[Federal Register Volume 60, Number 196 (Wednesday, October 11, 1995)]
[Proposed Rules]
[Pages 52874-52889]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-25179]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[AD-FRL-5313-4]
RIN 2060-AC06
National Ambient Air Quality Standards for Nitrogen Dioxide:
Proposed Decision
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed decision.
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SUMMARY: The level for both the existing primary and secondary national
ambient air quality standards (NAAQS) for nitrogen dioxide (NO2)
is 0.053 parts per million (ppm) (100 micrograms per meter cubed
(g/m3)) annual arithmetic average. In accordance with the
provisions of sections 108 and 109 of the Clean Air Act (Act), as
amended, the EPA has conducted a review of the criteria upon which the
existing NAAQS for NO2 are based. The revised
[[Page 52875]]
criteria are being published simultaneously with the issuance of this
proposed decision. After evaluating the revised health and welfare
criteria, under section 109(d)(1) of the Act, the Administrator has
determined that it is not appropriate to propose any revisions to the
primary and secondary NAAQS for NO2 at this time.
DATES: Comments. Written comments on this proposal must be received on
or before January 9, 1996.
Public Hearing. Persons wishing to present oral testimony
pertaining to this proposal should contact EPA at the address below by
October 26, 1995. If anyone contacts EPA requesting to speak at a
public hearing, a separate notice will be published announcing the
date, time, and place where the hearing will be held.
ADDRESSES: Comments on this proposed action should be sent in duplicate
to: U.S. Environmental Protection Agency, Air and Radiation Docket and
Information Center (6102), Room M-1500, 401 M Street, SW, Washington,
DC 20460, ATTN: Docket No. A-93-06. The docket, which contains
materials relevant to this proposed decision, is available for public
inspection and copying (a reasonable fee may be charged) weekdays
between 8:00 a.m. and 5:30 p.m. in the Central Docket Section (CDS) of
EPA, South Conference Center, Room M-1500, telephone (202) 260-7548.
Public Hearing. Persons wishing to present oral testimony
pertaining to this proposal should notify Ms. Chebryll C. Edwards, U.S.
Environmental Protection Agency, Office of Air Quality Planning
Standards, Air Quality Strategies and Standards Division, Health
Effects and Standards Group (MD-15), Research Triangle Park, NC 27711,
telephone number (919) 541-5428.
FOR FURTHER INFORMATION CONTACT: Ms. Chebryll C. Edwards, U.S.
Environmental Protection Agency, Office of Air Quality Planning and
Standards, Air Quality Strategies and Standards Division (MD-15),
Research Triangle Park, NC 27711, telephone (919) 541-5428.
SUPPLEMENTARY INFORMATION: Availability of Related Information. The
revised criteria document, ``Air Quality Criteria for Oxides of
Nitrogen'' (three volumes, EPA-600/8-91/049aF-cF, August 1993: Volume
I, NTIS #PB95124533, $52.00; Volume II, NTIS #PB124525, $77.00; Volume
III, NTIS #PB95124517, $77.00), and the final revised OAQPS Staff
Paper, ``Review of the National Ambient Air Quality Standards for
Nitrogen Oxides: Assessment of Scientific and Technical Information,''
(EPA-452/R-95-005, September 1995) are available from: U.S. Department
of Commerce, National Technical Information Service, 5285 Port Royal
Road, Springfield, Virginia 22161, or call 1-800-553-6847 (a handling
charge will be added to each order). Other documents generated in
connection with this standard review, such as air quality analyses and
relevant scientific literature, are available in the EPA docket
identified above.
The contents of this action are listed in the following outline:
I. Background
A. Legislative Requirements
1. The Standards
2. Related Control Requirements
B. Existing Standards for Nitrogen Dioxide
C. Review of Air Quality Criteria and Standards for Oxides of
Nitrogen
D. Decision Docket
E. Litigation
II. Rationale for Proposed Decision
A. The Primary Standard
1. Basis for the Existing Standard
2. Proposed Decision on the Primary Standard
a. Sensitive Populations Affected
b. Health Effects of Concern
c. Air Quality Considerations
d. Proposed Decision on the Primary Standard
B. The Secondary Standard
1. Direct Effects of Nitrogen Dioxide
a. Vegetation
b. Materials
c. Conclusions Concerning Direct Effects on Vegetation and
Materials
d. Other Related Effects of Nitrogen Dioxide
2. Nitrogen Deposition
a. Terrestrial/Wetland
b. Aquatic
3. Direct Toxic Effects of Ammonia Deposition to Aquatic Systems
4. Proposed Decision on the Secondary Standard
III. Miscellaneous
A. Executive Order 12866
B. Regulatory Flexibility Analysis
C. Impact on Reporting Requirements
D. Unfunded Mandates Reform Act
I. Background
A. Legislative Requirements
1. The Standards
Two sections of the Act govern the establishment and revision of
NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify pollutants which ``may reasonably be anticipated to endanger
public health and welfare'' and to issue air quality criteria for them.
