorts (1.9 liter), six 
Ford Tauruses (3.0 liter), six Ford Crown Victorias (5.0 liter) and six 
Dodge Dynastys (3.0 liter)) that together represented a broad spectrum 
of then current (1988) technology vehicles. To accumulate mileage, 
Ethyl utilized the ``Alternative Mileage Accumulation Cycle'' (AMA) 
which is a standard procedure utilized to accumulate mileage for 
certification purposes.\24\ It utilized two laboratories to measure 
each vehicle's exhaust emissions of the regulated pollutants (HC, 
oxides of nitrogen (NOx) and carbon monoxide (CO)) at 5,000-mile 
intervals up to 75,000 miles in the case of most vehicles and up to 
100,000 miles in the case of several.\25\ It also tested a number of 
these vehicles for evaporative HC, particulate and manganese emissions, 
materials compatibility, driveability and catalyst durability.

    \24\A driving cycle is a description of how to drive a vehicle 
to accumulate mileage, including such things as a what percentage of 
driving should be done at what speed and what the overall average 
speed should be. The AMA cycle is described in EPA Mobile Source 
Advisory Circular 37-A, (See Docket A-91-46) and is essentially 
prescribed for use by manufacturers to accumulate mileage for 
certification of vehicles (See 40 CFR 86.092-26). A driving cycle is 
used so that test vehicles accumulate mileage in a manner that is 
supposedly representative of in-use vehicles. The emissions of a 
test vehicle that has accumulated mileage according to a driving 
cycle representative of in-use vehicles are more likely to be 
representative of in-use vehicles' emissions. There are actually 
three alternative cycles associated with the AMA; however, the 
average speeds of the three alternatives are very similar, ranging 
from 29.9 mph to 30.72 mph.
    \25\The ``useful life'' of model year 1993 and earlier light-
duty vehicles (LDV's) is 50,000 miles or five years, whichever 
occurs first (section 202(d)). However, the Clean Air Act Amendments 
of 1990 extended the useful life of LDV's to 100,000 miles or ten 
years, beginning with 1994 model year vehicles. For the standards 
that begin to take effect in model year 1994, section 207(c) 
provides for intermediate in-use standards for several years.

    Ethyl analyzed the data collected using EPA's previously used 
statistical tests (43 FR 41424, September 18, 1978) and additional 
tests developed by its consultants to further characterize the data. 
Its analysis indicated that, on average, MMT at the requested 
concentration would result in a 0.018 gpm increase in HC emissions and 
decreases in NOx and CO emissions. The analyses further indicated 
that, when EPA's previously used tests are applied, the increase in HC 
emissions would not cause or contribute to vehicles' failure to meet 
the current HC emission standard. The results of Ethyl's testing for 
materials compatibility, driveability and catalyst durability also 
indicated that MMT would have no significant adverse effects on 
vehicles' ability to meet current emission standards under average 
driving conditions. On that basis, Ethyl claimed that it had made its 
statutorily required showing.
    Ethyl also submitted data on the catalyst efficiency of the 
vehicles which it tested. Ethyl performed back-pressure tests\26\ on 
all its vehicle fleet except one model group after accumulation of 
75,000 miles. Back-pressure tests were also performed on a pair of Ford 
Crown Victorias, one operated on MMT-fuel and one on clear fuel, at 
speeds higher than those used in Ethyl's 48-vehicle test program.\27\ 
The results of these tests indicated that back-pressure was not 
significantly different in the MMT vehicles when compared to the clear 
fuel vehicles. Ethyl also operated two 5.7 liter Corvettes at extremely 
high speeds (100 mph) for 25,000 miles, one using MMT fuel and one 
using clear fuel. Although similar in magnitude, the back pressure for 
the MMT vehicle was slightly higher than that for the clear vehicle. 
Ethyl also presented catalyst efficiency\28\ data based on engine-out 
emissions of its fleet and based on ``slave engine'' testing\29\ for 
half of its fleet. Results of the slave engine testing indicated no 
statistically significant difference between the catalyst efficiencies 
for the MMT vehicle components when compared with the clear vehicle 
components. Finally, four Chevrolet Corsicas were operated to 100,000 
miles, two utilizing MMT fuel and two with clear fuel. The purpose of 
this testing was to investigate MMT's effect on the catalyst for a 
longer mileage interval than the 75,000 miles over which most of 
Ethyl's fleet had been driven. Catalyst efficiencies of the MMT 
vehicles were not significantly different when compared to the clear 
fuel vehicles.

    \26\Back pressure tests are used to determine if significant 
plugging has occurred in a vehicle's catalyst. The total pressure 
ahead of the catalyst is back pressure. This pressure is a measure 
of constriction in flow through the exhaust system caused by flow of 
the exhaust through the emissions control system and the noise-
reducing components of the vehicle. If plugging has occurred in a 
vehicle, the total pressure ahead of its catalyst, the back 
pressure, should be greater than expected (e.g., greater than a 
matching control vehicle).
    \27\In this program the maximum speed was 65 mph for the first 
25,000 miles and 80 mph for an additional 10,000 miles.
    \28\Catalyst efficiency is a measure of what fraction of the 
emissions entering the catalyst are actually removed (or catalyzed) 
by the catalyst.
    \29\``Slave engine'' testing is the testing of vehicle 
components on a single engine which is not in a vehicle. In this 
case, catalyst efficiencies between control and MMT vehicles were 
investigated using exhaust gases from this single engine which were 
routed through the removed catalysts. This would likely result in a 
more accurate analysis of catalyst efficiency, since one possible 
confounding factor, vehicle to vehicle variability, would be 

    Ford presented original test data which Ford said supported its 
contention that actual in-use MMT-induced HC emissions increases are 
potentially far greater than those reported by Ethyl.\30\ Ford 
conducted testing on a more limited scale utilizing eight vehicles, 
representing two model groups, run for 105,000 miles. Ford chose two 
model groups which were representative of its newest technology 
vehicles at the time. One (the Explorer) represented a technology that 
Ford believed may be especially prone to exhibit a buildup of 
manganese, due to significantly higher operating temperatures and loads 
than those of passenger cars. The other model group, the Escorts, had 
close-coupled catalysts, a design which is being incorporated into many 
new vehicles in order to meet tighter emissions standards. Like Ethyl, 
Ford operated part of its test fleet on clear fuel and part on fuel 
containing 1/32 gpg MMT. However, Ford's test program differed from 
Ethyl's program in several ways. When accumulating mileage, Ford 
utilized a commercial gasoline which contained all of the additives 
(detergents, etc.) typically found in such fuels. Ethyl utilized a very 
high quality test fuel with tight specifications and no additives. 
(Although used for actual emissions testing purposes, Ethyl's fuel 
would not be allowed for mileage accumulation when certifying vehicles 
since it is not representative of in-use fuel.) When accumulating 
mileage, Ford utilized what it called its ``durability cycle'' which it 
had previously developed. Compared to the AMA cycle used by Ethyl, 
Ford's driving cycle had a higher average speed (54 miles per hour 
(mph) versus 30 mph), and a higher percentage of high speed 
driving.\31\ (As previously mentioned, Ethyl utilized the AMA cycle 
used for certification purposes.) Additionally, in the Ford program, 
vehicles were tested for emissions at five mileage intervals (5,000, 
20,000, 55,000, 85,000\32\ and 105,000 miles) and six emissions tests 
were done at each testing interval. Ethyl, by comparison, conducted 
testing every 5,000 miles to 75,000 miles (15 intervals) and utilized 
two emissions tests at each interval.\33\ Ford's test vehicles showed 
an elevation of HC emissions with MMT that was substantially greater 
than the 0.018 gpm reported by Ethyl from its test program.

    \30\EPA's emissions testing lab and Ford's lab routinely undergo 
correlation testing and the data indicate that correlation is good 
between the labs. (See memorandum, with attached data, from Martin 
E. Reineman, EPA Manager of Correlation and Engineering Services, 
Office of Mobile Sources, January 3, 1992, Docket A-91-46.)
    \31\Ford indicated that drivers who accumulated mileage in its 
test program were asked to follow posted speed limits. Ford 
indicated that the cycle consisted of 5% city driving (25 to 45 
mph), 5% gravel or off road driving (25 to 45 mph), 20% rural 
driving (45 to 55 mph), and 70% highway driving (65 mph). Posted 
speed limits are shown in parentheses. By way of comparison, the AMA 
cycle consists of 16.1% of driving at 30 mph, 22.6 at 35 mph, 20.9 
at 40 mph, 6.4 at 45 mph, 17% at variable speed and one of the three 
following options: 16.7% at 50 mph or 16.5% at 55 mph or 8.6% and 
7.9% at 55 mph and 70 mph, respectively.
    \32\In fact, only two of the four Escorts were tested at 85,000 
    \33\Although Ethyl conducted additional emissions tests at some 
mileage intervals when the initial two tests showed high variation, 
these additional tests were not used in Ethyl's analysis of its 

    Toyota also submitted data on a single vehicle which was operated 
for 30,000 miles on MMT-containing fuel after which the oxygen sensor 
and catalyst were replaced with new components and then driven on fuel 
not containing MMT for 30,000 miles. Toyota also used a driving cycle 
with an average speed (41.7 mph) higher than that used by Ethyl for 
mileage accumulation and used fuel with what Toyota believed was a 
relatively high trace level of lead than that usually found in unleaded 
gasoline (0.0045 gpg lead) and oil with a relatively high phosphorus 
level (0.13 weight percent). Toyota referred to this test procedure as 
the ``Toyota 9-Laps'' and presented evidence which it said suggested 
that the catalyst degradation seen by vehicles using the Toyota 9-Lap 
test was very similar to in-use catalysts tested by Toyota. Hence, 
Toyota suggested, these ``adjustments'' made in creating the Toyota 9-
Lap make the testing of a vehicle more consistent with what would 
happen in actual in-use driving. Toyota's data indicated an HC level 
after the first 30,000 miles of vehicle use (on MMT fuel) about 0.1 gpm 
higher than the same vehicle after the vehicle was driven for a second 
30,000 mile interval with a new catalyst and oxygen sensor. Toyota also 
submitted data indicating that the efficiency at which the catalyst was 
operating for the MMT-exposed components was less than that for the 
non-MMT exposed components.
    Some time after EPA's January 8, 1992 denial decision, EPA and 
Ethyl entered into discussions concerning a possible settlement of the 
court case which Ethyl had filed. In the context of these discussions, 
Ethyl submitted to the Agency new data it had developed since the 
denial decision. Ethyl tested six 1991 Escorts, using both the 
relatively high-speed driving pattern similar to that utilized by Ford 
in its testing of 1991 Escorts (the Ford cycle) and, also, after 
changing emissions system components (catalyst and oxygen sensor), the 
driving cycle used by Ethyl in the original test program (EPA's 
durability certification cycle also known as the AMA). Half of the 
vehicles utilized MMT-containing fuel and half were run on clear fuel 
(fuel not containing MMT). Ethyl also performed some catalyst 
efficiency tests on these vehicles utilizing a ``slave engine.''
    Ethyl also tested six 1988 Escorts which were used in its original 
test program driven on the AMA cycle. In the new program, after 
replacing the catalyst and oxygen sensor, Ethyl continued mileage 
accumulation, from 75,000 to 100,000 miles, utilizing the Ford cycle. 
Likewise, Ethyl tested six 1988 Buicks from its original fleet 
accumulating mileage (100,000 to 115,000 miles) using the Ford cycle 
but without replacing any components. Ethyl also accumulated mileage on 
seven pairs of 1992 vehicles (four Crown Victorias, Six Buick Regals 
and four Ford Mustangs) in test programs covering from 45,000 to 
100,000 miles beyond break-in with and without MMT, using the Ford 
    Based on its inspection and analysis of the new Ethyl data, the 
Agency ultimately concluded that Ethyl's program had demonstrated 
driving cycle does not contribute significantly to MMT-induced 
increases in hydrocarbon emissions. However, in addition to addressing 
the issue of driving cycle, the Ethyl data appeared to confirm the 
finding by Ford that 1991 Escorts experienced a much higher MMT-induced 
HC increase than that observed in other models tested (either in 
Ethyl's 1992 fleet or in the original 1988 Ethyl fleet). The Agency was 
concerned that these data could indicate that certain engine and 
emissions control system configurations were more vulnerable to an MMT-
induced emissions increase irrespective of driving cycle.
    To further assist the Agency in developing a test program, EPA held 
a workshop in October of 1992 and presented a proposed test program 
which could address in a timely manner specific unresolved issues 
concerning the effect of MMT on emissions: (1) Whether other vehicles 
utilizing fuels containing MMT are likely to experience increases in 
hydrocarbon emissions similar to those observed in 1991 Ford Escorts; 
and (2) whether fuels containing MMT have significant adverse effects 
on emissions from vehicles utilizing the technologies most likely to be 
employed to meet future standards.
    Ultimately the court case was not settled; however, the test 
program presented by the Agency at the workshop was largely adopted by 
Ethyl and is the basis of its most recent test program involving the 
1993 fleet. These vehicles (with the previously mentioned 1992 
vehicles) comprise Ethyl's most recent dataset.\34\

    \34\On May 25, 1993, and on subsequent dates, Ethyl provided 
summaries of the 1992/93 test data to EPA staff and these have been 
placed in public docket A-93-26.

    Ethyl accumulated mileage on three 1992 model year vehicles (four 
Crown Victorias to 100,000 test miles,\35\ six Buick Regals to 65,000 
test miles and four Ford Mustangs to 45,000 test miles) and six 1993 
model year vehicles (six Toyota Camrys to 85,000 test miles, six 
Oldsmobile Achievas to 65,000 test miles, six Dodge Shadows to 55,000 
test miles, six TLEV Ford Escorts to 85,000 test miles, six Honda 
Civics to 80,000 test miles and four 49-state Ford Escorts to 30,000 
test miles) with and without MMT. The driving cycles used for these 
vehicles were an intermediate driving cycle of 45 mph on average for 
the 1993 model year vehicles, an average 55 mph driving cycle (i.e., 
the Ford Cycle) for all mileage accumulation on the 1992 Ford Mustangs 
and for the initial 45,000 miles of operation on the 1992 Crown 
Victorias and Buick Regals and an average driving cycle of 45 mph was 
utilized for these two models thereafter.

    \35\As referred to here, ``test miles'' indicates mileage 
accumulated after break-in (break-in mileages vary among these 
models) and during which some vehicles were run on fuel containing 
MMT while control vehicles were run on clear fuel.

B. Comments on Vehicle Emissions Issues

    EPA provided an opportunity for the public to submit written 
comments.\36\ Many comments were received from a wide variety of 
interests, including refiners, automakers, emission control 
manufacturers, states committees, environmental and public interest 
groups and private citizens. Taken together, the comments touched on 
every aspect of Ethyl's application. The following is a summary of the 

    \36\As mentioned previously, the comments received concerning 
Ethyl's remanded waiver application are available in public docket 

    Four automakers (Ford Motor Company (Ford), General Motors 
Corporation (GM), Toyota Technical Center, U.S.A., Inc. (Toyota), and 
Chrysler Motors Corporation (Chrysler)), the American Automobile 
Manufacturers Association (AAMA), and the Manufacturers of Emission 
Controls Association (MECA) all recommended denial of Ethyl's request 
and expressed several concerns with regard to the addition of MMT to 
unleaded gasoline. First, they noted that the use of MMT will cause an 
increase in HC emissions. Most indicated that the more stringent 
emissions standards that began taking effect in model year 1994 will 
make any increase in HC emissions particularly troublesome. Further, 
they stated that newer technology vehicles will likely be equipped with 
catalysts which are nearer the engine (more ``closely coupled'') and 
that such close coupling, they stated, results in higher catalyst 
temperatures that may make the catalyst more prone to the deposition of 
manganese. These commenters indicated that deposition of manganese 
compounds on the surface of the catalyst would impair the catalytic 
breakdown of emissions from the engine, thereby decreasing catalyst 
effectiveness. Additionally, they were concerned that MMT, even at the 
1/32 gpg Mn concentration requested, would plug catalysts and thus 
reduce the surface area of the catalyst available to break down 
emissions from the engine, especially in the case of vehicles operated 
under driving conditions which result in higher temperatures such as 
heavy load or high speed. Under such conditions, it was pointed out, 
the vehicle may be more prone to deposition of manganese.
    Ethyl indicated that the assertions that it must ``conclusively'' 
demonstrate the absence of negative effects is not required by the 
section 211(f)(4) standard. Ethyl believes that it need only 
demonstrate, by a preponderance of evidence, that the additive will not 
cause or contribute to the failure of emission control devices to 
comply with applicable emission standards, and, further, it believes 
that it has made this showing. Ethyl also stated that the EPA test 
program proposed at its October 1992 workshop involving the 
accumulation of 65,000 test miles, would be sufficient for purposes of 
gauging the effect of MMT on emissions. Ethyl commented that it 
followed this proposal in the 1992/93 test fleet, although mileage 
accumulation has continued beyond 65,000 miles for three of the eight 
model year vehicles tested without new emission results different from 
the trends established through 65,000 miles.
    With respect to the automakers' concerns about effect of MMT on 
newer emission technology such as close-coupled catalysts, Ethyl 
indicated that the use of the 1993 Transitional Low Emission Vehicle 
(TLEV) Honda Civic in its most recent test program was intended so as 
to introduce a vehicle which has the most physically possible close-
coupled emission technology (i.e., one connected directly to the 
exhaust manifold). Despite such close-coupling, Ethyl indicated that 
the differences in hydrocarbon emissions between clear and MMT-fueled 
1993 TLEV Honda Civics was minimal. Ethyl also indicated that this 
concern about close-coupled catalysts completely ignores that Ethyl 
tested two 1988 models and three 1993 models equipped with close- 
coupled catalysts without showing any significant adverse effects on 
    Toyota submitted data on catalysts and oxygen sensors from in-use 
customer vehicles from Canada where MMT is used as a fuel additive. 
Toyota believes that these catalysts and oxygen sensors indicate that 
exhaust emissions of hydrocarbons and carbon monoxide are higher from 
catalysts/oxygen systems collected in Canada than comparable catalyst/
oxygen systems from U.S. vehicles. Also Toyota submitted photographs of 
a catalyst taken from a high mileage Canadian Hilux pickup truck which 
showed plugging of the catalyst passages.
    Ethyl's response to Toyota's catalyst/oxygen system data is that it 
is not clear from the description of the Toyota test results precisely 
what can be concluded from the test program. Ethyl stated that, without 
a detailed vehicle history, there is no basis to conclude that MMT had 
an effect on the catalyst/oxygen system data.
    Chrysler submitted data on the analysis of four catalysts, which 
was completed by Johnson Matthey Incorporated (JMI) at Chrysler's 
request, that had various degrees of manganese deposition from Canadian 
vehicles exposed to MMT in the fuel. It indicated that the results of 
the analysis demonstrate that the washcoat of both the partially 
plugged catalysts and unaffected catalysts exhibit a clear layer of 
``densified'' washcoat containing large quantities of manganese oxides. 
Chrysler believes that the JMI report supports its concern that 
manganese oxides can fill the catalyst pores, thereby covering precious 
metal sites or decreasing wash coat surface area, consequently 
eventually decreasing catalyst activity. Regarding the automakers' 
concerns about the Additive's effect on emissions system components, 
such as exhaust oxygen sensors, exhaust gas recirculation valves, 
catalysts and oxygen sensors, Ethyl stated that it has already provided 
extensive data showing that the Additive does not adversely effect any 
of these emission system components.\37\ (Ethyl's test programs are 
discussed in the previous section.)

    \37\Public Docket A-92-41, No. IV-D-3 (summarizing Ethyl's 
emission control component testing.)

    Nineteen small refiners including the National Petroleum Refiners 
Association all recommended approval. They concurred in Ethyl's 
assessment of the economic benefits and reduced refinery and vehicle 
emissions that would accrue from the replacement of octane obtained 
through higher-severity refining with octane obtained from MMT. Several 
emphasized that MMT would be especially helpful to small refiners since 
octane enhancement from MMT requires less capital investment than other 
means of increasing octane. Many refiners also pointed out that 
refinery operations at lower severity would result in decreased 
aromatic and benzene emissions from vehicles and increased yield for 
each barrel of crude oil refined.