These air quality criteria are to ``accurately reflect the latest
scientific knowledge useful in indicating the kind and extent of all
identifiable effects on public health or welfare which may be expected
from the presence of [a] pollutant in the ambient air * * *.''
Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants
identified under section 108. Section 109(b)(1) defines a primary
standard as one ``the attainment and maintenance of which, in the
judgment of the Administrator, based on the criteria and allowing an
adequate margin of safety, (is) requisite to protect the public
health.'' A secondary standard, as defined in section 109(b)(2), must
``specify a level of air quality the attainment and maintenance of
which, in the judgment of the Administrator, based on (the) criteria,
is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of (the)
pollutant in the ambient air.'' Welfare effects as defined in section
302(h) (42 U.S.C. 7602(h)) include, but are not limited to, ``effects
on soils, water, crops, vegetation, manmade materials, animals,
wildlife, weather, visibility and climate, damage to and deterioration
of property, and hazards to transportation, as well as effects on
economic values and on personal comfort and well-being.''
The U.S. Court of Appeals for the District of Columbia Circuit has
held that the requirement for an adequate margin of safety for primary
standards was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified (Lead Industries Association v. EPA, 647 F.2d 1130, 1154
(D.C. Cir. 1980), cert. denied, 101 S. Ct. 621 (1980); American
Petroleum Institute v. Costle, 665 F.2d 1176, 1177 (D.C. Cir. 1981),
cert. denied, 102 S. Ct. 1737 (1982)). Both kinds of uncertainties are
components of the risk associated with pollution at levels below those
at which human health effects can be said to occur with reasonable
scientific certainty. Thus, by selecting primary standards that provide
an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree.
In selecting a margin of safety, the EPA considers such factors as
the nature and severity of the health effects involved, the size of the
sensitive population(s) at risk, and the kind and degree of the
uncertainties that must be addressed. Given that the ``margin of
safety'' requirement by definition only
[[Page 52876]]
comes into play where no conclusive showing of adverse effects exists,
such factors, which involve unknown or only partially quantified risks,
have their inherent limits as guides to action. The selection of any
numerical value to provide an adequate margin of safety is a policy
choice left specifically to the Administrator's judgment (Lead
Industries Association v. EPA, supra, 647 F.2d at 1161-62).
Section 109(d)(1) of the Act requires that ``not later than
December 31, 1980, and at 5-year intervals thereafter, the
Administrator shall complete a thorough review of the criteria
published under section 108 and the national ambient air quality
standards * * * and shall make such revisions in such criteria and
standards * * * as may be appropriate * * *.'' Section 109(d)(2) (A)
and (B) requires that a scientific review committee be appointed and
provides that the committee ``shall complete a review of the criteria *
* * and the national primary and secondary ambient air quality
standards * * * and shall recommend to the Administrator any * * *
revisions of existing criteria and standards as may be appropriate * *
*.''
The process by which the EPA has reviewed the existing air quality
criteria and standards for NO2 under section 109(d) is described
later in this notice.
2. Related Control Requirements
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards. Under title I of the Act
(42 U.S.C. 7410), States are to submit, for EPA approval, State
implementation plans (SIP's) that provide for the attainment and
maintenance of such standards through control programs directed to
sources of the pollutants involved. The States, in conjunction with the
EPA, also administer the prevention of significant deterioration
program (42 U.S.C. 7470-7479) for these pollutants. In addition,
Federal programs provide for nationwide reductions in emissions of
these and other air pollutants through the Federal Motor Vehicle
Control Program under title II of the Act (42 U.S.C. 7521-7574), which
involves controls for automobile, truck, bus, motorcycle, and aircraft
emissions; the new source performance standards under section 111 (42
U.S.C. 7411); and the national emission standards for hazardous air
pollutants under section 112 (42 U.S.C. 7412).
B. Existing Standards for Nitrogen Dioxide
The principal focus of this standard review is the health and
welfare effects associated with exposure to NO2 and other oxides
of nitrogen. Nitrogen dioxide is a brownish, highly reactive gas which
is formed in the ambient air through the oxidation of nitric oxide
(NO). Nitrogen oxides (NOX), the term used to describe the sum of
NO and NO2, play a major role in the formation of ozone in the
atmosphere through a complex series of reactions with volatile organic
compounds. A variety of NOX compounds and their transformation
products occur both naturally and as a result of human activities.
Anthropogenic (i.e., man-made) sources of NOX emissions account
for a large majority of all nitrogen inputs to the environment. The
major sources of anthropogenic NOX emissions are mobile sources
and electric utilities. Ammonia and other nitrogen compounds produced
naturally do play a role in the cycling of nitrogen through the
ecosystem.
At elevated concentrations, NO2 can adversely affect human
health, vegetation, materials, and visibility. Nitrogen oxide compounds
also contribute to increased rates of acidic deposition. Typical peak
annual average ambient concentrations of NO2 range from 0.007 to
0.061 ppm (``Air Quality Criteria for Oxides of Nitrogen,'' (Criteria
Document or CD), U.S. EPA, 1993, p. 7-10). The highest hourly NO2
average concentrations range from 0.04 to 0.54 ppm (CD, 1993, p. 7-10).
Currently, all areas of the U.S., including Los Angeles (which is the
only area to record violations in the last decade), are in attainment
of the annual NO2 NAAQS of 0.053 ppm. The origins, concentrations,
and effects of NO2 are discussed in detail in the ``Review of
National Ambient Air Quality Standards for Nitrogen Dioxide: Assessment
of Scientific and Technical Information,'' (Staff Paper or SP) (SP,
U.S. EPA, 1995) and in the revised Criteria Document (CD, 1993).
On April 30, 1971, under section 109 of the Act, EPA promulgated
identical primary and secondary NAAQS for NO2 at 0.053 ppm annual
average (36 FR 8186). The scientific and medical bases for these
standards are contained in the original criteria document, ``Air
Quality Criteria for Nitrogen Oxides,'' (CD, 1971).
On December 12, 1978 (43 FR 58117), the EPA announced the first
review and update of the 1971 NO2 criteria in accordance with
section 109(d)(1) of the Act as amended. In preparing the Air Quality
Criteria Document, the EPA provided a number of opportunities for
external review and comment. The Clean Air Scientific Advisory
Committee (CASAC) of the EPA Science Advisory Board held meetings in
1979 and 1980 before providing written closure on the revised criteria
document in June 1981 (Friedlander, 1981). This process resulted in the
production of the revised 1982 document, ``Air Quality Criteria for
Oxides of Nitrogen'' (U.S. EPA, 1982a).
A staff paper, which identified critical issues and summarized
staff interpretation of key studies, received verbal closure at a CASAC
meeting in November 1981 and formal written closure in July 1982
(Friedlander, 1982). In the Staff Paper (U.S. EPA, 1982), staff
recommended that the Administrator select an annual standard ``at some
level between 0.05 ppm and 0.08 ppm.'' Based on the analysis of the
criteria, staff concluded that choosing an annual standard within this
range would ``provide a reasonable level of protection against
potential short-term peaks.''
On February 23, 1984, the EPA proposed to retain both the annual
primary and secondary standards at 0.053 ppm annual average and to
defer action on the possible need for a separate short-term primary
standard until further research on health effects of acute exposures to
NO2 could be conducted (49 FR 6866). The CASAC met to consider the
Agency's proposal on July 19-20, 1984. In an October 18, 1984 closure
letter based on weight of evidence, CASAC concurred with the Agency's
recommendation to retain the annual average primary and secondary
standards at 0.053 ppm (Lippmann, 1984). The CASAC further concluded
that, ``while short-term effects from nitrogen dioxide are documented
in the scientific literature, the available information was
insufficient to provide an adequate scientific basis for establishing
any specific short-term standard * * *.'' After taking into account
public comments, the final decision to retain the NAAQS for NO2
was published by EPA in the Federal Register on June 19, 1985 (50 FR
25532).
C. Review of Air Quality Criteria and Standards for Oxides of Nitrogen
On July 22, 1987, in response to requirements of section 109(d) of
the Act, the EPA announced that it was undertaking plans to revise the
1982 Air Quality Criteria Document for Oxides of Nitrogen (52 FR
27580). The EPA held public workshops in July 1990 to evaluate the
scientific data being considered for integration into the CD.