C. Available Data Meet Previously Used Criteria

    The criteria and statistical tests previously used by EPA to 
examine durability waiver applications were used only once by the 
Agency prior to Ethyl's 1990 application for the use of MMT.\38\ These 
tests include a variety of approaches to durability data designed to 
determine whether the additive causes increases in regulated pollutants 
and, if so, whether those increases bring about failure of vehicles in 
the fleet to meet the standards to which they were certified. While EPA 
has some concerns regarding the appropriateness of these criteria and 
tests for current conditions, the Agency does not intend in this action 
to hold Ethyl to any new criteria and/or tests that are not currently 
in place. Accordingly, the following discussion is addressed primarily 
to the results of applying the most critical of the previously used EPA 

    \38\The test were used in EPA's examination of Ethyl's 1978 
application. For a description of these tests, see EPA's decision on 
the application at 43 FR 41424, September 18, 1978.
    \39\EPA has carefully reviewed Ethyl's application of the test 
to these data in various combinations and has concluded that the 
tests were conscientiously and accurately applied. This review 
focused particularly upon the application of the ``integrated 
emissions test'', the ``cause or contribute'' test, and the overall 
sign test.

    The earliest set of test results under consideration here (tests of 
1988 vehicles submitted with Ethyl's 1990 application) exhibit the most 
pronounced MMT-caused emissions increases of the data generated by the 
applicant (about 0.02 gm/mi\40\, but these increases fall substantially 
short of failure on the determinative ``cause or contribute'' test\41\ 
(3 of 8 vehicle models tested fail for HC and 4 of 8 models fail for 
CO, while 7 of the 8 models tested are required to fail before the 
additive fails this overall test on either pollutant).

    \40\Determined by integration of emissions test results gathered 
over the full range of mileage supplied by the applicant.
    \41\Ethyl's consultant, Systems Applications, Inc., describes 
this test on page 19 of a report that was included as Appendix 2A in 
Ethyl's May 9, 1990 application for waiver. This co-called ``cause 
or contribute'' test, really addresses the question of whether the 
additive ``causes'' a failure to meet the certified standard for a 
regulated pollutant for each model group and then looks to see if 
enough model groups failed the test to warrant the conclusion with 
high confidence that more than half of the models are caused to fail 
by operation on the additive.

    When the larger body of all available and appropriate\42\ long-term 
emissions data on High-Tech 3000 is evaluated using these previously 
used EPA tests, the conclusion is that these increases (averaging 0.02 
gm/mi for HC) bring about failure of the ``cause or contribute'' test 
in only 4 (for HC) or 5 (for CO) of the 19 model groups tested by the 
applicant and others. Failure of that test\43\ must occur in at least 
13 of the 19 model groups examined before the additive is deemed to 
have failed the overall test with 90% confidence. Fourteen of 19 must 
fail before the test is failed at the 95% confidence level.\44\

    \42\Appropriate data are considered to be those collected with 
Federal Test Procedure (FTP) testing using an experimental design 
with a control group and no obvious sources of bias. The data 
referred to here include the eight 1988 models tested by Ethyl, the 
two 1991 models tested by Ford, and the eight 1992 and 1993 models 
tested by Ethyl.
    \43\In order for a model to fail the test, emissions from the 
additive-fueled vehicles must be sufficiently high that then percent 
of the represented fleet of that model group using the additive is 
predicted to exceed the standard beofre the end of its useful life. 
The control vehicles must reach this failure rate at a higher 
mileage than the additive-fueled vehicles.
    \44\These ``confidence levels'' correspond, respectively, to the 
0.10 and 0.05 significance levels. The significance level is the 
probability that a decision to reject the null hypothesis (and find 
an increase) will be a result of sampling error and thus be 

    If the newer technology 1992 and 1993 vehicles tested by the 
applicant are examined in isolation from the earlier test programs, the 
data (with an average HC effect of 0.002 gm/mi) pass the historical 
tests even more easily than is the case for the data combinations 
examined above. None of the nine models failed the ``cause or 
contribute'' test for hydrocarbons and only one failed for carbon 
monoxide. Seven of nine models would have to fail for the additive to 
fail the overall sign test at the 90 percent confidence level and eight 
would have to fail for 95 percent confidence.
    The overall conclusion from the above analysis, then, is that 
Ethyl's additive passes the most critical of the historical tests with 
a comfortable margin.

D. Data on Newer-Technology Vehicles Meet More Stringent Criteria

    Notwithstanding the Agency's conclusion that it would not be 
appropriate to require Ethyl to satisfy new statistical tests 
concerning which it has not been given prior notice and the Agency's 
decision to evaluate Ethyl's application primarily according to the 
previously utilized statistical tests, the Agency nevertheless 
considers its existing tests and the criteria that they implement to be 
obsolete under current conditions.\45\

    \45\The tests are extremely conservative in that they place most 
of the burden of proof on the Agency rather than on the applicant. 
The ``cause or contribute'' test is failed by an engine family only 
under circumstances where emissions from the family are so high that 
an additive--caused increase in some pollutant pushes more than ten 
percent of the vehicle fleet into violation of the standard. 
Moreover, the final sign test that is applied to the model-specific 
results is failed by the additive only when it may be concluded with 
high confidence that more than half of the models in the represented 
population would fail the model-specific test. In practical 
situations with relatively small samples, this sign test permits a 
high percentage of the models in the sample to fail before the 
additive is declared to have failed the test. These tests, then, may 
permit the granting of waivers in the face of substantively 
significant emissions increments attributable to an additive--
increments that would tend to offset the benefits from an 
increasingly stringent regulatory program aimed at bringing the 
nation's most serious air quality problems under control. Agency 
concerns with these tests were addressed previously in its decision 
on Ethyl's 1990 application (57 FR 2535, January 22, 1992) and in 
(58 FR 64761, December 9, 1993).

    Therefore, EPA has gone beyond the historical tests to examine 
Ethyl's data on the use of the additive with newer technology vehicles 
under more stringent criteria of the sort that seem to be warranted by 
current conditions. For this analysis, EPA chose to examine the 
additive's performance against the most stringent of the possible 
criteria--a requirement that the additive cause no statistically 
significant increase in emissions.
    If one uses a one-sided null hypothesis that the additive causes no 
increase in HC emissions, one may employ various statistical tests to 
examine the credibility of that hypothesis in light of the test 
results. One such test is the computer-intensive ``permutation test'' 
in a form called an ``approximate randomization'' test.\46\ Application 
of this test to the full mileage range of HC emissions data from 
Ethyl's tests of 1992 and 1993 vehicles results in a failure to discern 
any ``real'' emissions increase at all--that is, no increase that we 
may not reasonably attribute to sampling error rather than to an 
additive effect on HC in the sampled vehicle population.\47\

    \46\The permutation test is built around the idea that, if the 
null hypothesis is correct, the increase due to the additive in the 
sample is only one member of a distribution of all possible such 
increases computed from assignments of vehicle emissions to fuel 
groups within models. Only if the fuel-related increase from the 
sample is an extremely unusual result in theis distribution of 
possible increases is it reasonable to reject the null hypothesis 
and conclude that the additive actually brought about an increase in 
emissions. The way that this method works in practice is that, on 
each iteration of the computer program, the computer randomly 
rearranges the fuel group assignments among the emission results 
within each model group separately. The emission values assigned 
(for that iteration) to the additive fuel group are then summed over 
the entire sample to form the test statistic. This process is 
repeated a very large number of times (one million in this case) and 
the resulting test statistics are tabulated. Only if the same test 
statistic, as computed from the empirical sample, exceeds a pre-
determined percentage of the simulated test statistics may we reject 
the ``no-difference'' hypothesis and conclude with the necessary 
degree of certainty that an increase has occurred.
    \47\This conclusion holds even when the test is performed at the 
0.10 significance level used in conducting the statistical testing 
on the data from Ethyl's 1978 application. It is important to note 
that the original Ethyl test fleet of 1988 model year vehicles that 
are now older than those representing the newest Ethyl data set did 
not fare as well and, as mentioned previously, do demonstrate 
statistically significant increases in HC emissions.

E. Finding

    Based on all of the information then available concerning the 
potential effect of use of MMT in unleaded gasoline on regulated 
emissions, as submitted by Ethyl and others, the Administrator of EPA 
determined on November 30, 1994 that, ``Ethyl has satisfied its burden 
under Clean Air Act 211(f)(4) to establish that use of HiTEC 3000 at 
the specified concentration will not cause or contribute to a failure 
of any emission control device or system (over the useful life of any 
vehicle in which such device or system is used) to achieve compliance 
by the vehicle with the emission standards with respect to which it has 
been certified.'' The basis for this determination was described 
briefly in the Administrator's November 30, 1994 notice, and has been 
reviewed in detail above.
    The November 30, 1994 determination was specific to Ethyl's present 
waiver application. As the Administrator made clear in the notice 
announcing the determination, it does not apply to any new application 
concerning either HiTEC 3000 or MMT in the event that this decision to 
deny Ethyl's application on the basis of concerns regarding potential 
health effects is upheld in any subsequent judicial review. Although 
Ethyl may be able to sustain its burden under Section 211(f)(4) in the 
context of any future waiver application, any such application must 
include satisfactory data addressing the effect on vehicles in 
production at that time and will be evaluated according to the 
statistical methods and criteria for evaluation of waiver applications 
in effect at that time.

V. The Onboard Diagnostics Issue

    Prior to the Administrator's November 30, 1994 finding concerning 
emission effects, three auto manufacturers, Ford, General Motors (GM), 
and Chrysler, and the American Automobile Manufacturers Association 
(AAMA), all commented on concerns about the impact of the oxidative 
products of MMT on onboard diagnostic (OBD II) systems employing 
before-catalyst and after-catalyst oxygen sensors.\48\

    \48\An Onboard Diagnostic System, with the present generation 
commonly known as OBD-II, monitors the activity of an automobile's 
emission control system, primarily the catalytic converter, and 
alerts the driver via a dashboard light in the event of a 
malfunction. Put simply, this aspect of the OBD system functions by 
utilizing devices before and after the catalyst which ``sense'' the 
presence of oxygen. If the catalyst is functioning properly, it will 
absorb a certain amount of oxygen and a specified decrease in oxygen 
content in the exhaust gases can be determined by comparing the 
oxygen ``sensed'' before and after the catalyst. If the catalyst is 
functioning improperly, oxygen storage by the catalyst is impaired 
and a drop in exhaust gas oxygen after the catalyst beyond the 
proper range is ``sensed'' by the OBD system.

    GM concerns regarding the OBD II system were two-fold. Its first 
concern was that since it is known that manganese oxide has the ability 
to store oxygen, a potential problem could occur with dual oxygen 
sensor systems. GM stated that, with manganese oxide covering the 
catalyst and the oxygen sensors, a false oxygen storage capacity of the 
catalyst could be indicated by the OBD II system, which could then 
indicate that the catalyst was still working properly while the 
opposite could be true. GM's second concern was that the catalyst would 
act as a ``filter'' and manganese oxide from MMT combustion passing 
through the exhaust system would coat the before-catalyst oxygen sensor 
and after-catalyst oxygen sensors unevenly, thus causing the OBD II 
system to malfunction. GM also stated that in the 1994 model year GM 
planned to market two engine families equipped with OBD II systems 
employing before- and after-catalyst oxygen sensors.
    With respect to the automakers' concern that use of MMT would 
adversely affect operation of the OBD II system, on July 15, 1993, 
Ethyl submitted data which it believed demonstrated that this concern 
has no basis.\49\ Ethyl stated that no production vehicles were then 
equipped with OBD-II systems and that the primary hardware approach 
being considered by the automobile manufacturers involves the use of 
exhaust gas oxygen (EGO) sensors before and after the catalytic 
converter to monitor converter efficiency. Ethyl further commented that 
test data generated by Ethyl showed that use of the additive would have 
no adverse effect on either the hardware component of these planned 
ODB-II systems (i.e., the oxygen sensors), or on the catalytic 
converter itself. Ethyl noted that, ``[s]ince these future systems are 
currently under development, it is impossible to consider the long term 
effects of MMT on these systems.''

    \49\See Public Docket A-93-26, Number II-D-8, Appendix 5.

    On November 4, 1994, only 26 days prior to the mandatory date for a 
decision on Ethyl's waiver application, Ford Motor Company submitted a 
report describing bench testing\50\ of catalysts, in which Ford 
measured the oxygen storage capacity of catalysts which had been 
deliberately degraded and then exposed to the emissions from MMT-
containing fuel. Ford's conclusion based on these tests was that the 
exhaust gas oxygen (EGO) sensors would be affected by the deposition of 
manganese oxides associated with MMT use, thus sending incorrect 
signals to the diagnostic control system in the vehicle.

    \50\Bench testing means the testing of components during which 
time the components are not actually in the vehicle. The details of 
this testing can be found in Document II-D-56 in Docket A-93-26. (An 
incomplete preliminary report of this information was submitted to 
the Agency in Document II-D-38, Docket A-93-26.)

    Although the Agency regarded the concerns expressed by Ford in its 
November 4, 1994 submission regarding the effect of MMT use on OBD 
systems as potentially very important, based on the very limited 
analysis which could be undertaken prior to the November 30, 1994 
deadline for a decision concerning Ethyl's application, the Agency 
concluded that the limited bench testing submitted by Ford did not 
allow a conclusion concerning the likelihood that a significant impact 
would actually occur during vehicle operation. In addition, the Agency 
had several questions regarding the procedures involved in the Ford 
testing which could not be resolved within the available time. The 
November 30, 1994 notice announcing the Administrator's determination 
concerning emission effects made it clear that EPA was concerned about 
this issue and would retain the authority to take appropriate action in 
the future pursuant to Clean Air Act Section 211(c).
    EPA met with staff of Ford in February of 1994 in order to discuss 
Ford's concerns raised in its November 4, 1993 submission. Ford 
generally expressed the same concerns as had been expressed by GM (and 
discussed above).\51\ According to Ford, its testing showed that 
combustion of gasoline containing MMT deposits a layer of manganese 
oxide on top of the catalyst washcoat and that this causes the EGO 
Sensor to measure a lower oxygen level, thereby indicating a higher 
oxygen storage capacity than that which would be indicated by the 
catalyst without MMT. As a result, a malfunctioning catalyst might not 
be detected. Ford expressed particular concern because it had just 
introduced three 1994 model year vehicle families employing OBD-II, 
whereas the other automakers will not have systems out until the 1996 
model year.

    \51\Ford's concerns are discussed in more detail in a memo to 
docket A-93-26, with an attachment submitted to the Agency entitled 
``Section 211(c) Impacts of MMT''.

    Recently, on May 3, 1993, Ford submitted additional information 
which the Agency is currently reviewing. This new information appears 
to provide further evidence to substantiate the concerns expressed by 
Ford regarding the impact of MMT use on OBD systems. Unlike the 
previously submitted Ford data, the new data address an actual 
production vehicle fitted with a failed catalyst and the effect use of 
MMT had on the OBD system's ability to detect failure of the catalyst.
    The Agency is continuing to investigate the question of the 
potential impact of use of MMT in unleaded gasoline on OBD systems. If 
after further investigation EPA concludes that the concerns expressed 
by the vehicle manufacturers are warranted, EPA intends to initiate an 
appropriate rulemaking under Section 211(c).

VI. Manganese Health Assessment\52\

    \52\The assessment presented here is taken from ``Reevaluation 
of Inhalation Health Risks Associated with Methylcyclopentadienyl 
Manganese Tricarbonyl (MMT) in Gasoline'' (United States 
Environmental Protection Agency, 1994b) which can be found in the 
docket in its entirety.

A. Introduction

    In 1990, the EPA Office of Research and Development (ORD) assessed 
the potential health risks associated with the use of 
methylcyclopentadienyl manganese tricarbonyl (MMT) as an additive in 
unleaded gasoline (U.S. Environmental Protection Agency, 1990).\53\ 
Later, ORD (Preuss, 1991) reaffirmed its assessment after considering a 
resubmitted waiver application for MMT from Ethyl Corporation. As 
identified in earlier ORD evaluations (U.S. Environmental Protection 
Agency, 1990; Preuss, 1991), a key issue is the potential health risk 
associated with inhalation exposure to manganese tetroxide 
(Mn3O4), which is the primary by-product resulting from the 
combustion of MMT in gasoline. New information on manganese (Mn) health 
effects and exposure is incorporated in this revised risk assessment. 
(United States Environmental Protection Agency, 1994b)

    \53\The many references in this section of the decision dealing 
with manganese health effects are referred to in parentheses and 
listed at the end of this section in subsection E.

    This reevaluation has four components: (1) a health effects 
assessment, (2) an exposure assessment, (3) a risk characterization 
relating the first two, and (4) a summary and conclusions. This 
evaluation summarizes earlier ORD assessments and incorporates 
information from certain other major new reports and analyses.\54\

    \54\The reader is referred to the appendices of the full EPA/ORD 
reevaluation for more detailed background information. This report, 
``Reevaluation of Inhalation Health Risks Associated with 
Methylcyclopentadienyl Manganese Tricarbonyl (MMT) in Gasoline'', 
can be found in its entirety, including the appendices, in docket A-
91-46. Appendix A presents dose-response analyses, Appendix B 
presents an exposure assessment, and Appendix C contains the current 
verified Mn inhalation reference concentration (RfC) as it appears 
in the U.S. EPA Integrated Risk Information System (IRIS, 1993).

B. Health Effects Assessment

1. Background
    The toxicity of Mn varies according to the route of exposure. By 
ingestion, Mn has relatively low toxicity at typical exposure levels 
due in part to a low rate of absorption from the gastrointestinal tract 
and in part to efficient regulation by homeostatic mechanisms. 
Manganese is considered a nutritionally essential trace element and is 
required for certain enzymes important for normal functioning of the 
central nervous system and other body organs. However, by inhalation, 
Mn has been known since the early 1800s to be toxic to workers. It 
should be noted that Mn occupational studies predominantly (and 
sometimes exclusively) involve men. Neurobehavioral, respiratory, and 
reproductive effects are the primary features of excessive occupational 
exposure to Mn. Manganism is characterized by various psychiatric and 
movement disorders, with some general resemblance to Parkinson's 
disease in terms of difficulties in the fine control of some movements, 
lack of facial expression, and involvement of underlying 
neuroanatomical and neurochemical factors. Neurobehavioral effects of 
Mn intoxication are generally more clinically prominent than 
respiratory or reproductive effects. However, respiratory effects 
(e.g., pneumonitis) and reproductive dysfunction (e.g., reduced libido) 
are also frequently reported features of occupational Mn intoxication. 
The available evidence is inadequate to determine whether or not Mn is 
carcinogenic; some reports suggest that it may even be protective 
against cancer. Based on this mixed but insufficient evidence, EPA has 
placed Mn in a Group D weight-of-evidence category, which signifies 
that it is not classifiable as to human carcinogenicity. Given these 
features of Mn toxicity, the health assessment focuses on the potential 
for chronic noncancer effects.
    Various epidemiological studies of male workers exposed to Mn at 
average levels below the current American Conference of Governmental 
Industrial Hygienists Threshold Limit Value (TLV) (5 mg/m\3\)\55\ have 
shown neurobehavioral, reproductive, and respiratory effects, both by 
objective testing methods and by workers' self-reported symptoms on 
questionnaires. Neurobehavioral effects generally have reflected 
disturbances in the control of hand movements (e.g., tremor, reduced 
hand steadiness) and/or the speed of movement (e.g., longer reaction 
time, slower finger-tapping speed). Reproductive effects have included 
a decrease in the number of children born to Mn-exposed workers 
(compared to matched controls) and various self-reported symptoms of 
sexual dysfunction. In recent studies at low to moderate occupational 
exposure levels, respiratory effects have been reflected primarily in 
self-reported symptoms of respiratory tract illnesses rather than in 
differences between objective pulmonary function measurements in Mn-
exposed and control workers. However, the lack of studies using more 
sensitive investigational methods and the existence of some limited 
evidence from an epidemiological study of school children raise a 
degree of concern about pulmonary function effects in relation to lower 
level Mn exposure.