[[Page 52877]]
In November 1991, the EPA released the revised CD for public review and
comment (56 FR 59285).
The revised CD provides a comprehensive assessment of the available
scientific and technical information on health and welfare effects
associated with NO2 and NOX. The CASAC reviewed the CD at a
meeting held on July 1, 1993 and concluded in a closure letter to the
Administrator that the CD ``* * * provides a scientifically balanced
and defensible summary of current knowledge of the effects of this
pollutant and provides an adequate basis for EPA to make a decision as
to the appropriate NAAQS for NO2'' (Wolff, 1993).
In the summer of 1995, the Office of Air Quality Planning and
Standards (OAQPS) finalized the document entitled, ``Review of the
National Ambient Air Quality Standards for Nitrogen Dioxide: Assessment
of Scientific and Technical Information,'' (SP, U.S. EPA, 1995). The
Staff Paper summarizes and integrates the key studies and scientific
evidence contained in the revised CD and identifies the critical
elements to be considered in the review of the NO2 NAAQS.
The Staff Paper received external review at a December 12, 1994
CASAC meeting. The CASAC comments and recommendations were reviewed by
EPA staff and incorporated into the final draft of the Staff Paper as
appropriate. The CASAC reviewed the final draft of the Staff Paper in
June 1995 and responded by written closure letter (see docket A-93-06).
D. Decision Docket
In 1993, the EPA created a docket (Docket No. A-93-06) for this
proposed decision. This docket incorporates by reference a separate
docket established for the criteria document revision (Docket No. ECAO-
CD-86-082).
E. Litigation
On July 21, 1993, the Oregon Natural Resources Council and Jan
Nelson filed suit under section 304 of the Act to compel the EPA to
complete its periodic review of the criteria and standards for NO2
under section 109(d)(1) of the Act (Oregon Natural Resources Council v.
Carol M. Browner, No. 91-6529-HO (D.Or.)). The plaintiffs and the EPA
agreed to a consent decree establishing a schedule for review of the
NO2 NAAQS, which was subsequently modified pursuant to a further
agreement between the parties. The U.S. District Court for the District
of Oregon entered an order on February 8, 1995 requiring the EPA
Administrator to publish a Federal Register notice announcing her
decision on whether or not to propose any modification of the NAAQS for
NO2 by October 2, 1995. The order also requires the Administrator
to sign a notice to be published in the Federal Register announcing the
final decision whether or not to modify the NO2 NAAQS by October
1, 1996.
II. Rationale for Proposed Decision
A. The Primary Standard
1. Basis for the Existing Standard
The current primary NAAQS for NO2 is 0.053 ppm (100
g/m\3\), averaged over 1 year. In selecting the level for the
current standard, the Administrator made judgments regarding the lowest
reported effect levels, sensitive populations, nature and severity of
health effects, and margin of safety. After assessing the evidence, the
Administrator concluded that the annual standard of 0.053 ppm
adequately protected against adverse health effects associated with
long-term exposures and provided some measure of protection against
possible short-term health effects. The June 19, 1985 Federal Register
notice (50 FR 25532) provides a detailed discussion of the bases for
the existing standard.
2. Proposed Decision on the Primary Standard
The Administrator has determined that it is not appropriate to
propose any revisions of the existing NO2 primary standard at this
time. In reaching this proposed decision, the Administrator has
carefully considered the health effects information contained in the
1993 CD, the 1995 Staff Paper, and the advice and recommendations of
the CASAC as presented both in discussion of these documents at public
meetings and in its 1995 closure letter (see docket A-93-06).
The EPA staff identified several factors that the Administrator
should consider in reaching a decision on whether or not to revise the
current primary standard to protect against exposures to NO2.
These factors include: the sensitive populations affected by nitrogen
dioxides, the nature and severity of the health effects, and the
protection afforded by the current standards.
a. Sensitive Populations Affected. Two general groups in the
population may be more susceptible to the effects of NO2 exposure
than other individuals. These groups include persons with pre-existing
respiratory disease and children 5 to 12 years old (SP, 1995, p. 39).
Individuals in these groups appear to be affected by lower levels of
NO2 than individuals in the rest of the population.