    \55\The American Conference of Governmental Industrial 
Hygienists (1992) has given notice of intent to lower the TLV to 0.2 

    The precise mechanisms of Mn neurotoxicity are not well understood, 
but it appears that Mn can affect several different aspects of central 
nervous system (CNS) function and structure. Some experimental evidence 
suggests that the mechanisms of Mn toxicity may depend on the oxidation 
state of Mn. However, both the trivalent form (Mn3+) and the 
divalent form (Mn2+) have been demonstrated to be neurotoxic.\56\ 
Also, both forms of Mn can cross the blood-brain barrier, although 
research suggests that Mn3+ is predominantly transported bound to 
the protein transferrin (Aschner and Gannon, 1994), whereas Mn2+ 
may enter the brain independently of such a transport mechanism (Murphy 
et al., 1991). Unlike ingested Mn, inhaled Mn is transported directly 
from the respiratory system to the vicinity of the brain before its 
first pass by the liver. Depending on the form of Mn inhaled, its 
conversion to other oxidation states (e.g., oxidation of Mn2+ to 
Mn3+ or reduction of Mn4+ to Mn3+), and its ability to 
enter the brain (through a protein transport mechanism or otherwise), 
it is quite possible that a significant fraction of even small amounts 
of inhaled Mn would be able to reach target sites in the CNS. Thus, the 
apparently greater toxicity of inhaled versus ingested Mn may reflect 
important pharmacodynamic and pharmacokinetic differences of Mn that 
enters the body by different routes. A more definitive understanding of 
these issues will require more empirical information.

    \56\Various elements can exist in more than one form of charged 
atom, depending on the number of negatively charged and positively 
charged particles contained in the atom. Manganese is one such 
element where, depending on the number of charged particles 
associated with the atom, the atom may have a net charge of two or 
three ``plus'' charges resulting in a ``divalent'' or ``trivalent'' 
form, respectively.

2. Earlier Assessments
    Earlier ORD health assessments have been based on the RfC, which is 
defined as an estimate (with uncertainty spanning about an order of 
magnitude) of a continuous inhalation exposure level for the human 
population (including sensitive subpopulations) that is likely to be 
without appreciable risk of deleterious noncancer effects during a 
lifetime. The basic procedure for derivation of an RfC entails 
identifying a no-observed-adverse- effect level (NOAEL) and a lowest-
observed-adverse-effect level (LOAEL) from a ``principal'' study, 
generally defined as the available study that best defines the highest 
NOAEL or lowest LOAEL for the most sensitive endpoint affected by a 
chemical. When an investigation of occupationally exposed humans is the 
principal study (as in the case of the Mn RfC), the NOAEL or LOAEL is 
adjusted for differences in ventilation rates and exposure durations 
between the occupational exposure scenario (10 m\3\ air breathed per 8-
h workday, 5 days/week) and the ``general public'' scenario (20 m\3\ 
air breathed per 24-h day, 7 days/week). The adjusted NOAEL or LOAEL is 
then divided by uncertainty factors and a modifying factor. In the case 
of the original (1990) RfC for Mn, uncertainty factors of 10 each were 
used for extrapolating from a healthy worker population to the general 
population (including sensitive subpopulations) and for extrapolating 
from a LOAEL to a NOAEL. Also, an uncertainty factor of 3 
(approximately one-half of 10 on a log scale) was used for 
extrapolating from subchronic to chronic exposure. A modifying factor 
of 3 was used because of statements by the authors of the principal 
study (Roels et al., 1987) that past exposure levels of workers in the 
subject study were probably lower than those measured at the time the 
study was conducted. The resulting RfC of 0.4 g Mn/m\3\ was 
used for the earlier ORD risk assessment (U.S. Environmental Protection 
Agency, 1990) and was entered on EPA's IRIS computer database of human 
health risk and regulatory information in December 1990.
3. 1993 Revised RfC
    The original RfC for Mn was revised, in part, because newer 
information supplied in conjunction with the resubmittal of the MMT 
waiver application by Ethyl indicated that the workers' exposure levels 
in the principal study had probably not increased over time, and thus 
the modifying factor could be ``eliminated'' (i.e., set equal to 1). 
Another reason for revising the original RfC was that more recent 
studies (Roels et al., 1992; Mergler et al., 1994) provided additional 
evidence of health effects in workers at relatively low airborne 
concentrations of Mn.
    Independently of their earlier study of Mn-exposed workers (Roels 
et al., 1987), Roels et al. (1992) conducted a cross-sectional study of 
neurobehavioral and other endpoints in another group of workers from a 
different factory-namely, 92 male alkaline-battery plant workers 
exposed to manganese dioxide (MnO2) dust--who were compared to a 
matched control group of 101 male workers without industrial Mn 
exposure. The geometric mean occupational-lifetime integrated 
respirable dust concentration was 793 g Mn/m\3\  x  years 
(range: 40 to 4,433). The equivalent value for total dust was 3,505 
g Mn/m\3\  x  years (range: 191 to 27,465). The authors noted 
that the monitored concentrations were representative of the usual 
exposures of the workers because work practices had not changed during 
the last 15 years of the plant's operation. No data on particle size or 
chemical purity were provided in the report by Roels et al. (1992), but 
based on information provided by Roels et al. (1992) and Roels (1993), 
the median cut point for the respirable dust fraction was 5 m 
aerodynamic diameter. The respirable fraction is more representative of 
the toxicologically significant particles (i.e., the smaller particles 
that are inhaled and deposit predominantly in the lower respiratory 
tract). Total dust measurements comprised the respirable dust as well 
as larger particles that deposit predominantly in the nose and throat 
region (via nasal breathing) and would be cleared more rapidly from the 
respiratory tract than the smaller particles retained in the lower 
regions. Therefore, the respirable dust measurements were considered to 
be a more accurate indicator of exposure in relation to the observed 
health effects.
    Manganese-exposed workers in the 1992 study by Roels et al. 
performed significantly worse than matched controls on several measures 
of neurobehavioral function, particularly visual reaction time, eye-
hand coordination, and hand steadiness. Similar neurobehavioral 
impairments were also found in the earlier study by Roels et al. (1987) 
of a different occupational population exposed to mixed Mn oxides and 
salts at approximately the same levels of total dust (respirable dust 
was not measured). In addition, a recent study in Canada by Mergler et 
al. (1994) indicated that, among other effects, performance on tests of 
the ability to make rapid alternating hand movements, to maintain hand 
steadiness, and to perform other aspects of fine motor control was 
significantly worse, compared to matched controls, in workers who were 
exposed to even lower concentrations of respirable dust (35 g 
Mn/m\3\ at the time of the study). If Mergler et al. had included 
information on integrated past exposure levels (which they have since 
provided to ORD in a preliminary form not yet submitted for 
publication), their study would have provided a fivefold lower LOAEL 
for the derivation of the RfC. In addition, reports of a Swedish study 
of Mn-exposed steel workers (Iregren, 1990; Wennberg et al., 1991, 
1992) provided compelling evidence of comparable neurobehavioral 
impairments, including slower reaction time and finger-tapping speed. 
The median total dust concentration in the Swedish study was 140 
g Mn/m\3\, with respirable dust reported as constituting 20 to 
80% of individual workers' total dust exposures. Thus, the LOAEL from 
this study would be somewhat lower than that from Roels et al. (1992), 
but the less fully characterized exposure histories in the Swedish 
study made it more appropriate as a supporting (rather than principal) 
study for deriving the Mn RfC.
    Taken together, the above epidemiological studies provide a 
consistent pattern of evidence indicating that neurotoxicity is 
associated with low-level occupational Mn exposure. The fact that speed 
and coordination of motor function are especially impaired is 
particularly noteworthy, given its consistency with other 
epidemiological, clinical, and experimental animal evidence of higher 
concentration Mn intoxication.
    Differences among these studies in the duration of workers' 
exposure to Mn raise another issue of relevance to this discussion. In 
the Roels et al. (1992) study, the mean period of exposure was 5.3 
years (range: 0.2 to 17.7 years). In the other studies, the mean 
durations of exposure were longer: 7.1 years in Roels et al. (1987), 
9.9 years in Iregren (1990), and 16.7 years in Mergler et al. (1994). 
The indications of lower LOAELs in the Canadian and Swedish studies 
suggest that neurobehavioral effects might occur at lower 
concentrations of Mn if the exposure periods were longer. In addition, 
the age of the workers may be an important factor in interpreting these 
findings. The oldest worker in the Roels et al. (1992) study was less 
than 50 years old; also, the average age in that study was only 31.3 
years, versus 34.3 years in Roels et al. (1987), 43.4 years in Mergler 
et al. (1994), and 46.4 years in Iregren (1990). These points suggest 
that longer exposure and/or testing later in life might result in the 
detection of effects at lower concentrations than is possible after 
shorter periods of exposure and/or in younger workers. On the other 
hand, it is also evident from these studies that a much shorter period 
than a full lifetime of occupational Mn exposure may be sufficient to 
induce Mn neurotoxicity.
    As Roels et al. (1992) and other investigators have noted, a 
threshold for the neurotoxic effects of Mn has not been reported in the 
epidemiological literature. Therefore, instead of a NOAEL, a LOAEL was 
obtained from the study by Roels et al. (1992) by dividing the 
geometric mean integrated respirable dust concentration (793 g 
Mn/m3  x  years) by the average period of worker exposure (5.3 
years) to eliminate time (in years) from the time-weighted average, 
thereby yielding a LOAEL of 150 g Mn/m3. (The geometric 
mean concentration was used to represent the average exposure because 
the workers' exposure measurements were log-normally distributed, and 
the arithmetic mean exposure period was used because it was the only 
value reported by Roels et al. (1992).) The workplace-based LOAEL of 
150 g Mn/m3 was then adjusted for nonoccupational 
lifetime exposure by multiplying it by (1) the quotient of 10 m3/
day divided by 20 m3/day (for worker versus nonworker ventilation 
rates) and (2) the quotient of 5 days divided by 7 days (for work week 
versus full week). The resulting adjusted LOAEL, labeled the human 
equivalent concentration (HEC), was 50 g Mn/m3, which was 
then divided by a total uncertainty factor of 1,000 to yield an RfC of 
0.05 g/m3. The total uncertainty factor of 1,000 
incorporated the following factors: 10 to protect sensitive 
individuals; 10 for using a LOAEL in lieu of a NOAEL; and a composite 
factor of 10 for database limitations reflecting the less-than-chronic 
periods of exposure and the lack of reproductive and developmental 
toxicity data, as well as potential but unquantified differences in the 
toxicity of different forms of Mn. A modifying factor was not used 
(i.e., it was set equal to 1).
    Each RfC is assigned an overall rating of low, medium, or high 
confidence level, based on two subsidiary confidence ratings reflecting 
the quality of the evidence from the principal studies and the quality 
of the overall database for the chemical in question, respectively. The 
revised Mn RfC was assigned a medium level of confidence. The evidence 
for the neurobehavioral effects of low-level Mn exposure by inhalation 
was compelling and consistent across several well-conducted studies. 
However, the limited duration of exposure and the lack of a NOAEL for 
neurotoxicity in any of the principal or supporting studies prevented 
assigning a confidence level greater than medium. Also, the lack of 
definitive data on the concentration-response relationship and on the 
potential reproductive and developmental toxicity of inhaled Mn limited 
the degree of confidence in the database to a medium rating. Virtually 
all of the human health evidence is based on healthy, adult male 
workers. No known studies have investigated human female reproductive 
function, and even though male worker reproductive function is known to 
be affected by Mn exposure, it has not received adequate investigation. 
The limited available information concerning the developmental toxicity 
of inhaled Mn suggests the possibility that prenatal exposure of 
laboratory rodents to MnO2 (via the air supplied to the pregnant 
mother) may depress neurobehavioral activity in neonatal rats and that 
continued postnatal exposure of the pups may intensify this depression. 
In addition, several studies have demonstrated alterations in 
neurochemical (dopamine) levels in young mice and rats exposed during 
early postnatal development to Mn via other routes. Thus, the potential 
for developmental toxicity due to Mn exposure exists. The 
concentrations and durations of exposure sufficient to induce such 
effects are not known. Although adequate epidemiological studies of 
children and the elderly have not been conducted, it is known that 
certain populations, such as children, pregnant women, elderly persons, 
iron- or calcium-deficient individuals, and individuals with liver 
impairment, may have an increased potential for excessive Mn body 
burdens due to increased absorption or altered clearance mechanisms.
    Another concern raised by the lack of studies involving longer 
periods of exposure and/or older subjects is that the compensatory or 
reserve capacity of certain neurological mechanisms may be stressed by 
Mn exposure earlier in life, with manifestations of impairments only 
becoming evident much later, perhaps at a geriatric stage. One reason 
for the latter concern is that Parkinson's disease is typically a 
geriatric disease in which symptoms are only seen when the loss of 
brain cells that produce dopamine (which is also apparently involved in 
Mn toxicity) reaches 80% or more. Indeed, some neurologists think that 
a long latency period of perhaps several decades may precede various 
parkinsonian syndromes. These points lead to a concern that if Mn 
reduces the compensatory or reserve capacity of the nervous system, 
parkinsonian-type effects might occur earlier in life than they would 
otherwise. Thus, several questions remain to be answered before higher 
confidence in the accuracy of the RfC can be achieved.
    The two studies of Roels et al. (1992, 1987) were considered 
coprincipal studies for the derivation of the revised RfC, with 
supporting evidence in the reports of Mergler et al. (1994), Iregren 
(1990), and Wennberg et al. (1991, 1992). Given the fact that these 
studies involved exposure to various oxides and salts of Mn, the RfC is 
designated as applying to Mn and Mn compounds (including 
Mn3O4). The previous RfC of 0.4 g Mn/m3 applied 
to Mn only, due to undifferentiated forms of Mn in the principal study. 
Given that different forms of metals may have different toxic 
properties (due to different oxidation states, different solubilities, 
and possibly other factors), it is likely that different compounds of 
Mn vary in toxicity. However, sufficient data on the comparative 
toxicity of various compounds of Mn are not available to judge the 
relative toxicity of Mn3O4 specifically.
    As noted above, Mn affects multiple organ systems, including the 
respiratory and reproductive systems as well as the CNS. However, 
because the only available evidence suggests that the CNS is the most 
sensitive target for Mn toxicity, neurobehavioral endpoints were the 
focus of the RfC derivation. Although other types of effects remain a 
concern, it is presumed, based on the limited data now available, that 
protecting against neurotoxicity provides protection against these 
other, apparently less sensitive endpoints.
    In revising the RfC for Mn, a draft version was subjected to peer 
review by external experts (from academic and non-EPA governmental 
institutions) as well as internal experts. Following this peer review, 
a further-revised version was submitted to and verified by an EPA-wide 
RfD/RfC work group in September 1993. The current RfC for Mn was made 
available through IRIS in early November 1993 through two mechanisms. A 
special notice beginning November 1 in the news section of EPA's 
internal IRIS2 database announced the availability of a hard copy of 
the text to EPA requesters who contacted the Risk Information Hotline; 
also, the text was obtainable through the National Library of 
Medicine's publicly accessible on-line computer database, TOXNET, 
beginning November 10, 1993. It also became available on line via the 
EPA IRIS database beginning December 1, 1993.\57\

    \57\The complete text of the revised RfC as it exists on IRIS2 
may be found in Appendix C in the docket.

4. Alternative Approaches to Deriving RfCs
    After the revised RfC for Mn became available to the public, Ethyl 
Corporation and other interested parties submitted comments on the RfC 
and issues related to it. One of the primary comments concerned the 
availability of various statistical techniques for deriving a NOAEL 
from the study by Roels et al. (1992) and/or from supplementary data 
for that study provided to ORD by Roels (1993). In response to Ethyl 
Corporation's request that EPA consider alternative approaches to 
analyzing these data and deriving an RfC for Mn, further analyses of 
the subject data were undertaken using a variety of statistical 
methods. These approaches may be identified as (1) conventional NOAEL- 
or LOAEL- based analyses, (2) ``no statistical significance of trend'' 
(NOSTASOT) analyses of the type described by Tukey et al. (1985), (3) 
benchmark dose analyses of the type described by Crump (1984), and (4) 
Bayesian analyses of the type described by Jarabek and Hasselblad 
(1991). These analyses and their results\58\ yield several possible RfC 
estimates, so designated because the current and only verified RfC for 
Mn is that which has been verified by the EPA-wide RfD/RfC work group 
and entered on IRIS. It must be emphasized that the RfC estimates 
developed for the purpose of this risk assessment do not represent a 
revision of the current verified RfC for Mn. Reexamination of the 
current Mn RfC, and any decision to revise or reaffirm the current RfC, 
will be under the purview of the EPA-wide RfD/RfC work group at some 
future date.

    \58\See ``Reevaluation of Inhalation Health Risks Associated 
with Methycycloentadienyl Manganese Tricarbonyl (NMT) in Gasoline'', 
Appendix A, Docket A-91-46.

    A fundamental issue pertaining to all of the approaches presented 
here is the selection of a measure of exposure. Roels et al. (1992) 
described two measures of respirable dust, the occupational lifetime 
respirable dust concentration (LIRD), expressed as g/m3 x 
years, and the current concentration of respirable dust (CRD), 
expressed as g/m3. The CRD concentration was measured at 
the time the study was conducted by Roels et al. and refers to a 
representative concentration measured for the type of job performed by 
a worker (e.g., electrician, maintenance worker). The LIRD value for 
each worker was a cumulative exposure measure derived by adding the CRD 
values over the worker's entire period of employment. If a worker 
changed jobs within the plant during his period of employment, the CRD 
for each job held was multiplied by the number of years the worker 
performed that job. Thus, if more than one job classification was 
worked, the worker's LIRD was the sum of the products of CRD multiplied 
by years of performance of the respective jobs. However, if a worker 
held only one job classification, his LIRD was simply equal to his CRD 
multiplied by the number of years employed. Another measure of exposure 
may be derived from LIRD by dividing an individual worker's LIRD value 
by his total number of years of employment. The latter measure, 
designated as the average concentration of respirable dust (ACRD), 
reflects a worker's time-weighted cumulative exposure level but removes 
years from the unit of measurement of LIRD and is expressed as 
g/m3. Although Roels et al. (1992) did not refer to ACRD, 
this value could be calculated for each individual and for the entire 
cohort by using the unpublished data provided to ORD by Roels (1993). 
For reasons to be discussed later, ACRD offers advantages for certain 
analyses and, unless otherwise noted, is the exposure measure used in 
the alternative RfC estimates discussed here.
    a. Conventional NOAEL- or LOAEL-Based Approach. The conventional 
method, and only method used thus far by EPA, to derive an RfC has been 
to identify a NOAEL or LOAEL from a study and divide that concentration 
by uncertainty factors, as described above for the Mn RfC. In the case 
of the study by Roels et al. (1992), the geometric mean LIRD 
concentration of the Mn-exposed workers was used as a LOAEL. Roels et 
al. (1992) also performed an exposure-response analysis of their data 
by grouping the exposed workers into three exposure categories and 
comparing the prevalence of abnormal neurobehavioral scores for each of 
the three groups to those of controls. As indicated in the summary 
sheet for the Mn RfC (see Appendix C), the results of this exposure-
response analysis were not used in deriving the revised Mn RfC because 
the reported analysis did not correct for multiple comparisons. 
However, ORD's analyses of additional data provided by Roels (1993) 
suggest a possible RfC estimate of 0.03 g/m3 (versus the 
current RfC of 0.05 g/m3), if a one-tailed test of 
statistical significance is accepted (see Appendix A, ``Reevaluation of 
Inhalation Health Risks Associated with Methyl- cyclopentadienyl 
Manganese Tricarbonyl (MMT) in Gasoline'' as referenced in the 
reference section below, hereafter referred to as Appendix A.). Because 
it was based on an exposure-response analysis, this RfC estimate is 
labeled as such in Figure 1, which could not be reproduced in the 
Federal Register. It is available by calling the person listed in the 
FOR FURTHER INFORMATION CONTACT section of this notice. It is also 
available in docket A-93-26, item number II-A-17, page 12 (see ADDRESS 
section of this notice for docket location).
    b. NOSTASOT Approach. Another approach to analyzing dose-response 
data makes use of a procedure known as NOSTASOT, described by Tukey et 
al. (1985). In essence, the procedure applies a trend test sequentially 
to determine the highest noneffective dose of a series of doses by 
eliminating one dose at a time. In this manner, the dose level at which 
the response is not significantly different from controls is determined 
to be the NOSTASOT dose, which could therefore be considered a NOAEL. 
Applied to Roels' (1993) epidemiologic data by beginning with the 
highest individual ACRD exposure and moving downward (i.e., a ``top-
down'' approach), the procedure yielded a NOSTASOT of 285 g/
m3 for eye-hand coordination (see Table A-4, Appendix A). This 
approach implies that once nonsignificance is reached, further 
application of trend tests to lower dose groups would also yield 
nonsignificance. However, this was not the case with Roels' (1993) 
epidemiologic data, and thus it was important to determine not simply 
the highest NOAEL but the highest NOAEL below the lowest LOAEL. By this 
``bottom-up'' approach, the highest nonsignificant exposure below the 
lowest statistically significant exposure was 21 g/m3, 
for visual reaction time. Using the latter value as a NOAEL and a total 
uncertainty factor of 100 (the same as that used for the current Mn 
RfC, except omitting a factor of 10 for extrapolating from a LOAEL to a 
NOAEL), one would obtain a value of 0.07 g/m3 for an RfC 
estimate (Figure 1). Disparities in the NOSTASOTs obtained for various 
endpoints by the top-down and bottom-up approaches raise questions 
about the suitability of this technique for deriving a NOAEL from the 
data of Roels (1993).
    c. Benchmark Analyses. Another approach to deriving an RfC estimate 
is the benchmark dose (BMD) approach, which has been described by Crump 
(1984) and others (e.g., Kimmel and Gaylor, 1988; Faustman et al., 
1994; Allen et al., 1994). A BMD is an estimate of the dose (the term 
dose is used interchangeably here with concentration, although the 
latter is more appropriate for inhalation exposure) that will produce a 
specified effect (e.g., a 10% increase in the prevalence of abnormal 
scores on a neurobehavioral test in the case of the study by Roels et 
al. (1992)). The BMD is calculated by fitting a mathematical model to 
the available data and obtaining a maximum likelihood estimate of the 
dose associated with a specified increase in response (typically 10, 5, 
or 1%). A lower confidence limit is then calculated for the BMD 
(usually the 95th percentile), and the result is denoted as a benchmark 
dose level (BMDL), which has been proposed as a substitute for a NOAEL 
in deriving RfDs or RfCs (Crump, 1984; Barnes et al., 1994). Subscripts 
designate the effect level (10, 5, or 1%) for which the BMDL has been 
calculated, as in BMDL10, BMDL5, or BMDL1.
    A large number of mathematical models could be used for deriving 
BMDLs, but six frequently used models have been selected for the 
present exercise (as discussed in Appendix A). In applying these models 
to the dataset provided by Roels (1993), it appears that the models fit 
the CRD and ACRD data better than the LIRD data. (As explained in 
Appendix A, it made little difference whether the LIRD values were 
obtained from the group data provided in the report by Roels et al. 
(1992) or from the individual exposure data supplied by Roels (1993), 
so the latter LIRD data were used here.) In principle, LIRD is superior 
to CRD as a measure of long-term or cumulative exposure. One reason for 
the difference in goodness of fit between LIRD and either CRD or ACRD 
is that two workers with low LIRD values had abnormal eye-hand 
coordination responses (exceeding the 95th percentile of control 
scores). These two subjects appear to have had rather short exposure 
durations (0.3 and 0.4 years) and moderately high CRD values (201 
g/m\3\ each). Thus, these two data points suggest an LIRD 
exposure-response relationship that is better fit by a supralinear 
curve with a power term <1 (see Figures A-3 and A-5 in Appendix A) than 
by the more nearly linear curve such as that produced by the quantal 
linear or restricted Weibull model (see Figures A-1 and A-4 in Appendix 
A). If CRD or ACRD exposure data are used, however, the two individuals 
tend to fall in line better with a linear model (see Figures A-7 to A-
10 in Appendix A). Another factor contributing to difficulty in fitting 
the Roels (1993) data with linear models is a tendency for the 
prevalence of abnormal eye-hand coordination responses to decline 
slightly at the highest LIRD, CRD, and ACRD concentrations (evident in 
Figures A-1 to A-6).
    The supralinear exposure-response curve for abnormal eye-hand 
coordination scores suggests a possible corollary to the healthy worker 
phenomenon; namely, the existence of newly employed, relatively 
sensitive workers vis-a-vis long-term, relatively nonsensitive workers. 
It may be that the two above-noted workers happened to be rather 
susceptible to Mn toxicity but had not been employed long enough for 
their greater sensitivity to become otherwise evident. There could be a 
tendency for such workers to move to other types of employment, leaving 
a greater proportion of relatively less sensitive individuals among the 
older workers. Although irregularities of the type posed by the data 
for these two workers create complexities for model fitting, it is 
important to recognize that statistical curve fitting is secondary to 
the objective of selecting the most biologically appropriate exposure 
variable. However, given that the final results obtained with LIRD, 
CRD, and ACRD are roughly equivalent, ACRD has been selected for 
discussion here because it estimates an average exposure over time and 
yet provides as good or better goodness of fit as CRD or LIRD for most 
of the models considered.
    Of the six models considered, the quantal linear model fits the 
data reasonably well and is the least complex (see Appendix A). It also 
gives equivalent results to the restricted Weibull model for BMDL 
calculations (although the two models differ slightly when used in the 
Bayesian analyses, to be described below). Although much more 
conservative results would be obtained if the unrestricted Weibull or 
unrestricted log-logistic model were used, the NOAEL/LOAEL surrogates 
obtained with the latter models are so small as to be practicably 
incalculable and extend far below the range of actual measurements. 
Therefore, the following discussion is focused on the results obtained 
with the quantal linear model.
    In addition to choosing a model, a specified rate of increase in 
the effect of concern must be selected in using the BMD approach. This 
percentage increment is expressed in terms of the effective 
concentration that would yield the stated increase. Increases of 10, 5, 
and 1% in the incidence of abnormal eye-hand coordination scores (as 
dichotomized by Roels et al., 1992) have been considered, with the 
concentrations associated with these levels called ``effective 
concentrations'' and designated as EC1, EC5, and EC10, 
respectively. One guide to the choice of an effect level is that the 
resulting BMD (before calculating the lower confidence limit) is 
preferably near or within the range of observed exposure concentrations 
(cf. Barnes et al., 1994). Because the BMD for EC1 falls outside 
this range of observed concentrations, the primary focus in this 
discussion is devoted to the BMDL5 and the BMDL10.
    It should be kept in mind that the BMDL represents the lower 95th 
percent confidence interval for the effective concentration in 
question, and therefore the BMDL probably inherently reflects some 
degree of conservatism. However, the degree of conservatism obviously 
varies with the effective concentration for different percentage effect 
levels and with the nature of the effect (e.g., severe versus moderate 
impairment). For the purposes of this assessment, if one treats the 
BMDL10 derived from the dichotomized (quantal) data of Roels as if 
it were a minimal (less severe) LOAEL and the BMDL5 as if it were 
a NOAEL, uncertainty factors of 3 and 1, respectively, would be 
warranted. On this basis, as shown in Figure 1 and in Table A-39 of 
Appendix A for the quantal linear model using ACRD, an RfC estimate of 
0.09 g/m\3\ would be obtained by using the quantal BMDL10 
as if it were a LOAEL and a total uncertainty factor of 300 (10 for 
intraspecies sensitivity, 10 for database limitations, and 3 for a 
minimal severity LOAEL). Similarly, the quantal BMDL5 would yield 
an RfC estimate of 0.1 g/m\3\, based on a total uncertainty 
factor of 100 (10 for intraspecies sensitivity, 10 for database 
limitations, and 1 for a NOAEL). As applied here, the benchmark 
approach yields candidate RfC estimates of 0.09 to 0.1 g/m3.
    d. Bayesian Analyses. Another approach to deriving a substitute for 
a conventional LOAEL or NOAEL, which bears some resemblance to the BMD 
approach just described, is known as the Bayesian approach (see 
Appendix A). In essence, the Bayesian approach yields a distribution of 
concentrations (rather than a point estimate) associated with a 
specified effect. Some features of the BMD approach are common to the 
Bayesian approach: a mathematical model must be fit to the data, an 
effect level must be selected, and a confidence bound on the estimated 
concentration associated with a given effect level must be calculated 
(although the calculation procedures are different). If these choices 
are consistent with those for the BMD approach, the results are quite 
similar. By the Bayesian analysis, for a 10% increase in abnormal eye-
hand coordination scores, the lower 90% credible set limit (roughly 
equivalent to the quantal BMDL 95% confidence limit\59\ based on the 
estimated median concentration obtained with the quantal linear model 
is 73 g/m\3\. Adjusting this value to a human equivalent 
concentration (HEC) and treating the result (26 g/m\3\) as if 
it were a LOAEL(HEC), one may divide by a total uncertainty factor of 
300 (10 for intraspecies sensitivity, 10 for database limitations, and 
3 for a minimal severity LOAEL) and obtain an RfC estimate of 0.09 
g/m\3\. Similarly, the 5% effect level yields an RfC estimate 
of 0.1 g/m\3\, based on a total uncertainty factor of 100. 
Thus, as applied here, the Bayesian approach yields candidate RfC 
estimates of 0.09 to 0.1 g/m\3\, essentially identical to the 
results of the benchmark analysis (Figure 1).

    \59\U.S. Environmental Protection Agency. (1994a) ``Reevaluation 
of inhalation health risks associated with methylcyclopentadienyl 
manganese tricarbonyl 9MMT) in gasoline.'' Washington, DC: Office of 
Research and Development; EPA report no. 600/R-94/062. For further 
information see Air Docket A-93-26, II-A-12.

    One advantage of the Bayesian approach is that it lends itself well 
to using continuous as well as dichotomous data. Although Roels et al. 
(1992) did not provide individual continuous data (i.e., actual raw 
scores instead of designations of normal/abnormal) on the performance 
of the workers in their study, they did report mean differences and 
standard deviations. With this information, it is possible to estimate 
the concentration at which certain effect levels would occur based on 
the Bayesian posterior distribution. For example, a 10% increase in the 
proportion of subjects with abnormal scores would be associated with a 
median concentration of 112 g/m\3\, which has a lower 90% 
credible set limit of 90 g/m\3\. Adjusting the latter value as 
if it were a LOAEL(HEC) yields a concentration of 32 g/m\3\ 
and an RfC estimate value of 0.1 g/m\3\, based on a total 
uncertainty factor of 300 (10 for intraspecies sensitivity, 10 for 
database limitations, and 3 for a minimal severity LOAEL). Note that 
these calculations based on continuous data essentially approximate the 
quantal BMD and Bayesian calculations for a 10% effect level based on 
dichotomous data (see Figure 1). Similar calculations for the actually 
observed difference (i.e., 13%) between the Mn-exposed and control 
workers in the Roels et al. (1992) study yield an RfC estimate of 0.2 
g/m\3\, based on a total uncertainty factor of 300 (including 
a factor of 3 for a minimal severity LOAEL). Calculating the 
concentration associated with the difference between the exposed and 
control mean values that just achieves statistical significance (a 4% 
difference in this case) also results in a candidate RfC value of 0.2 
g/m\3\, based on a total uncertainty factor of 100 
(eliminating the minimal LOAEL factor of 3). Thus, as applied here, the 
Bayesian analyses of continuous data yield candidate RfC estimates of 
0.1 to 0.2 g/m\3\.
    e. Summary of RfC Estimates. Figure 1 displays the current, 
verified RfC along with over 100 possible Mn RfC estimates based on 
various exposure measures, models, effects measures, and uncertainty 
factors. Not all of these RfC estimates are equally plausible or worthy 
of consideration in assessing the potential health risks associated 
with Mn inhalation exposure due to MMT usage. As discussed above, some 
combinations of the three exposure measures and six mathematical models 
fit one another better than other combinations. Based primarily on 
considerations of cumulative dose toxicity, statistical goodness-of-
fit, and parsimony, ACRD and the quantal linear model appear to achieve 
the best results in this respect. Given the similarities of the 
benchmark and Bayesian analytic results using ACRD and the quantal 
linear model, little distinction can be made between the two analytic 
approaches in the present application. As for the results obtained for 
different effect levels, using a severity uncertainty factor of 3 with 
a 10% effect level (for either benchmark or Bayesian analyses) is 
essentially equivalent to using a severity UF of 1 with a 5% effect 
level. Note that the terms LOAEL and NOAEL do not actually correspond 
to the results for 10% and 5% effect levels, and therefore neither is 
preferable to the other in the sense that a NOAEL is preferable to a 
LOAEL in deriving an RfC. Therefore, benchmark and Bayesian results for 
10% and 5% effect levels (using ACRD with the quantal linear model) are 
regarded as equally worthy of consideration here. These particular 
analyses yield Mn RfC estimates of 0.09 to 0.1 g/m\3\.
    In general, continuous response data are preferred to dichotomized 
data, primarily because they provide more information and avoid the 
basically arbitrary division of effect measurements into categories 
(e.g., normal versus abnormal). The Bayesian analysis based on mean 
differences between exposed and control groups offers some of the 
advantages of using continuous data, in that the reported means and 
standard deviations (from Roels et al., 1992) provide a basis for 
estimating the distribution of continuous response measures. However, 
this use of continuous data is not immune to certain common problems, 
such as the issue of statistical power associated with studies of 
limited size, for the approaches calculating observed or just-
statistically significant differences. Also, whereas the dichotomous 
data analyses yield more precision in estimating the effective 
concentration associated with a somewhat imprecise response variable, 
the continuous data analyses offer the opposite trade off (i.e., more 
precision in the response variable but less in the exposure estimate). 
Nevertheless, the continuous data analyses appear to merit 
consideration as well as the analyses based on dichotomous data. By the 
Bayesian analyses of continuous data, Mn RfC estimates of approximately 
0.1 to 0.2 g/m\3\ are obtained.
    Based on the available data and on decisions and assumptions 
involved in analyses of these data, the leading candidate estimates for 
an alternative Mn RfC appear to fall in a range of approximately 0.09 
to 0.2 g/m\3\. (Ethyl Corporation (1994) has proposed an 
alternative RfC estimate based on a BMDL10 value of 87 g/
m\3\. Treating this value as essentially a NOAEL (thereby eliminating 
an uncertainty factor for use of a LOAEL), Ethyl Corporation divided 
the adjusted NOAEL(HEC) by a single uncertainty factor of 10 for 
sensitive subpopulations to derive a Mn RfC estimate of 3 g/

    \60\The RfC listed here is not simply the BMDL10 of 87 
g/m\3\ reduced by the uncertainty factor of 10 because the 
BMDL10 must first be adjusted to produce an adjusted NOAEL(HEC) 
(i.e., to go from an occupational exposure scenario to a scenario 
for the general public) prior to reduction by the uncertainty 

C. Exposure Assessment

1. Background
    Very limited data have been available by which to estimate 
potential Mn personal exposure levels likely to be associated with the 
use of MMT as an additive in unleaded gasoline. For example, after the 
completion of ORD's 1990 exposure assessment for Mn (U.S. Environmental 
Protection Agency, 1990), Ethyl Corporation provided EPA a brief report 
of a personal monitoring study as part of Ethyl Corporation's 
resubmittal of a waiver application for MMT. The study focused on 6 
taxi drivers and 17 office workers in Toronto, ON, where the allowable 
MMT concentration in gasoline is \1/16\ (0.062) g Mn/gal. (In the 
Toronto study, the actual concentration was reported as \1/26\ (0.039) 
g Mn/gal, which is only slightly greater than the \1/32\ (0.031) g Mn/
gal concentration proposed for the United States. As confirmed by 
Kirshenblatt (1993), MMT concentrations in Canadian gasoline average 
well below the allowable limit there.) In comments on Ethyl's 
resubmittal, ORD considered the Toronto data in conjunction with 
results from independent field studies of personal exposures to carbon 
monoxide to develop a revised Mn exposure assessment (Preuss, 1991). A 
key element of the 1991 ORD assessment was the assumption that taxi 
drivers (six of whom were monitored in Toronto within a 2-week period) 
were members of a high-exposure cluster reflecting the upper 4% of the 
population in a model based on the carbon monoxide field studies. The 
result of the 1991 assessment was an estimate that 4% of the general 
public might be exposed to Mn at approximately 0.09 g/m\3\, 
although this estimate had an undetermined amount of uncertainty due to 
the inadequacies of the available data.
2. Additional Canadian Studies
    Since the 1991 ORD assessment, additional personal exposure studies 
have been completed in Montreal and Toronto (described in Appendix B, 
``Reevaluation of Inhalation Health Risks Associated with Methyl 
cyclopentadienyl Manganese Tricarbonyl (MMT) in Gasoline'' as 
referenced in the reference section below, hereafter referred to as 
Appendix B.) As shown in Figure B-10 of Appendix B, the average 
concentrations reported in the Canadian studies vary by as much as an 
order of magnitude for small groups (5 to 19 persons each) of garage 
mechanics, taxi drivers, and office workers. The highest average Mn 
personal exposure level was 0.25 g/m\3\ for Montreal garage 
mechanics while at work; the other averages ranged from 0.002 to 0.035 
g/m\3\ for various particle size fractions. Although it is 
impossible to extrapolate the results of these studies to the 
distribution of Mn exposure levels for the general population, it does 
appear that there is a general relationship between personal exposure 
levels of Mn and proximity to vehicular emissions of combusted MMT. 
Thus, populations living near high traffic-volume areas such as inner 
cities and expressways would probably tend to experience higher Mn 
exposure levels in relation to MMT usage.
    Some of the limitations of the Canadian studies with respect to 
development of a quantitative exposure assessment are reviewed in 
detail in Appendix B and may be summarized briefly as follows.

     The studies did not have adequate sample sizes and did 
not sample according to a probabilistic statistical design that 
would help ensure the representativeness of the sampled individuals.
     The sampling periods were relatively short, 1 to 2 
weeks at most. Meteorological and other factors that would be 
expected to influence ambient measurements over relatively short 
periods of time were not characterized.
     Because the studies did not use ambient monitors 
collocated with reference monitors (such as the dichotomous samplers 
used by Canadian agencies), it is difficult to relate data from the 
studies to larger databases from the government monitoring networks.
     Because the studies did not use identical monitors to 
measure personal exposure levels and outdoor ambient levels, it is 
difficult to distinguish between personal exposures and ambient 
levels or to relate one to the other.
     Quality assurance and certain other important 
methodological details are not fully provided in the available 

    Because of the substantial limitations of the above exposure 
studies, no quantitative assessment of personal exposures to Mn in a 
Canadian population is possible at present.
3. The PTEAM Study
    The only published study that has used a probability-based 
representative sampling design for evaluating exposure levels of Mn in 
a general population is the Particle Total Exposure Assessment 
Methodology (PTEAM) study, which was conducted in Riverside, CA, over a 
7-week period in the fall of 1990 (Pellizzari et al., 1992). This study 
used personal and stationary monitors to measure Mn concentrations 
indoors and outdoors. The personal samplers collected PM10, and 
the stationary samplers collected PM2.5 as well as PM10 (see 
glossary of terms in Attachment B-6 to Appendix B). Of the 139,000 
nonsmoking residents age 10 years and older in Riverside, 178 
individuals were selected through a stratified sampling plan to 
represent the general population and were monitored over two 12-hour 
periods (daytime and nighttime). More than 2,750 particle samples were 
collected. Quality assurance and other procedures are summarized in 
Appendix B and are described extensively elsewhere (e.g., Pellizzari et 
al., 1992; Clayton et al., 1993; Thomas et al., 1993). The PTEAM study 
has been presented in various peer-reviewed publications and discussed 
in several scientific forums (see Attachment B-5 to Appendix B). It 
represents the best available information on an actual distribution of 
general population exposures to Mn. It also provides valuable 
information on potential Mn exposure associated with MMT use, because 
MMT was used in leaded gasoline in California prior to and during the 
period of the PTEAM study.
4. Estimated Mn Exposure Levels Associated with MMT Usage
    As noted above, the substantial limitations of the available 
Canadian exposure studies make them unsuitable for estimating 
population exposure levels of Mn in relation to MMT usage. In addition, 
ambient monitoring data typically underestimate and may be uncorrelated 
with personal exposure levels of automotive-source pollutants. 
Therefore, of the currently existing published evidence pertaining to 
Mn exposure levels in relation to MMT usage, only the PTEAM Riverside 
study (Pellizzari et al., 1992) provides a reasonable basis for 
estimating potential future exposure levels in relation to a scenario 
where 100% of unleaded gasoline contains \1/32\ g Mn/gal as proposed by 
Ethyl Corporation.
    In the PTEAM study, measurements of personal exposure levels of 
PM10 Mn indicated that approximately half of the population in 
Riverside in the 1990 study period had 24-hour personal exposures to 
PM10 Mn above 0.035 g/m\3\, with the highest 1% of the 
population having exposures above 0.223 g/m\3\ PM10 Mn. 
However, given the use of PM5 Mn exposure measurements in the 
study of Roels et al. (1992), it would be preferable to consider a 
population distribution of personal exposure levels of PM5 Mn. Due 
to limitations in the available data, the exposure assessment in 
Appendix B focuses on estimated personal exposure levels for PM4 
Mn, not PM5. Although the difference is probably small, PM4 
levels are an underestimate of PM5 levels. The derivation of the 
projected exposure estimates involved several steps, which may be 
summarized as follows.
    The automotive and nonautomotive contributions to particulate Mn 
exposures in the PTEAM study were estimated using data from Lyons et 
al. (1993), who reported particle size distributions up to PM4 of 
selected trace metals, including Mn, at two locations near Riverside in 
the winter and summer of 1989. They attributed most of the PM4 Mn 
to automotive sources. Based on their findings and data from other 
sources, it is possible to estimate that 69% of the PM2.5 fraction 
of PM4 Mn they measured was derived from automotive sources 
(namely the combustion of MMT in motor vehicle fuel, as then allowed in 
leaded gasoline in California) and that 31% was derived from paved road 
dust (mostly earth crustal material). Next, the PTEAM Mn measurements 
from stationary indoor monitors (SIMs) were used to estimate personal 
exposure levels by adjusting the SIM PM2.5 Mn data to reflect the 
typically higher levels of all elements measured by personal exposure 
monitors (PEMs). This adjustment was made in two ways, either by the 
PEM:SIM ratio obtained for Mn or by the ratio obtained for lead (Pb), 
another element related to automotive fuel usage. These two methods of 
adjusting the SIM data to PEM values resulted in two projected 
distributions, as will be described below. The next step in the 
derivation procedure involved adjusting the PM2.5 personal 
exposure estimates to reflect PM in the size range from 2.5 to 4 
m (based again on data from Lyons et al., 1993).
    With these estimates of PM4 Mn personal exposure levels due to 
automotive sources, it was then possible to project from the situation 
in Riverside around the time of the PTEAM study (when leaded-MMT 
gasoline constituted about 14% of the gasoline sold and contained an 
average of 0.048 g Mn/gal) to a future scenario that assumes 100% of 
the unleaded gasoline contains MMT at \1/32\ (0.031) g Mn/gal. This 
aspect of the derivation is described in detail in Attachment B-4 to 
Appendix B. In essence, a factor was calculated to reflect the 
estimated increase in MMT usage between 1990 and 1995 (i.e., the first 
full year in the near future). This projection factor assumed an 
increase of 1% per year in gasoline usage and no difference in the Mn 
emission rate (grams Mn emitted per gram Mn in fuel combusted) for 
noncatalyst vehicles using leaded-MMT gasoline in 1990 versus catalyst 
vehicles using unleaded-MMT gasoline in 1995.
    Next, the nonautomotive contribution to PM4 Mn was estimated 
and added to the estimated automotive contribution to obtain the 
projected personal exposure levels of total PM4 Mn. Assuming the 
estimated PM4 Mn distribution has the same form as the PM10 
Mn distribution from PTEAM (approximately lognormal with equal 
geometric standard deviations), the ratio of the PM4:PM10 
arithmetic mean personal exposure levels yields a scaling factor that 
can be applied to the PM10 distribution to obtain the PM4 
distributions. Because of the alternative bases for adjusting the SIM 
PM2.5 data for personal exposures (as noted above), two different 
scaling factors were multiplied by the PM10 distribution, thereby 
producing a higher and a lower estimate of the distribution of 24-hour 
average PM4 Mn personal exposure levels.
    In addition, because long-term exposures are likely to have less 
variance than 24-hour exposures, it is appropriate to adjust the 
distributions of 24-hour average exposure levels to better reflect 
longer periods of exposure. Of various methods that may be used for 
this purpose (Wallace et al., 1994), two approaches were applied to 
adjust the geometric standard deviations of the two projected exposure 
distributions, based on data from either the PTEAM study or the smaller 
pilot study that preceded the PTEAM Riverside study. These alternative 
methods were applied to the two estimated distributions of 24-hour 
average exposures to yield the distributions of long-term average 
PM4 Mn personal exposure levels depicted as lines 1 and 2 in 
Figure 2 (only the highest and lowest of the four resulting estimates 
are shown). It must be emphasized that these two distributions do not 
represent upper and lower bounds, because even higher or lower 
estimates could be produced by alternative assumptions and adjustments 
of the data. Moreover, if data were available for another time of the 
year (e.g., spring in addition to fall), the estimates would not be 
season-specific and could possibly be much higher or much lower. 
Nevertheless, given the limited available data, lines 1 and 2 in Figure 
2 represent two reasonable estimates of the projected long-term 
(autumnal) personal exposure levels of PM4 Mn in relation to MMT 
usage at \1/32\ g Mn/gal in 100% of unleaded gasoline.
    By examination of the logarithmic-probability plot of long-term 
personal exposure levels of Mn, it is estimated that half of the 
population would be exposed to PM4 Mn levels of more than 
approximately 0.045 to 0.050 g/m\3\. Also, based on the two 
projection estimates, approximately 5 to 10% of the population would 
have personal exposure levels around 0.1 g/m\3\ PM4 Mn or 
higher. The highest 1% would be predicted to have PM4 Mn exposure 
levels above 0.15 g/m\3\. It should be noted that these 
projections refer specifically to Riverside, CA, with a population of 
more than 139,000 persons. However, in many significant respects (e.g., 
meteorology and traffic volume), Riverside is reasonably representative 
of the greater metropolitan area of Los Angeles, which has a total 
population of over 14.5 million persons. The exposure projection 
estimates for Riverside imply the possibility that hundreds of 
thousands of persons in the Los Angeles area alone could be exposed to 
PM4 Mn levels exceeding 0.1 g/m\3\. To the extent that 
any other U.S. cities (e.g., in the Southwest) share some degree of 
resemblance in meteorology, vehicle miles traveled (VMT), and possibly 
other characteristics of relevance to automotive Mn levels, the 
estimated exposure levels for Riverside could be pertinent, at least 
qualitatively, to other locales or portions of locales as well. 
Similarities and differences in point-source contributions to Mn 
exposure would also figure into comparisons with other communities. The 
presence of a major point source or sources of Mn in a community (which 
was not a factor in Riverside) would add some increment to the level of 
Mn exposure experienced by the persons in that community. Although 
these Riverside estimates cannot be applied quantitatively to any other 
U.S. metropolitan areas, the total population of the U.S. counties with 
VMT levels greater than that of Riverside (apart from the four counties 
Los Angeles comprises) is approximately 15 million persons. Possibly, 
then, several hundreds of thousands of persons could be exposed to 
PM4 Mn levels of approximately 0.1 g/m\3\ or higher if 
MMT were used in 100% of the unleaded gasoline in all of these areas. 
However, it must be emphasized that because of the limited available 
data, a great deal of uncertainty surrounds such estimates. The actual 
exposure levels could be much higher or lower.




D. Risk Characterization

    To assess the public health risk associated with the use of MMT in 
gasoline in the United States, the available qualitative and 
quantitative health effects information on Mn must be related to the 
available exposure information. From the standpoint of a qualitative 
hazard identification, the available evidence amply demonstrates that 
inhaled Mn is toxic to the nervous system, the respiratory system, and 
the male reproductive system. The toxicity of Mn by different routes of 
exposure has been demonstrated by numerous medical reports and 
epidemiological and experimental studies. However, available data do 
not allow quantitative estimation of the relative toxicological potency 
of different Mn compounds or permit quantitative route-to-route 
extrapolations for predicting the effects of Mn3O4.
    The focus of the above health assessment discussion has been on the 
RfC and the types of risks associated with chronic Mn exposures 
because, for the most part, acute effect levels appear to be 
considerably higher than the highest projected exposure levels. 
However, the issue of less-than-chronic exposures does arise with 
respect to the potential for developmental toxicity. It is widely 
recognized that the human CNS develops over a period of several years, 
prenatally and postnatally, and can be vulnerable to long-term or 
irreversible effects if damage occurs during certain ``critical 
stages'' of development. Recent evidence from ongoing longitudinal 
studies of children indicates that lead (Pb) exposure (measured as 
blood Pb level) around 2 years of age in particular is associated with 
reduced cognitive performance at 4 to 10 years of age. Such evidence 
raises the concern that exposure to another neurotoxic metal such as Mn 
during part of early development might also be capable of inducing 
permanent or irreversible damage to the developing CNS. Moreover, the 
ramifications of such damage might extend to other important functions, 
such as reproduction.
    Children may also be at higher risk in terms of exposure because of 
biomedical and metabolic differences at a young age (greater uptake and 
retention) and/or because of the longer duration of their exposure over 
a lifetime. Over time, small impairments in neurobehavioral function 
may accumulate. For this reason, the elderly, whose neurobehavioral 
function may already be compromised by normal aging processes and 
possibly by other disease states (e.g., parkinsonism or preclinical 
parkinsonism), also represent a special population of concern. The 
ability of the elderly or other subpopulations to compensate for such 
declines in neurobehavioral function may be overwhelmed eventually by 
additional, albeit possibly quite small, insults due to Mn. If so, the 
effect could be manifested as a more severe or earlier onset of 
declining function in senescence, with consequent implications for 
increased societal health-care costs.
    Special subpopulations at increased risk may be defined not only in 
terms of their biological susceptibility, as exemplified above by the 
young and the elderly, but also by their increased risk of exposure. In 
this respect, inner city residents and others who live near high 
traffic areas such as expressways (e.g., low-income and minority 
communities) would possibly have a disproportionate likelihood of 
higher Mn exposure levels due to their closer proximity to vehicular 
    The nature of the neurobehavioral effects observed in occupational 
studies such as Roels et al. (1992) should be understood as effects 
that probably would not be treated by, or even be readily evident to, a 
clinical physician. Nonetheless, they are significant from a public 
health standpoint when considered in terms of population effects. This 
concept is illustrated by the well documented findings on low-level Pb 
neurotoxicity in children, where changes of as little as 1 or 2 points 
in IQ have been repeatedly demonstrated in several independent, 
prospective, epidemiological studies in recent years. Such changes 
could not be reliably demonstrated either in a clinical setting or in 
earlier cross-sectional epidemiological studies; yet they are now well 
established to be ``real'' and significant from a public health 
standpoint. With regard to the reductions in neurobehavioral function 
observed in various epidemiological studies of Mn-exposed workers, 
these studies independently converge on findings of impaired motor 
function (e.g., reductions in eye-hand coordination, slower hand or 
finger movements, and less control of fine movement). As recently 
expressed in a document prepared by the Subcommittee for Risk 
Assessment of the Federal Coordinating Council for Science, Engineering 
and Technology (Federal Register, 1993), an adverse effect can include 
``both unwanted effects and any alteration from baseline that 
diminishes the ability to survive, reproduce or adapt to the 
environment.'' Thus, it can be argued that these effects in themselves 
warrant consideration as adverse health effects. They may also have 
ramifications for health and safety of an even more serious nature, if 
a person's ability to react quickly and accurately to a situation 
(e.g., traffic conditions) was impaired.
    Another aspect of the findings from available occupational studies 
concerns the temporal relationship between exposure and effect. As 
noted above, the geometric mean average period of Mn exposure of the 
workers in the Roels et al. (1992) study was only 4 years, with the 
longest period of Mn exposure for any one individual being less than 18 
years. Also, the oldest worker in the Roels et al. (1992) study was 
less than 50 years old. This relatively limited period of exposure 
along with the absence of older subjects in the Roels study raises the 
question of whether sufficient time had elapsed for the full expression 
of the toxic effects of Mn. Some reports in the literature indicate 
that Mn toxicity may not be clinically evident until some years after 
exposure occurred or terminated (e.g., Cotzias et al., 1968; Rodier, 
1955), and other reports point to a greater sensitivity of elderly 
persons, compared to middle-aged or young adults, for acute as well as 
chronic Mn toxicity (e.g., Kawamura et al., 1941). An uncertainty 
factor for extrapolation from a subchronic exposure to chronic exposure 
was included in deriving the RfC estimates shown in Figure 1. However, 
a ``half-factor'' of 3 was used for this area of uncertainty. If the 
average period of Mn exposure (geometric mean: 4 years) in the Roels et 
al. (1992) study is compared to an assumed lifetime of 70 years, one 
could argue that a factor of 70/4=17.5, or at least 10, would be a more 
appropriate adjustment for subchronic to chronic exposures. Given the 
limited available data, this area of uncertainty is difficult to 
express in a quantitative manner, but in general practice, EPA has used 
an uncertainty factor of 3 for other chemicals with comparable 
databases. Nevertheless, this area of concern suggests that the Mn RfC 
estimates derived here with an uncertainty factor of 3 for subchronic 
to chronic exposure probably do not tend to err in the direction of 
being too conservative.
    Another qualitative concern that should be recognized in 
considering the potential health effects of Mn is the possibility of 
certain types of effects that are suggested by clinical and other 
evidence but are difficult to measure quantitatively or demonstrate 
with currently available methods. Specifically, much of the clinical 
literature on manganism refers to a psychiatric component of the 
illness, which often involves striking emotional or mood changes that 
tend to appear before changes in motor function are evident. Such 
effects are inherently difficult to measure in a quantitative manner. 
However, the possibility of such effects at lower levels of exposure 
than those at which motor control is affected should not be discounted 
out of hand. Some reports in the literature (e.g., Gottschalk et al., 
1991) suggest that aggressive behavior may be associated with Mn 
exposure (as reflected in the concentration of Mn in hair of prison 
inmates). Such reports require substantiation by further studies, and 
the validity and relevance of hair Mn levels to environmental Mn 
exposure remains to be established, but the suggestion that an 
association might exist between hair Mn and behavior cannot be totally 
    Quantitative analyses of neurobehavioral data obtained from an 
occupational cohort provide a range of possible RfC estimates in 
addition to the current, verified Mn RfC value of 0.05 g/m\3\. 
In ORD's judgment, the leading candidates for a possible alternative 
RfC estimate are approximately 0.09 to 0.2 g/m\3\, based on 
currently available information. By definition, RfC analyses do not 
yield a precise concentration that defines a demarcation between safety 
and hazard. Rather, interpretation of a Mn RfC estimate is best made in 
relation to an assessment of population exposures to Mn, with the 
understanding that the RfC is a protective level, not a predictive 
    The exposure assessment, based largely on data from the PTEAM 
study, provides some reasonable but necessarily uncertain estimates of 
personal exposure levels of Mn that might result from the use of MMT in 
gasoline. These estimates indicate that if MMT (at \1/32\ g Mn/gal) 
were used in all unleaded gasoline in Riverside, CA (or the greater Los 
Angeles metropolitan area), approximately 40 to 50% of the population 
could experience PM4 Mn exposures exceeding the current RfC of 
0.05 g/m\3\ (derived from PM5 Mn health effects data), 
and approximately 5 to 10% could experience PM4 Mn exposure levels 
around 0.1 g/m\3\ or higher (see Figure 2). In terms of the 
Los Angeles area population of 14.5 million persons, even an estimate 
of 5% of the population implies over 700,000 persons.
    Uncertainties are inherent in any risk assessment. In this case, on 
the health assessment side, the numerical uncertainty factors used in 
the RfC analyses presented here have been explicitly described and 
explained in and summarized above. These factors are intended to 
provide a reasonable degree of public health conservatism reflecting 
areas of biological knowledge as well as areas of information deficit. 
In addition, an RfC estimate by definition reflects uncertainty 
spanning perhaps an order of magnitude, and thus there is no 
significant difference between the verified RfC of 0.05 g/m\3\ 
and alternative estimates of 0.09 to 0.2 g/m\3\. Other 
qualitative uncertainties are also discussed above. On the exposure 
assessment side, the primary uncertainties are related to projections 
from the PTEAM data, rather than the PTEAM data per se. Inferences 
about the relative contributions of crustal and automotive sources to 
PM4 Mn were drawn from studies conducted in geographical and 
temporal proximity to the PTEAM study, but both the data from these 
studies and the inferences based on them introduce uncertainties. 
Attempts to adjust the PTEAM data in various ways, including any 
extrapolation from the 24-hour average distribution obtained in the 
fall of 1990 to a long-term average for other seasons, introduce 
progressively greater uncertainties at each step. Some adjustments have 
not even been attempted. For example, a weighting of the daytime and 
nighttime PTEAM exposure data to reflect a higher average ventilation 
rate during daytime activities and a lower ventilation rate during 
nighttime activities (e.g., sleeping) would have resulted in higher 
personal exposure estimates. Thus, given other approaches or 
assumptions, different projection estimates are possible. It must 
therefore be emphasized that the two projections of Mn exposure levels 
in Figure 2 should not be interpreted as upper and lower bound 
estimates, for even the higher projection could possibly underestimate, 
or the lower projection overestimate, the PM4 Mn exposure levels 
associated with MMT.
    As for the relevance of the PTEAM Riverside personal exposure 
estimates to other communities, the PTEAM study was, strictly speaking, 
only designed to statistically represent Riverside, CA. In that 
respect, the design and conduct of the PTEAM study provide a high 
degree of confidence that it does accurately represent 24-hour average 
Mn exposure concentrations for the Riverside population in the fall of 
1990. In ORD's judgment, the PTEAM study provides a reasonable 
representation of the Los Angeles Basin as well, given the 
commonalities in geography, vehicle usage, and meteorology. However, 
the relevance of the Riverside data to other U.S. communities depends 
upon their similarities or differences in the most relevant 
characteristics or dimensions. For example, to the extent that several 
other major U.S. metropolitan areas (or subcommunities in these areas) 
also have a high level of vehicle usage, the Riverside projections may 
have greater relevance. To the extent that these same areas do not 
share the meteorological conditions that contribute to the Mn exposure 
levels measured in Riverside, the Riverside projections have lesser 
relevance. It is also important to consider other sources of Mn 
exposure apart from automotive and crustal sources. Although Riverside 
had no major point sources of Mn contributing to the personal exposure 
levels measured in the PTEAM study, other communities may have such 
sources, and thus the personal exposure levels of Mn from all sources 
might be higher in other communities than in Riverside.
    The exposure estimates shown in Figure 3 are in the range of or 
exceed some candidate RfC estimates as well as the current RfC. 
Exceeding the RfC does not necessarily indicate that a public health 
risk will occur. At present, it is impossible to state whether 
projected exposures above the RfC would result in an adverse health 
effect for either an individual or the general population. At a 
sufficiently high level of exposure, adverse effects would be expected 
to occur, first in any sensitive subpopulations, then with greater 
prevalence in the general population and extending to other types of 
effects (e.g., reproductive and/or respiratory as well as 
neurobehavioral effects in the case of Mn). However, the relationship 
between such ``sufficiently high'' levels and the population exposure 
levels estimated by the projection methods employed here is unknown. 
Expressed differently, given the gap between observed or modeled effect 
levels and the RfC values obtained by applying uncertainty factors of 
orders of magnitude, it is impossible to state whether projected 
population exposures would lie above or below a presumed threshold 
level on the actual concentration-response curve for Mn neurotoxicity. 
This gap between projected exposure levels and the lowest 
concentrations obtained by modeling the concentration-response 
relationship (at least, by the quantal linear model) makes it 
impossible to make any assertion regarding the likelihood of a health 
risk at projected exposure levels. However, this conclusion should not 
be interpreted to imply that, therefore, no health risk is expected to 
exist at exposure levels exceeding the RfC.




E. References

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Columbia, Canada. Pittsburgh, PA: Air and Waste Management 
Association; paper no. 91-173.3.
Kawamura, R.; Ikuta, H.; Fukuzami, S.; Yamada, R.; Tsubaki, S.; 
Kodama, T. (1941) Intoxication by manganese in well water. Kitasato 
Arch. Exp. Med. 18: 145-169.
Kimmel, C. A.; Gaylor, D. W. (1988) Issues in qualitative and 
quantitative risk analysis for developmental toxicology. Risk Anal. 
8: 15-20.
Kirshenblatt, M. (1993) (Letter to Ms. M. T. Smith, U.S. EPA, 
regarding the August 30, 1993, rebuttal comments from Ethyl 
Corporation in support of their waiver request for HITEC 3000 fuel 
additive.) Ottawa, Ontario, Canada: Environment Canada, 
Transportation Systems Division, Industrial Programs Branch; 
September 23.
Lyons, J. M.; Venkataraman, C.; Main, H. H.; Friedlander, S. K. 
(1993) Size distributions of trace metals in the Los Angeles 
atmosphere. Atmos. Environ. Part B 27B: 237-249.
Mergler, D.; Huel, G.; Bowler, R.; Iregren, A.; Belanger, S.; 
Baldwin, M.; Tardif, R.; Smargiassi, A.; Martin, L. (1994) Nervous 
system dysfunction among workers with long-term exposure to 
manganese. Environ. Res. 64: 151-180.
Murphy, V. A.; Wadhwani, K. C.; Smith, Q. R.; Rapoport, S. I. (1991) 
Saturable transport of manganese (II) across the rat blood-brain 
barrier. J. Neurochem. 57: 948-954.
Pellizzari, E. D.; Thomas, K. W.; Clayton, C. A.; Whitmore, R. W.; 
Shores, R. C.; Zelon, H. S.; Perritt, R. L. (1992) Particle total 
exposure assessment methodology (PTEAM): Riverside, California pilot 
study, volume I (final report). Research Triangle Park, NC: U.S. 
Environmental Protection Agency, Atmospheric Research and Exposure 
Assessment Laboratory; EPA report no. EPA/600/R-93/050. Available 
from: NTIS, Springfield, VA; PB93-166957/XAB.
Preuss, P. W. (1991) ORD's comments on Ethyl Corporation's July 12, 
1991 resubmittal of a waiver application for the use of 
methylcyclopentadienyl manganese tricarbonyl (MMT) in unleaded 
gasoline (memorandum to Richard Wilson). Washington, DC: U.S. 
Environmental Protection Agency, Office of Research and Development; 
December 12.
Rodier, J. (1955) Manganese poisoning in Moroccan miners. Br. J. 
Ind. Med. 12: 21-35.
Roels, H. (1993) (Letter to Dr. M. Davis, U.S. EPA, on definitions 
of ``respirable,'' ``total,'' and ``inhalable'' dusts). Bruxelles, 
Belgium: Universite Catholique de Louvain, Unite de Toxicologie 
Industrielle et Medecine du Travail; October 19.
Roels, H.; Lauwerys, R.; Genet, P.; Sarhan, M. J.; de Fays, M.; 
Hanotiau, I.; Buchet, J.-P. (1987) Relationship between external and 
internal parameters of exposure to manganese in workers from a 
manganese oxide and salt producing plant. Am. J. Ind. Med. 11: 297-
Roels, H. A.; Ghyselen, P.; Buchet, J. P.; Ceulemans, E.; Lauwerys, 
R. R. (1992) Assessment of the permissible exposure level to 
manganese in workers exposed to manganese dioxide dust. Br. J. Ind. 
Med. 49: 25-34.
Thomas, K. W.; Pellizzari, E. D.; Clayton, C. A.; Whitaker, D. A.; 
Shores, R. C.; Spengler, J. D.; Ozkaynak, H.; Wallace, L. A. (1993) 
Particle total exposure assessment methodology (PTEAM) study: method 
performance and data quality for personal, indoor, and outdoor 
aerosol monitoring at 178 homes in southern California. J. Exposure 
Anal. Environ. Epidemiol. 3: 203-226.
Tukey, J. W.; Ciminera, J. L.; Heyse, J. F. (1985) Testing the 
statistical certainty of a response to increasing doses of a drug. 
Biometrics 14: 295-301.
U.S. Environmental Protection Agency. (1990) Comments on the use of 
methylcyclopentadienyl manganese tricarbonyl in unleaded gasoline. 
Washington, DC: Office of Research and Development.
U.S. Environmental Protection Agency. (1991) Information needed to 
improve the risk characterization of manganese tetraoxide 
(Mn3O4) and methylcyclopentadienyl manganese tricarbonyl 
(MMT). Washington, DC: Office of Research and Development; December 
Wallace, L. A.; Duan, N.; Ziegenfus, R. (1994) Can long-term 
exposure distributions be predicted from short-term measurements? 
Risk Anal. 14: 75-85.
Wennberg, A.; Iregren, A.; Struwe, G.; Cizinsky, G.; Hagman, M.; 
Johansson, L. (1991) Manganese exposure in steel smelters a health 
hazard to the nervous system. Scand. J. Work Environ. Health 17: 
Wennberg, A.; Hagman, M.; Johansson, L. (1992) Preclinical 
neurophysiological signs of parkinsonism in occupational manganese 
exposure. Neurotoxicology 13: 271-274.

F. Comments on Health Assessment and EPA Response

    The focus of most of Ethyl's comments regarding the effects on 
public health and welfare resulting from the use of MMT in unleaded 
gasoline deal with issues related to EPA's risk assessment and 
specifically to the calculation of an RfC for manganese by the 
utilization of various analytical methods, to the uncertainty factors 
used by the Agency in determining the RfC for manganese, and to the 
exposure assessment for manganese. A summary discussion of the 
significant issues related to these comments follows.\61\

    \61\For a complete description of the EPA response to Ethyl's 
comments, the reader is referred to Air Docket A-93-26, (Docket A-
93-26, II-A-15) ``ORD's Response to Public Comments on the Reference 
Concentration for Manganese and on the Draft ORD Risk Assessment of 

    Generally, Ethyl states that ``ORD openly concedes that after four 
years of efforts it is unable to conclude that a public health risk 
would exist for even the most highly exposed segment of the 
population.'' The Agency indicated to Ethyl in 1991 the research it 
could conduct to provide for a more quantitative assessment of the MMT 
issue. Thus, assessment of the risks or benefits of MMT could be, or 
perhaps could already have been, improved if more empirical information 
were provided by Ethyl to the Agency.
    Ethyl commented on ORD's April 28, 1994 draft MMT Risk Assessment 
that, depending on the methodology used for the derivation of the RfC 
for manganese, the level of manganese concentrations ``without 
appreciable risk'' lies somewhere within the range of 0.1 to 3.0 
g/m3 and that all of ORD's RfC calculations fall within 
this range. Contrary to Ethyl's statement, the inclusion of Ethyl's 
proposed RfC estimate of 3.0 g/m3 in tables and figures 
summarizing the RfC estimates obtained by different methods was merely 
an attempt by the Agency to illustrate all of the RfC estimates. The 
Agency did not imply or state a conclusion that ``The majority of ORD's 
RfC calculations fall in the 0.1-3.0 g/m3 range.'' ORD 
focused on a leading candidate RfC estimate of approximately 0.1 
g/m3, while explicitly noting that the current official 
RfC of 0.05 g/m3 is not meaningfully different from a 
possible alternative RfC estimate of 0.1 g/m3.
    To more effectively avoid possible ambiguity that is reflected in 
Ethyl's comment, Figure 1 of the final MMT Risk Assessment (U.S. 
Environmental Protection Agency, 1994b) shows all of the RfC estimates 
contained in Table A-39 of Appendix A. Ethyl's preferred RfC estimate 
of 3 g/m3 is now omitted from Figure 1 because it is not 
a value obtained by any of ORD's analyses. Moreover, it is ORD's view 
that the best estimate of an alternative RfC estimate for Mn, based on 
the analyses and data considered to date, probably lies in the range of 
0.09 to 0.2 g/m3. This judgment is based on the strengths 
and weaknesses of the various analytic approaches considered by ORD in 
the April 28 draft assessment.\62\

    \62\The reader is referred to Air Docket A-93-26, II-A-15, 
``ORD's Response to Public Comments on the Reference Concentration 
for Manganese and on the Draft ORD Risk Assessment of MMT, for 
further evaluation of the various analytical approaches that were 
investigated by the Agency. ORD views the 10 and 5% benchmark and 
Bayesian analyses of dichotomous data and the Bayesian analyses of 
continuous data using the quantal linear model as having greater 
scientific strengths than the other analyses considered by ORD or by 

    Ethyl has stated that EPA has failed to address issues regarding 
both the statistical treatment of the data underlying the RfC and EPA's 
decision to employ a 1000-fold factor of uncertainty in calculating the 
RfC. EPA disagrees. Appendix A of the ORD Risk Assessment addresses 
each of these issues. The extensive analyses the Agency has conducted 
represent a concerted effort to respond to the statistical issues 
raised by Ethyl. The fact that EPA does not adopt Ethyl's advocated 
position is not a failure to address the issues. In fact, no scientific 
consensus or policy exists on many of these issues, such as the 
selection of a specific mathematical dose-response model, the selection 
of an effect level, how to address continuous data, and the appropriate 
magnitude of severity regarding production of uncertainty factors for 
the various effect levels. As is specifically described in the 
``Response to Comments'' document, ORD utilized scientific judgment in 
determining the appropriateness of certain models and the inclusion of 
specific uncertainty factors.
    Ethyl also expressed concerns that EPA had not adequately addressed 
alternative analyses for estimating an RfC by utilizing different 
groupings of subjects in the Roels analysis. ICF Kaiser, a consultant 
for Ethyl, commented that ``ORD ignores alternative analyses which most 
closely resemble the conventional method for calculating RfCs.'' The 
alternative approach Ethyl employed to group the data from the study by 
Roels et al. (1992) has no evident rationale other than providing a 
statistically nonsignificant effect level. The groupings employed by 
Roels et al. (1992) and used by ORD appear to have been selected 
because they provided approximately equal numbers of subjects per 
group. The groupings selected by Ethyl result in both smaller and more 
disparate numbers of subjects per group, with a consequent reduction in 
statistical power to detect a significant difference between a Mn-
exposed group and controls. Further, as explained in the Response to 
Comments Document previously cited, ORD does not view the use of the 
upper boundary of the concentration associated with a group as a 
scientifically appropriate or representative metric to describe the 
exposure of the entire group.
    Ethyl also took issue with the Agency for focusing on analyses 
using the average concentration of respirable dust (ACRD). The decision 
to focus on ACRD was not arbitrary or designed to yield lower RfC 
estimates but, as explained in Appendix A of the ORD Risk Assessment, 
was based on both an explicit consideration of the advantages ACRD 
offered for fitting dose-response models and its ability to reflect 
average past exposure, thereby obviating some of the apparent 
disadvantages of the two exposure measures reported by Roels et al. 
(1992), LIRD and CRD. Although the Agency views ACRD as the best choice 
of these three available exposure measures for the analytical 
approaches employed in the ORD risk assessment, Figure 1 (See Section 
VI-B, above.) illustrates all of the RfC estimates presented for all 
three exposure measures and all analytical approaches.
    Ethyl recommends that certain analyses should be omitted from 
consideration on the ORD risk assessment. To some extent, the Agency 
believes the difference between Ethyl's and EPA's positions is related 
to differing interpretations of what is reasonable versus unreasonable, 
biologically plausible versus implausible, or valid versus invalid. As 
addressed before, the Agency's basic approach in the ORD risk 
assessment was to present all of the results of its analyses and then 
identify a subset of the total that have a scientifically stronger 
basis for being considered preferentially.\63\

    \63\For specific discussion related to these issues the reader 
is referred to Air Docket A-93-26, II-A-12.

    Ethyl offers several arguments directed at reducing the overall 
uncertainty and specifically the composite uncertainty factor (UF) of 
10 for database limitations applied in deriving estimates of an RfC for 
manganese. Ethyl's first argument concerns the basic ``biological 
plausibility'' of the RfC, suggesting that inhaled manganese cannot be 
as toxic (compared to ingested manganese) as the roughly 100-fold 
difference in RfC and RfD estimates would indicate.\64\ The Agency 
believes that a fundamental problem with Ethyl's comparison is that it 
is limited to what Ethyl estimates is the ``systemic dose.'' The key 
issue is not systemic dose but delivered dose, i.e., the amount of 
manganese that actually reaches and enters a critical target organ such 
as the brain. Furthermore, the Agency believes that, depending on the 
form of manganese inhaled, and its ability to enter the brain, it is 
quite possible that a significant fraction of even small amounts of 
inhaled manganese would be able to reach target sites in the central 
nervous system. Thus, the apparently greater toxicity of inhaled versus 
ingested manganese may reflect important pharmacodynamic and 
pharmacokinetic differences of manganese that enters the body by 
different routes of exposure. Ethyl's analysis of systemic dose does 
not adequately address this issue.\65\

    \64\See Air Docket A-93-26. The Reference Dose or RfD is 
analogous to the RfC but for ingestion as opposed to inhalation. 
Document II-A-13 (in the docket), Table 1, compares the ``systemic 
dose'' of manganese for the Recommended Daily Intake (now termed 
``Recommended Daily Allowance'' by FDA) of ingested manganese, the 
RfC for inhaled manganese, and the RfD for manganese in drinking 
water. This tabular comparison does not address several issues that 
the Agency has identified as factors in its assessment on inhaled 
manganese toxicity.
    \65\In fact, as is indicated in ``ORD's Response to Comments on 
MMT'' document, many studies indicate that inhalation exposure does 
result in a greater delivered dose. This evidence includes, for 
example, a report by Coulston and Griffin (1977), who exposed 
monkeys to the whole combustion products of MMT (Mn3O4) 
for 23 hr/day for up to 66 weeks. According to the authors, monkeys 
breathed 0.86 L/min, or 119 L in a 23-hr exposure day. AT 100 
g/m3, approximately 120 g Mn would be inhaled 
daily, which would equate to a systemic dose of about 24 g/
day (assuming 20% deposition and 100% absorption). In addition, both 
control and inhalation-exposed monkeys ingested about 4-5 mg Mn/day 
in their diet, which would equate to a systemic dose of 
approximately 135 g/day (assuming 3% absorption). Even 
though these calculations indicate that the diet accounted for about 
six times more Mn entering the body of the exposed animals, the 
relatively small inhalation fraction doubled the delivered dose of 
Mn in the brains of the inhalation-exposed monkeys. Furthermore, the 
authors found blood levels of Mn to be equivalent in the exposed and 
control monkeys, despite the higher brain levels in the inhalation-
exposed animals. Thus, extrapolations based on estimated systemtic 
dose by ingestion underpredict doses delivered to the brain by 

    The more specific critique Ethyl offers regarding the composite UF 
of 10 for database limitations deals with three areas of inadequate 
information represented by the composite factor: chronic exposure 
effects, reproductive/developmental effects, and differences in 
toxicity of different forms of manganese. In deriving RfCs for other 
chemicals, any one of these three areas of uncertainty has, in one case 
or another, warranted a full UF of 10.\66\

    \66\The decision whether to utilize the full uncertainty factor 
of 10 is based on a judgement by the RfC workgroup on a case-by-case 

    The Agency believes that given the possible accumulation of 
manganese, the effects of chronic exposure to inhaled manganese are of 
concern. The maximum average exposure duration of the available 
occupational studies cited by both ORD and Ethyl is 16.7 years (Mergler 
et al. 1994),\67\ which does not constitute chronic exposure. Ethyl's 
comments ignore the point that the RfC is in fact aimed at 
extrapolation to 70 exposure years. Reproductive and developmental 
toxicity are widely recognized as significant public health concerns 
and therefore warrant specific consideration as uncertainties in RfC 
derivation. The available animal studies on reproductive and 
developmental effects almost exclusively involved oral dosing of 
manganese and are inadequate for inhalation RfC purposes. Studies of 
such effects in humans are limited for males and nonexistent for 

    \67\Mergler D.; Huel, G.; Bowler, R.; Iregren, A.; Belanger, S.; 
Baldwin, M.; Tardif, R.; Smargiassi, A.; Martin, L.; (1994) Nervous 
system dysfunction among workers with long-term exposure to 
manganese. Environ. Res. 64: 151-180.

    The potential difference in toxicity potency of Mn3O4 in comparison 
to MnO2 or other compounds of manganese is a significant uncertainty 
because no existing study has directly compared the toxicities of these 
compounds by inhalation exposure. Ethyl's comparison of studies 
involving not only different routes of exposure but significant 
differences in methods, subjects/subject populations, and other key 
features does not constitute an adequate quantitative assessment.
    In summary, the three areas of uncertainty reflected in the 
composite UF of 10 used in deriving the manganese RfC estimates are 
appropriate because they are not adequately addressed by available 
    Ethyl stated that ORD has overlooked evidence from the Canadian 
exposure studies that indicate personal exposure levels of manganese 
are likely to be lower than indicated by the PTEAM Riverside Study. The 
Agency considers Ethyl's exposure assessment based on Canadian data to 
be deficient in several respects. Given the fundamental differences in 
the sampling procedures for the Canadian and Riverside studies, as well 
as other differences in the design and conduct of the studies in 
question, the Agency continues to judge the PTEAM Riverside data to be 
far more useful to exposure assessment purposes that the Canadian data. 
The ``consistency'' in the Canadian data that Ethyl emphasizes is a 
subjective matter that can be considered from more than one 

    \68\For specific discussion related to these issues the reader 
is referred to Air Docket A-93-26, II-A-15.

    Ethyl also disagrees with certain assumptions and features of ORD's 
draft exposure assessment. In particular, Ethyl claims that ORD's draft 
``PTEAM Riverside exposure model'' overestimates manganese exposure 
levels for selected Canadian cities by a factor of approximately three 
when used with a factor for projected increases in MMT usage. Ethyl's 
application of the ``Riverside model'' is incorrect for several 
reasons, including Ethyl's use of ambient monitoring data to estimate 
personal exposure levels (ORD used stationary indoor monitoring data 
collected as part of a personal exposure study). The Agency notes that 
qualitative differences in meteorology and the degree of automobile 
usage provide a more likely basis for the differences between Riverside 
and the Canadian cities cited by Ethyl.
    Ethyl also states that the manganese emission rate from catalyst 
vehicles should be a factor of three lower than that for noncatalyzed 
vehicles, which would lower ORD's projected exposure estimates by 
almost the same factor. After reevaluating the limited data on 
manganese emissions from noncatalyst and catalyst vehicles, ORD has 
concluded that no specific emission rate can be justified by the 
available data and that, consequently, an assumption of equivalent 
emission rates for both types of vehicles is reasonable and 
    Ethyl cited data pertaining to the amount of iron and manganese in 
soil and paved road dust (PRD) in Riverside that indicate the 
percentage of automotive contribution to personal exposure levels of 
PM4 manganese should be lower than the value ORD used. ORD agrees 
that the iron:manganese data for Riverside PRD provide a more 
geographically specific basis for estimating automotive contributions, 
and therefore the projection estimates of personal exposure levels of 
manganese would be slightly lower because of this adjustment.\69\ This 
adjustment in the exposure assessment was incorporated into the final 
assessment as described above in section VI-C.

    \69\This and other adjustments in the exposure assessment are 
incorporate in to the final ORD assessment. For further information 
regarding this matter, see Air Docket A-93-26, II-A-12.

    Ethyl states that Riverside is not representative of other U.S. 
cities, and ORD does not disagree. Nevertheless, ORD believes Riverside 
is reasonably representative of the greater Los Angeles metropolitan 
area. Consequently, the PTEAM Riverside exposure assessment can be 
considered representative for at least 14.5 million people (the 
population of the greater Los Angeles area). To the extent that other 
U.S. cities share some degree of resemblance in meteorology, vehicles 
miles traveled, and possibly other characteristics of relevance to 
automotive manganese levels, the estimated exposure levels for 
Riverside may be pertinent to other locales and portions of locales as 
well. However, ORD can only point to qualitative similarities and is 
unable to make any quantitative statement on the degree of relevance of 
the Riverside data to other U.S. cities.
    In its May 27 submission,\70\ Ethyl has noted that a distribution 
of 24-hour average exposure levels is likely to have higher exposures 
at the upper portion of the distribution than would a longer term 
average for the same population. ORD agrees that this is likely, but 
the lack of adequate exposure data make it impossible to determine what 
the actual distribution would be. Although Ethyl attempts to correct 
the distribution of 24-hour average data to represent a longer term 
average, ORD notes unstated assumptions and possible errors in Ethyl's 
calculations and prefers alternative approaches to estimating such a 
correction to the distribution.\71\

    \70\See Air Docket A-93-26, Category II-D-98.
    \71\For further information regarding this matter, see Air 
Docket A-93-26, Category II-A-15, page 40.

    Ethyl commented that ORD's newly released PTEAM exposure modeling 
shows that no (emphasis in original) portion of the population would 
experience manganese exposures above the 0.1-0.3 g/m3 
range. The Agency believes that Ethyl is mistaken in making this 
conclusion. Statements and graphical information in ORD's risk 
assessment indicate that an estimated 5-10% of the Riverside/Los 
Angeles population would be projected to experience personal exposure 
levels of PM4 manganese at or above 0.1 g/m3. 
Ethyl's statement regarding EPA's exposure assessment would be correct 
only if it were to replace ``0.1-0.3 g/m3 range'' with 
``0.3 g/m3 range''.
    Ethyl commented that the ``ORD risk assessment projects that the 
99th percentile manganese exposure would be about 0.2 g/
m3 or lower, while the 90th to 95th percentiles would experience 
manganese exposures below even 0.1 g/m3.'' On the 
contrary, as shown in the ORD risk assessment, by one estimate, the 
98th percentile exposure level would be approximately 0.2 g/
m3, and both estimates are above 0.1 g/m3 at the 
95th percentile. (See Figure 2, Section VI-C of this Decision.)
    Finally, Ethyl raises issues from comparing ORD's assessment of MMT 
with ORD's assessment of methyl tertiary butyl ether (MTBE). Ethyl 
reduces the official EPA RfC for MTBE by inappropriate uncertainty 
factors, compares the result to both appropriate and inappropriate 
exposure values for MTBE, finds an overlap of these health and exposure 
assessments for MTBE, and concludes that, by comparison, MMT was 
treated too conservatively or erroneously. Basically, because of 
substantial differences between the databases and characteristics of 
MTBE (a volatile organic compound that remains in the body for a matter 
of hours to days) and Mn (a metal that accumulates in the body), 
Ethyl's direct comparisons are not valid.\72\

    \72\For further discussion regarding this matter, the reader is 
referred to Air Docket A-93-26, II-A-15, Section IV.

    Comments from other interested parties regarding potential health 
concerns related to manganese exposure are addressed in ``ORD's 
Response to Public Comments'' found in Docket A-93-26.

VII. Fuels and Fuel Additives Registration and Research Needs

    To more accurately define an RfC for manganese and to more 
accurately predict the distribution of expected manganese exposures 
associated with MMT use, additional research will have to be completed. 
Such research would ultimately lead to a risk assessment which could 
better define the risk associated with MMT use. Furthermore, 
regulations under sections 211(a), (b), and (e) of the Act, set forth a 
procedure under which required health and exposure data must be 
submitted prior to any new use of a fuel or additive. Given the basis 
for today's decision and the purpose of the testing requirements 
imposed by these regulations, the Agency expects that the testing 
required to resolve the concerns upon which today's decision is based 
will also be required under these regulations. The following section 
explains these newly promulgated requirements and how they would apply 
to the use of MMT in unleaded gasoline.
    Under section 211 of the Clean Air Act, certain fuels and additives 
must be registered with EPA as a precondition to introduction into 
commerce. Section 211(a) of the Act authorizes EPA to designate any 
fuel or fuel additive for registration. Upon designation, fuels or 
additives may not be introduced into commerce unless they have been 
registered by EPA in accordance with section 211(b). In 1975, EPA 
issued regulations (40 CFR Part 79) implementing basic registration 
requirements, as stipulated by section 211(b)(1), that required 
applicants to submit certain information, such as commercial 
identifying information, range of concentration, purpose-in-use, and 
chemical composition, in order to register a fuel or fuel additive.
    The 1970 Clean Air Act also gave EPA discretionary authority to 
establish additional registration requirements under section 211(b)(2). 
This section authorized EPA to require fuel and fuel additive 
manufacturers ``to conduct tests to determine potential public health 
effects of such fuel(s) or additive(s) (including but not limited to, 
carcinogenic, teratogenic, or mutagenic effects),'' and to further 
furnish other ``reasonable and necessary'' information to identify fuel 
and fuel additive emissions and determine their effects on vehicular 
emission control performance and on public health and welfare.
    EPA did not exercise its discretionary authority to require testing 
of fuels and fuel additives under section 211(b)(2) when general 
registration regulations were first adopted in 1975. However, in the 
Clean Air Act Amendments of 1977 (Public Law 95-95, August 7, 1977), 
Congress added section 211(e), which required EPA to take certain 
actions to implement section 211(b)(2). A final rule was signed by the 
Administrator of EPA on May 27, 1994 and was published in the Federal 
Register on June 27, 1994 (59 FR 33042). The final rule, ``Fuels and 
Fuels Additives Registration Regulations'', implemented additional 
registration requirements under sections 211(b)(2) and 211(e) of the 
Clean Air Act. The rule requires manufacturers to provide EPA with 
information to assist EPA in identifying and evaluating potential 
adverse health effects of motor vehicle fuel and fuel additive 
emissions, and to support and guide related regulatory actions in the 
    The recently promulgated health effects testing requirements 
incorporate a three-tiered health effects evaluation structure. Under 
Tier 1, fuel and fuel additive manufacturers are required to perform a 
literature search on the health and welfare effects of fuel and fuel 
additive emissions, characterize the emissions, and provide exposure 
information. Tier 2 includes short-term biological testing to screen 
for specific health effects endpoints, involving the exposure of 
laboratory animals to the whole emissions of fuels or fuel/additive 
    Where appropriate, EPA has retained authority to modify or augment 
the standard Tier 2 test requirements. Under ``alternative Tier 2'' 
procedures set forth in Sec. 79.58(c), EPA may substitute alternative 
tests for standard Tier 2 tests or impose additional testing 
requirements. Alternative Tier 2 procedures may be utilized to modify 
standard Tier 2 requirements, but may not be utilized to entirely 
delete testing for any of the standard endpoints. The alternative Tier 
2 provision affords the Agency flexibility when available information 
indicates that another testing regimen is preferable to the standard 
set of Tier 2 tests. Instances where Alternative Tier 2 requirements 
may be appropriate include scenarios where previously available 
information may cause EPA to be concerned about potential health 
effects related to an endpoint not specifically addressed in Tier 2, or 
when otherwise available information identifies a potentially 
significant public health risk related to a Tier 2 endpoint such that 
more definitive testing will be required for this endpoint than is 
ordinarily required. In both of these scenarios, alternative Tier 2 
testing can facilitate earlier and potentially more efficient 
acquisition of the required data.
    After receipt and review of a manufacturer's Tier 1 and Tier 2 
submittals, EPA determines, on a case-by-case basis, if additional 
testing is needed under Tier 3 to evaluate the risk of a particular 
fuel or fuel additive (or group of fuels or fuel additives) on human 
health or welfare. Tier 3 testing could include any emission analysis, 
health effects, welfare effects, and/or exposure testing or analysis 
deemed necessary by EPA for this purpose.
    The Agency has considerable discretion to formulate and impose 
appropriate testing requirements under either alternative Tier 2 or 
Tier 3 procedures. In practice, EPA will be more likely to utilize 
alternative Tier 2 procedures in instances where additional data needs 
are apparent even prior to completion and submission of the standard 
Tier 2 tests, and prompt formulation of appropriate alternative testing 
requirements will result in a more efficient use of industry and 
governmental resources. Tier 3 procedures will be preferred in those 
instances where standard Tier 2 tests are necessary to assist EPA in 
identification of potential hazards and/or in design or selection of 
appropriate follow-up studies.
    For fuels and fuel additives registered as of the date of 
promulgation of the final rule, registrants must submit Tier 1 and Tier 
2 test data within six years of that date. On the other hand, 
manufacturers seeking to register new fuel and fuel additive products 
after the date of promulgation must satisfy all testing requirements 
before registration will be granted.
    In this regard, the new rule clarifies what constitutes a ``new'' 
fuel or fuel additive, distinguishing between two types of unregistered 
products which a manufacturer might seek to register after the 
promulgation of the final rule: (1) fuel and fuel additive products 
similar in composition and usage to those already allowed wide 
commercial distribution (e.g., registered for general use by other 
manufacturers), and (2) fuel and fuel additive products which differ 
significantly in composition and/or usage from such current products. 
To effectuate this distinction, EPA's final rule, promulgated under 
sections 211(b) and (e), makes use of grouping system concepts and 
definitions. Specifically, a fuel additive product not registered by 
its manufacturer\73\ for a specific type of fuel as of the date of 
promulgation of this rule is designated as ``registrable'' if the fuel/
additive mixture resulting from use of the additive in the specific 
fuel is in the same fuel/additive group as one or more currently 
registered fuels or bulk additives.\74\ The grouping system establishes 
various fuel/additive groups within each fuel family.\75\ Conversely, a 
fuel additive product not registered by its manufacturer for a specific 
type of fuel as of the date of promulgation is designated as ``new'' if 
the fuel/additive mixture resulting from use of the additive in the 
specific fuel cannot be grouped with one or more currently registered 
fuels or bulk additives. In these definitions, the term ``currently 
registered'' refers to the date on which the prospective applicant 
seeks registration for the fuel or fuel additive in question.

    \73\For purposes of these definitions, registration is product-
specific. Thus, if a particular fuel or fuel additive product has 
not been registered by its manufacturer, then that manufacturer does 
not have the right to introduce, market, and/or sell this product, 
even if a compositionally similar or identical product has been 
registered by another manufacturer.
    \74\A ``bulk additive,'' sometimes called a ``general use'' 
additive was defined as a product added to fuel at the refinery as 
part of the original blending stream or after the fuel is 
transported from the refinery, but before the fuel is purchased for 
introduction into the fuel tank of a motor vehicle. In contrast, an 
``aftermarket additive,'' sometimes called a consumer additive, is 
an additive product marketed for introduction directly into the fuel 
system of a motor vehicle.
    \75\``Fuel Family'' refers to the primary categorization of 
fuels and fuel additives within the proposed grouping system. A fuel 
family was defined as a set of F/FAs which share basic chemical and 
physical formulation characteristics and can be used in the same 
engine or vehicle. Six such fuel families were proposed (unleaded 
gasoline, leaded gasoline, diesel, methanol, ethanol, methane, and 
propane). EPA did not include a leaded gasoline family in the final 
rule because Clean Air Act section 211(n) prohibits on-road use of 
leaded fuel after December 31, 1995. In the definition of 
``registrable,'' the restriction ``in the same fuel family'' means 
that the similarity of an applicant F/FA to a bulk additive 
currently registered for use in another fuel family would not 
suffice to make the applicant F/FA registrable. This restriction is 
consistent with the general principles of the grouping system, which 
permits grouping of F/FAs only within defined fuel families.

    According to these definitions, an unregistered fuel additive which 
meets the criteria for grouping only with a currently registered 
aftermarket additive (and not also with a currently registered fuel 
and/or bulk additive) is not registrable. This does not preclude an 
unregistered aftermarket additive from being registrable (since 
aftermarket additives can group with fuels and bulk additives), nor 
does it affect the registration status of currently registered 
aftermarket additives.
    For example, an unregistered detergent additive (either bulk or 
aftermarket) intended for use in unleaded gasoline and conforming to 
the ``substantially similar'' criteria for unleaded gasoline (56 FR 
5352) would be registrable, since it would be able to group with 
currently registered baseline unleaded gasoline fuels and bulk 
additives.\76\ On the other hand, MMT is considered ``new'' rather than 
``registrable'' for unleaded gasoline, because there are no currently 
registered manganese-containing fuels or bulk additives in the unleaded 
gasoline family with which a mixture of MMT and unleaded gasoline could 
be grouped. This is true even though products containing MMT are 
currently registered for bulk use in leaded gasoline and as aftermarket 
additives grandfathered prior to the ban on such aftermarket 

    \76\The ability to join the unleaded gasoline baseline group 
assumes that the detergent additive does not exceed oxygen and 
sulfur limits applicable to the baseline unleaded gasoline category.
    \77\Until the 1990 CAA Amendments went into effect, the 
statutory language of section 211(f) was interpreted as applying 
only to unleaded gasoline fuels and related bulk additives. Thus, 
prior to November 15, 1990 (the effective date of the CAA 
Amendments), aftermarket additives intended for use in unleaded 
gasoline and containing elements other than carbon, hydrogen, 
oxygen, nitrogen, and sulfur were allowed to be registered. Under 
the 1990 CAA Amendments, all types of motor vehicle fuels and fuel 
additives were placed under 211(f) jurisdiction. All aftermarket 
additives that were not ``substantially similar'' and were 
introduced on or after November 15, 1990, were banned. However, this 
ban does not apply to products first introduced into commerce prior 
to November 15, 1990 (CAA section 211(f)(1)(B)). Thus, ``non-sub-
sim'' gasoline aftermarket additives which had been registered prior 
to that date were allowed to retain their registrations. These are 
so-called ``grandfathered'' aftermarket additives.

    The recently finalized rule requires that manufacturers of new fuel 
and fuel additive products (i.e., fuel and fuel additive products not 
registered by their specific manufacturers as of the date of 
promulgation and not fitting the registrable criteria) submit all 
testing requirements prior to registration, including any alternative 
Tier 2 tests or Tier 3 tests prescribed by the Agency. As discussed 
above, under the fuel and fuel additive testing rule, MMT is designated 
as ``new'' for purposes of registration for use in unleaded gasoline. 
Accordingly, any data required to register MMT for use in unleaded 
gasoline must be submitted prior to registration.
    As noted above, Tier 1 requires manufacturers of designated fuels 
or fuel additives (or groups of manufacturers pursuant to Sec. 79.56) 
to supply to the Administrator: (1) the identity and concentration of 
certain emission products of such fuel and fuel additives, (2) an 
analysis of potential emissions exposures, and (3) any available 
information regarding the health and welfare effects of the whole and 
specified emissions.
    Under the General Provision of the emission characterization 
requirements of Tier 1, it is stated in Sec. 79.52(b)(1)(ii) that the 
emissions shall be generated three times (on three different days) 
without a functional aftertreatment device and, if applicable, three 
times (on three different days) with a functional aftertreatment 
device, and each such time shall be analyzed according to the remaining 
provisions in this section (b). Measurement of background emissions, 
under Sec. 79.52(b)(1)(iii), states that it is required that ambient/
dilution air be analyzed for levels of background chemical species 
present at the time of emission sampling (for both combustion and 
evaporative emissions) and that background chemical species profiles be 
reported with emissions speciation data. Ethyl has yet to provide the 
Agency with data on the characterization of emissions resulting from 
the use of MMT in unleaded gasoline which meets the requirements of 
Tier 1 under these subsections. Thus, to EPA's knowledge, data has not 
yet been collected which would fully satisfy Tier 1 requirements under 
Sec. 79.52(b)(1) (ii) and (iii).
    Standard Tier 2 testing includes certain specific types of short-
term biological testing to screen for specific health effects 
endpoints, involving the exposure of laboratory animals to the whole 
emissions of the fuel additive (when added to a base fuel). Where 
appropriate, the Agency may impose an alternative Tier 2 test in lieu 
of a standard Tier 2 test or prescribe additional Tier 2 testing in 
addition to the standard Tier 2 tests.
    EPA's Office of Research and Development has prepared a report that 
specifically identifies research which would allow a more accurate 
evaluation of the risk involved in utilizing MMT in unleaded 
gasoline.\78\ Some of the research described in the ORD report is 
intended to address toxicologic endpoints which are also addressed by 
standard Tier 2 tests,\79\ while other research described in the report 
is intended to address other endpoints. EPA presently has an adequate 
basis to conclude that there are public health concerns associated with 
the potential use of MMT in unleaded gasoline, and anticipates that 
research addressing each of the areas identified in the ORD report will 
be necessary to support registration of MMT for use in unleaded 
gasoline. It is not necessary, and would be inefficient, to wait for 
the completion and submission of all of the standard Tier 2 tests in 
order to identify the requisite test data, and use of alternative Tier 
2 procedures is therefore appropriate.

    \78\ORD originally reviewed the information needed to improve 
the risk characterization in: Preuss, P.W. (1991) ORD Document on 
Information Needed to Improve the Risk Characterization of Manganese 
Tetraoxide (Mn3O4) and Methylcyclopentadienyl Manganese 
Tricarbonyl, December 12, 1991 [memorandum to Richard Wilson]. 
Washington, DC: U.S. Environmental Protection Agency, Office of 
Research and Development; December 16, 1991. For further information 
the reader is referred to Air Docket A-93-26, II-A-16. ORD has 
reevaluated these information needs in light of new information. See 
Memo from Peter W. Preuss to Richard Wilson dated July 13, 1994, 
Docket A-93-26, II-A-18.
    \79\For example, it is clear that the standard Tier 2 
neurotoxicity will not address adequately the potential 
neurotoxicity of inhaled manganese to humans. Although the precise 
mechanisms of such neurotoxicity are not fully understood, it has 
been shown that manganese toxicity may be associated with 
degeneration of certain tissues in the brain. These brain tissues 
include the substantia nigra which is found only in primates and not 
in lower mammals. Laboratory rodents typically employed in the 
standard Tier 2 neurotoxcity test do not physically possess this 
target for manganese toxicity in the brain.

    If Ethyl or any other party seeks to register MMT for use in 
unleaded gasoline, it will need to submit the required test data in 
addition to obtaining the required waiver. In that case, EPA expects to 
identify specific alternative Tier 2 tests which must be performed 
prior to registration.\80\ In the case of endpoints addressed by the 
ORD report, that report will serve as the starting point for EPA in 
identifying appropriate alternative Tier 2 tests. In each instance 
where EPA does not adopt any alternative Tier 2 requirement for an 
endpoint which is addressed by standard Tier 2 tests, standard 
information or testing requirements will continue to apply. The process 
for establishment of alternative Tier 2 testing requirements for a 
product includes specific procedures for notification of the 
manufacturer and an opportunity for comment, and is set forth in 40 
C.F.R. 79.58(c).

    \80\If there is any reason why Ethyl would prefer to complete 
the standard set of Tier 2 tests and submit such tests prior to 
imposition of further data requirements, EPA is willing in the 
alternative to impose additional testing requirements under Tier 3 

    Submission of any data which is required by the registration 
regulations prior to registration of MMT for use in unleaded gasoline 
is not itself a specific legal prerequisite to the granting of a waiver 
under section 211(f)(4). However, in these circumstances, since the 
Agency is declining to grant the requested waiver based on potential 
health effects, the Agency considers it reasonable to require that 
Ethyl submit the same health effects and exposure test data which will 
be required prior to registration of MMT for unleaded gasoline before 
it will consider taking favorable action on another waiver application.
    As the health effects testing rule indicates, EPA believes that it 
should exercise particular caution in registering new fuel or fuel 
additive products that are significantly different from, or have a 
usage pattern which is significantly different in scope or character 
from, currently registered fuel or fuel additive products. EPA also 
believes that the same cautious approach is appropriate in evaluating 
waiver applications under section 211(f)(4).
    In the event that Ethyl wishes to undertake the testing required to 
register MMT for unleaded gasoline, EPA will work with Ethyl to resolve 
the details of all necessary test data. Further, once the data 
requirements for registration have been duly established, EPA will not 
require submission of any additional health effects and exposure data 
as part of a new waiver application.

VIII. Other Issues

    Ethyl has stated that certain other considerations and data 
associated with MMT use should be taken into account by the 
Administrator in utilizing the discretion authorized under section 
211(f)(4). These issues include (1) decreased emissions of toxic air 
pollutants due to decreases in aromatic use when MMT is substituted as 
an octane enhancer; (2) decreased refinery emissions due to decreased 
reformer severity; (3) decreased carbon monoxide (CO) and oxides of 
nitrogen (NOX) vehicle emissions, (4) increased crude oil yield 
and associated energy savings, and (5) decreased levels of tropospheric 
ozone associated with lower levels of reactive hydrocarbon emissions 
and NOX emission decreases.
    Regarding CO emission decreases, EPA's analysis indicates that 
examination of all of the available test data on the CO effects of MMT 
shows a small (0.07 gpm or 2% of applicable standards\81\) decrease 
attributable to the additive.\82\ With the exceptions of two vehicle 
models, the CO effects in both directions were relatively small. The 
most significant exception to this general pattern is the test data 
from the 1988 Ford Crown Victoria 5L, which showed a decrease of 0.72 
gpm (or 21% of the standard). The other unusual vehicle was also a Ford 
Crown Victoria--a 1992 model with a 0.36 gpm decrease (11% of 
standard). The small average size and somewhat erratic nature of the CO 
decreases seen in the vehicle sample lead to questions about whether 
this small potential benefit is likely to actually occur in the vehicle 

    \81\This effect was calculated by integrating emissions, 
separately for clear and MMT fuels, over the full range of mileage 
for which data were available, taking the difference between fuel 
groups within models to get a model-by-model effect, and then 
averaging these effects over model groups.
    \82\For a complete description the reader is referred to Air 
Docket A-93-26, II-A-14, ``Evidence of CO and NOX Emission 
Decreases Attibutable to MMT in Durability Testing Data''.

    EPA also examined NOX emissions changes demonstrated by the 
data which had been submitted by both Ethyl and Ford. The test data for 
NOX show a more substantial and more consistent decrease for this 
pollutant than was the case for CO. The average across models was 0.08 
gpm or 8% of the standard. The largest decreases were seen in the 1992 
Ford Mustang 5.0L model and the 1988 Ford Crown Victoria 5L (each 
showing a 0.30 gpm decrease or 30% of the standard). Other models with 
substantial decreases were the 1988 Ford Taurus (0.22 gpm or 22% of the 
standard) and the 1992 Buick Regal 3.8L (0.19 gpm or 19% of the 
    It is difficult to clearly determine the effects which NOX 
decreases of this magnitude might have on tropospheric ozone 
concentrations, since such effects are influenced heavily by the mix of 
ozone precursors and the atmospheric chemistry of particular non-
attainment areas. Modeling studies performed by EPA have shown that 
NOX reductions in NOX limited areas can significantly reduce 
regional ozone levels. However, although the regional ozone decreased, 
some urban areas showed slight increases in ozone levels. It is not 
possible to quantify the degree of reduction in ozone associated with 
the introduction of MMT in general because many factors affect ambient 
ozone. In order to achieve such a quantification, individual cities 
would need to be analyzed to determine their VOC/NOX ratios and 
the mobile source contribution to those ratios.
    EPA has therefore concluded that it is likely that in certain 
NOX-limited areas, ambient levels of ozone would decrease based on 
the expected NOX reductions resulting from MMT use. It is 
impossible to quantify the exact degree of these decreases or the 
magnitude of the area which would be affected.\83\

    \83\For a complete description and response to Ethyl's comments, 
the reader is referred to Air Docket A-93-26, II-A-15, ``ORD's 
Response to Public Comments on the Reference Concentration for 
Manganese and on the Draft ORD Risk Assessment of MMT''.

    Ethyl indicates that ``use of the Additive will result in as many 
as 25 fewer cancer cases [emphasis in the original] attributable to 
automotive exhaust over the 1995-2010 span.'' Although ORD has not 
independently verified Ethyl's current analysis, it is nevertheless 
possible to evaluate their analysis by taking it at face value. 
Significant uncertainties are inherent in any prediction of a net 
change in cancer risk. For example, there are various models and 
assumptions underlying estimated changes in carcinogen exposures. 
Assumptions must be made regarding the composition of gasoline at 
different times in the future; projected fleet emissions must be 
estimated based on measurements from a few vehicles; the impact of MMT 
on such emissions must be estimated. In addition, the relationship of 
changes in emissions is assumed to be related to population exposure 
    Cancer risk, whether characterized in terms of the number of cases 
in the entire U.S. population or in terms of individual unit risk 
estimates, also has inherent uncertainties. According to Ethyl, the 
predicted reduction in estimated cancer cases is 0.6 to 2.0 cases per 
year. These represent upper bound estimates, and thus the true impact 
is very unlikely to be higher and may in fact be less. Ethyl also 
predicts that the changes in the probability of an individual 
contracting cancer is lowered with MMT use from about 12 cases per 
100,000 to 6 cases per 100,000. Again these estimates represent upper 
bounds, so the true risk to an individual would not be expected to be 
higher and could well be lower. In any case, the individual risk level 
remains in the 10-5 range. Relative to the uncertainties in the cancer 
risk estimates as well as in the emission and exposure aspects of 
Ethyl's analysis, the magnitude of the claimed change in risk is 
comparatively quite small.
    The accuracy of these estimates also depends very much on the 
estimates of decreased aromatics use for individual refiners. Very 
significant changes are taking place in the refining industry to comply 
with various aspects of programs being implemented pursuant to the 1990 
Clean Air Act amendments. Programs such as the introduction of 
``reformulated gasoline'' will dramatically change the overall chemical 
and physical properties of gasoline. Thus, estimates of aromatics usage 
associated with refining are even more highly speculative over the next 
several years as these programs are implemented.
    In any case, it is not possible to weigh these predicted changes in 
cancer incidence or in ozone formation against the concerns which have 
been explained above in relation to increased manganese emissions. 
Therefore, the Agency disagrees with Ethyl's claim that it can be shown 
scientifically that a net public health benefit results from the use of 
MMT in unleaded gasoline. While the Agency agrees that such risk-
benefit comparisons can be important, the present data are inadequate 
for this purpose. With regard to the potential risks associated with 
inhalation exposure to manganese, the Agency's Risk Assessment as 
described above indicates that there are insufficient data to conclude 
quantitatively whether the increased use of MMT will (or will not) 
increase public health risk. With regard to the potential reductions in 
cancer incidence and tropospheric ozone levels associated with MMT use 
claimed by Ethyl, the above analysis indicates that, while reductions 
are possible, the data are too sparse to provide reasonable 
quantitative estimates of the magnitude of such reductions. The 
estimated cancer reduction is not truly discernable as a quantifiable 
benefit. This is because relative to the uncertainties in the cancer 
risk estimates as well as in the emission and exposure aspects of 
Ethyl's analysis, the estimated magnitude of change in risk is quite 
small, even when based on the ``upper bound'' cancer estimates. For 
ozone, it is not possible in general to quantify the degree of 
reduction associated with the introduction of MMT because many factors, 
including local volatile organic compound (VOC)/NOX ratios affect 
ambient ozone levels. To achieve a quantification, all appropriate 
individual cities would have to be analyzed.

IX. Decision

    As previously discussed, the Agency interprets section 211(f)(4) of 
the Act as establishing a two-stage process for evaluating waiver 
applications. The first stage requires that EPA determine whether an 
applicant has met its burden of demonstrating that a fuel does not 
cause or contribute to a failure to meet regulated emission standards. 
Unless EPA finds that the waiver applicant has met this burden, a 
waiver may not be granted. The second stage of the process reflects the 
discretionary nature of the waiver authority provided to the EPA 
Administrator by the statute. In this second stage, the Administrator 
may consider other factors in determining whether granting a waiver is 
in the public interest and consistent with the objectives of the Clean 
Air Act. In this stage, the Administrator has broad discretion in 
selecting the issues to be examined and in balancing the potential 
positive and negative impacts of a waiver.
    For purposes of the pending waiver application, as remanded to EPA 
by the D.C. Circuit Court of Appeals and resubmitted by Ethyl, I 
determined on November 30, 1993 that use of Ethyl's product HiTEC 3000 
in unleaded gasoline at the specified concentration will not cause or 
contribute to a failure to achieve compliance with vehicle emission 
standards. The data and analysis upon which this determination was 
based are described in Section IV above. The data submitted by Ethyl in 
connection with this waiver application satisfy all of the quantitative 
criteria for determining whether an additive will ``cause or 
contribute'' to a failure to meet emission standards previously 
utilized by EPA. As noted above, EPA has serious reservations 
concerning the present suitability of these criteria. In light of the 
intractability of the nation's air pollution problems, EPA is 
considering adoption of new criteria which would be utilized in 
assessing future waiver applications. However, for purposes of Ethyl's 
present application, EPA concluded that it would not be appropriate to 
make the required determination concerning emission effects by 
utilizing a new methodology or new criteria concerning which Ethyl was 
not afforded any prior notice. My decision that Ethyl has met the 
requisite statutory burden for purposes of the present waiver 
application is also based on the Agency's assessment of the newest data 
provided by Ethyl on newer-technology vehicles which do not, when 
evaluated separately, indicate any statistically significant increase 
in regulated vehicle emissions.
    The Agency remains concerned about the possible effects of fuels 
containing MMT on the functioning of onboard diagnostics (OBD) 
equipment. At the time that EPA completed the assessment of emissions 
issues underlying my finding concerning emissions on November 30, 1993, 
the evidence to support the assertions by automakers that MMT would 
prevent the oxygen sensors in OBD systems from operating properly was 
quite limited and these assertions had not at that time been tested in 
any actual vehicles utilizing OBD systems. I expressed concern about 
the effect of MMT on OBD systems at that time, and indicated that if 
EPA ultimately concluded that MMT could prevent proper functioning of 
OBD systems, the Agency would consider appropriate action under Clean 
Air Act section 211(c). Since that time, the Ford Motor Company has 
submitted further research involving use of fuel containing MMT in an 
actual vehicle equipped with an OBD system, which Ford contends 
demonstrates that manganese oxides resulting from MMT use prevent the 
oxygen sensors in its OBD systems from functioning properly. EPA 
intends to fully investigate the significance of these findings. If 
further investigation supports the concerns expressed by Ford and other 
automakers, EPA intends to initiate an appropriate rulemaking under 
section 211(c).
    Since I determined that Ethyl had met its burden regarding the 
``cause or contribute'' finding required by the statute, the Agency has 
been reviewing other issues bearing on the exercise of my discretionary 
authority to grant or deny Ethyl the requested waiver for use in 
unleaded gasoline, focusing in particular on the potential effects of 
manganese emissions resulting from use of MMT on public health and 
    The Agency remains concerned regarding the issue of manganese 
emissions from vehicles utilizing fuels containing MMT. From a hazard 
identification perspective, manganese can clearly be toxic to the 
central nervous system, the respiratory system, and the male 
reproductive system. The level of manganese which may safely be 
breathed over a lifetime is not known with precision. EPA has assessed 
potential health risks associated with potential exposures utilizing a 
Reference Concentration (RfC), which is defined as an estimate (with 
uncertainty spanning perhaps an order of magnitude) of a continuous 
inhalation exposure level for the human population (including sensitive 
subpopulations) that is likely to be without appreciable risk of 
deleterious noncancer effects during a lifetime of exposure.
    Beginning with existing data on the effects of manganese exposures, 
the Agency utilized its standard procedure to develop a verified 
Reference Concentration (RfC) of 0.05 g/m\3\. In response to 
comments and criticisms by Ethyl and others, the Agency then utilized a 
variety of additional techniques to consider potential alternative 
RfC's based on additional data from the occupational exposure study on 
which the current RfC is based (discussed in detail in Section VI-B-4 
above). The techniques judged by EPA scientists to be most appropriate 
produced alternative candidate RfC's ranging from 0.09 to 0.2 
g/m\3\. Since any formal revision of the present verified RfC 
of 0.05 g/m\3\ based on the ORD reassessment will occur in the 
future and could not be completed prior to the deadline for this 
decision, I have evaluated the potential health effects associated with 
use of MMT in unleaded gasoline based on all of the likely values, 
focusing in particular on a potential alternative RfC of 0.1 
g/m\3\ and the present RfC of 0.05 g/m\3\.
    Utilizing the newest and most accurate data available on personal 
exposures to particulate emissions of manganese from vehicular sources, 
Agency scientists also prepared an exposure analysis in which they 
attempted to predict the range of increases in personal exposure to 
particulate manganese that would occur in Riverside, California urban 
areas if MMT were to be used in all unleaded gasoline. If MMT were 
utilized in unleaded gasoline at the specified concentration in urban 
areas similar to Riverside, California, the Agency's exposure 
assessment predicts that the exposures of forty to fifty percent of the 
population in such areas to airborne manganese levels would exceed the 
present verified RfC of 0.05 g/m\3\, and the exposures of five 
to ten percent would exceed manganese levels exceeding a potential 
alternative RfC of 0.1 g/m\3\. Although it is impossible to 
state whether a health risk would definitely exist at the projected 
exposure levels, neither can the possibility of such a risk be ruled 
    It is reasonable to anticipate that persons living near major 
thoroughfares and in inner cities, often among the most economically 
disadvantaged Americans, will be disproportionately represented among 
persons with higher manganese exposure. Moreover, there is reason to 
believe certain subpopulations such as the young, the elderly, and 
persons with certain preexisting conditions, such as Parkinson's 
disease, might be more susceptible to any adverse effects of manganese 
    Certain occupational groups who work in proximity to vehicular 
emissions such as toll takers, parking garage attendants, traffic 
policemen, taxi dispatchers, and service station attendants, are 
represented in the Agency's exposure estimates only to the extent of 
their prevalence in the general population. These occupational groups 
would likely experience manganese exposures attributable to MMT use 
near or even exceeding the highest predicted exposures\84\. I realize 
that if Ethyl is granted the requested waiver, MMT would not be 
utilized immediately in all congested urban areas. Some of the urban 
areas with higher vehicular usage are areas which will be required to 
begin using a cleaner gasoline referred to as ``reformulated gasoline'' 
beginning in 1995. If Ethyl were to obtain the waiver under section 
211(f)(4) which is the subject of this decision, MMT could not be used 
in reformulated gasoline unless and until Ethyl obtained an additional 
waiver under Clean Air Act section 211(k)(2)(D). However, I am 
nevertheless concerned about the effects of potential MMT use in areas 
required to utilize reformulated gasoline, both because a waiver under 
section 211(f)(4) of the Act is a necessary prerequisite to the use of 
MMT in either conventional or reformulated gasoline and because Ethyl 
has made it clear that it does not seek the present waiver solely to 
permit use in conventional gasoline.

    \84\The Agency realizes that exposures to manganese deemed 
acceptable in particular industrial or occupational contexts will 
often significantly exceed the levels considered by EPA to pose 
acceptable risks for the general population. The establishment of 
appropriate limits for occupational exposures to manganese is an 
entirely separate process from the establishment of an RfC for the 
general population. Nevertheless, I do not believe it would be 
appropriate to completely disregard the likelihood of substantial 
incremental additions to manganese exposure for certain categories 
of working Americans where such increased exposure would be 
attributable solely to approval of MMT use.

    I have concluded based on the assessments prepared by EPA 
scientists that, if I were to approve use of MMT in unleaded gasoline 
at the specified concentration, a significant number of persons could 
thereby be exposed to manganese concentrations in the ambient air which 
approach or exceed the current RfC or the candidate RfCs described in 
the risk assessment. Although all risk assessments have some degree of 
uncertainty, in some cases it is reasonable to conclude that the risk 
of adverse health effects is either very great or very small because 
estimated exposure levels are either far above or far below a potential 
health effect level. However, this is not the case with MMT.
    Although it is not possible based on the present information to 
conclude whether specific adverse health effects will be associated 
with manganese exposures in the vicinity of or exceeding the RfC, 
neither is it possible to conclude that adverse health effects will not 
be associated with such exposures. Moreover, it is likely that, if 
adverse effects do occur as a result of MMT usage, such effects will be 
subtle and difficult to detect. In these circumstances, I am very 
reluctant to conduct a massive experiment in which the citizens of 
numerous American cities are subjected to the additional exposures to 
particulate manganese associated with MMT use.
    I am aware of the proposal by Ethyl that EPA conditionally grant 
its waiver application for HiTEC 3000, with conditions which would 
require Ethyl to develop in a specified period the additional health 
effects and exposure data necessary to address present uncertainties in 
the risk assessment, require Ethyl to conduct ambient monitoring of 
particulate manganese levels in certain cities where unleaded gasoline 
containing MMT would be sold, and provide for prompt withdrawal of 
HiTEC 3000 from the market in the event that ambient monitoring were to 
demonstrate airborne manganese levels exceeding a specified level of 
concern (e.g. 0.1 g/m\3\). However, I believe that the 
additional information on health effects and exposure necessary to 
provide greater assurance that manganese emissions from MMT use will 
not jeopardize public health should be provided before I decide whether 
to expose Americans to such emissions on even a temporary basis. 
Moreover, Ethyl's proposal to monitor ambient exposure levels would not 
assure that personal exposures exceeding the specified threshold do not 
occur. The EPA risk assessment makes it clear that a substantial 
portion of the population exposed to airborne manganese as a result of 
MMT use would be expected to experience personal exposures exceeding 
measured ambient exposure levels.
    I recognize that there are some benefits that will be likely to 
accrue from approval of MMT use. In addition to the obvious economic 
benefits associated with reductions in petroleum use and in fuel 
prices, there might also be some favorable health and environmental 
effects. It is probable that, if MMT use were to result in reduced 
NOX emissions from motor vehicles, this would be accompanied by 
some site-specific decreases in ozone formation. The small decreases in 
cancer risks claimed by Ethyl are considerably more speculative and 
cannot be quantified with existing information. In any case, it is not 
possible for me to weigh any modest hypothetical decrease in cancer 
risk or limited site-specific ozone benefit against a risk of effects 
on the nervous system that could significantly decrease the quality of 
life for a large number of individuals. In addition, EPA is or will be 
taking a number of other actions under the Clean Air Act to reduce 
emissions of carcinogenic air pollutants and to limit ozone 

    \85\For example, reductions in levels of carcinogenic 
particulates will accrue due to controls placed on diesel sulfur (55 
FR 34120, August 21, 1990). Reductions in carcinogenic toxic 
emissions (and NOX emissions after the year 2000) will accrue 
as a result of regulations requiring the introduction of 
reformulated gasoline in 1995 promulgated under section 211(k) of 
the Act as amended in 1990 (59 FR 7716, February 16, 1994) and as a 
result of the Agency's vehicle-based refueling emissions regulations 
(59 FR 16262, April 6, 1994). EPA has already issued tighter 
tailpipe standard for NOX as required under the new amendments, 
and future ozone state implementation plans will address other 
NOX control strategies. In addition, the recently-promulgated 
enhanced emission I/M program regulations for the first time 
establish a performance standard to reduce in-use NOX emissions 
in the more serious ozone nonattainment areas. Regarding stationary 
source control strategies, other provisions of the Act call for a 
two million ton NOX reduction from certain utilities.

    Based on the EPA analyses described in this decision and on the 
administrative record compiled by EPA as part of this decision, I have 
concluded that there is a reasonable basis for concern about the 
effects on public health that could result if EPA were to approve use 
of MMT in unleaded gasoline pursuant to Ethyl's application. I have 
also concluded that the actual and hypothetical benefits which could 
accrue from MMT use are insufficient to outweigh the concerns regarding 
potential adverse health effects. In circumstances where there is 
substantial uncertainty regarding the nature of potential health 
effects which would result if a fuel additive waiver were approved, the 
burden of resolving such uncertainties should fall on the waiver 
applicant rather than on the public. This policy is consistent with the 
general policy established by the recently promulgated health effects 
testing rule, which requires that manufacturers of new fuels or fuel 
additives complete all necessary testing prior to registration.
    Therefore, for the reasons set forth above, utilizing the 
discretion afforded me under section 211(f)(4) of the Act, I am today 
denying Ethyl's request for a waiver of the section 211(f)(1) 
prohibitions against the use of MMT in unleaded gasoline. As previously 
stated in this decision, should Ethyl wish to undertake research 
intended to alleviate the present uncertainties concerning the adverse 
health effects which could be associated with the use of MMT in 
unleaded gasoline, the Agency will work with Ethyl to resolve the 
details of the needed research. If Ethyl decides to undertake the 
research which is needed, and which would likely be required in any 
case to register MMT for unleaded gasoline under the Agency's recently 
promulgated fuel and fuel additive health effects testing regulations, 
EPA will reconsider granting a waiver to Ethyl at that time.
    EPA has determined that this action does not meet any of the 
criteria for classification as a significant regulatory action under 
Executive Order 12866. Therefore, no regulatory impact analysis is 
required. This action is not a ``rule'' as defined in the Regulatory 
Flexibility Act, 5 U.S.C. 601 et seq., because EPA has not published, 
and is not required to publish, a Notice of Proposed Rulemaking under 
the Administrative Procedure Act, 5 U.S.C. 553(b), or any other law. 
Therefore, EPA has not prepared a supporting regulatory flexibility 
analysis addressing the impact of this action on small entities.
    This is a final Agency action of national applicability. 
Jurisdiction to review this action lies exclusively in the U.S. Court 
of Appeals for the District of Columbia Circuit. Under section 
307(b)(1) of the Act, judicial review of this action is available only 
by the filing of a petition for review in the U.S. Court of Appeals for 
the District of Columbia Circuit within 60 days of August 17, 1994. 
Under section 307(b)(2) of the Act, today's action may not be 
challenged later in a separate judicial proceeding brought by the 
Agency to enforce the statutory prohibitions.

    Dated: July 13, 1994.
Carol M. Browner,
[FR Doc. 94-18941 Filed 8-16-94; 8:45 am]