Both the 1993 CD and the 1995 Staff Paper support the hypothesis
that those with pre-existing respiratory disease have an enhanced
susceptibility from exposure to NO2. Since these individuals live
with reduced ventilatory reserves, any reductions in pulmonary function
caused by exposure to NO2 have the potential to further compromise
their ventilatory capacity. Compared to healthy individuals with normal
ventilatory reserves who may not notice small reductions in lung
function, those with pre-existing respiratory disease may be prevented
from continuing normal activity following exposure to NO2.
Asthmatic individuals are considered one of the subpopulations most
responsive to NO2 exposure (CD, 1993, p. 16-1). The National
Institutes of Health (1991) estimates that approximately 10 million
asthmatics live in the U.S. Because asthmatics tend to be much more
sensitive to inhaled bronchoconstrictors than nonasthmatics, there is
the added concern that NO2-induced increase in airway response may
exacerbate already existing hyperresponsiveness caused by pre-exposure
to other inhaled materials.
Patients with chronic obstructive pulmonary disease (COPD)
constitute another subpopulation which is more responsive to NO2
exposure than the average population. This group, which is estimated to
be 14 million in the U.S. (U.S. Department of Health and Human
Services, 1990), includes persons with emphysema and chronic
bronchitis. One of the major concerns for COPD patients is that they do
not have an adequate ventilatory reserve and, therefore, would tend to
be more affected by any additional loss of ventilatory function as may
result from exposure to NO2. The available data also indicate that
NO2 might further damage already impaired host defense mechanisms,
thus putting COPD patients at increased risk for lung infection.
Numerous epidemiological studies conducted in homes with gas stoves
provide evidence that children (5-12 years old) are at increased risk
of respiratory symptoms/illness from exposure to elevated NO2
levels (Melia et al., 1977, 1979, 1983; Ekwo et al., 1983; Ware et al.,
1984; Ogston et al., 1985; Dockery et al., 1989a; Neas et al., 1990,
1991, 1992; Dijkstra et al., 1990; Brunekreef et al., 1989; Samet et
al., 1993). Because childhood respiratory illness is very common (Samet
et al., 1983; Samet and Utell, 1990), any impact which NO2 might
have in
[[Page 52878]]
increasing the probability of respiratory illness in children is a
matter of public health concern. This is particularly true in light of
evidence that recurrent childhood respiratory disease may be a risk
factor for later susceptibility to lung damage (Glezen, 1989; Samet et
al., 1983; Gold et al., 1989). In the U.S., there are approximately 35
million children in the age group 5 to 14 years (Centers for Disease
Control, 1990).
b. Health Effects of Concern. Based on the health effects
information contained in the 1993 CD (which evaluates key studies
published through early 1993) and the 1995 Staff Paper, EPA has
concluded that NO2 is the only nitrogen oxide sufficiently
widespread and commonly found in ambient air at high enough
concentrations to be a matter of public health concern. Exposure to
NO2 is associated with a variety of acute and chronic health
effects. The health effects of most concern at ambient or near-ambient
concentrations of NO2 include changes in airway responsiveness and
pulmonary function in individuals with pre-existing respiratory
illnesses and increases in respiratory illnesses in children (5-12
years old).
The changes in airway responsiveness and pulmonary function are
mostly associated with short-term exposures (e.g., less than 3 hours).
Investigations of long-term exposures of animals to NO2 levels
higher than those found in the ambient air provide evidence for
possible underlying mechanisms of NO2-induced respiratory illness
such as those observed in the indoor epidemiological studies described
below. Furthermore, animal studies have also provided evidence of
emphysema caused by long-term exposures to greater than 8 ppm NO2.
The key evidence regarding these effects is summarized below.
(1) Increase in airway responsiveness. There is little, if any,
convincing evidence that healthy individuals experience increases in
airway responsiveness when exposed to NO2 levels below 1.0 ppm.
However, studies of asthmatics have reported some evidence of increased
airway responsiveness caused by short-term exposures (e.g., less than 3
hours) to NO2 at relatively low concentrations (mostly within the
range of 0.2 to 0.3 ppm NO2) which are of concern in the ambient
environment.
Responsiveness of an individual's airways is typically measured by
evaluating changes in airway resistance or spirometry following
challenge with a pharmacologically-active chemical (e.g., histamine,
methacholine, carbachol), which causes constriction of the airways.
Airway hyperresponsiveness is reflected by an abnormal degree of airway
narrowing caused primarily by airway smooth muscle shortening in
response to nonspecific stimuli. Asthmatics experience airway
hyperresponsiveness to certain chemical and physical stimuli and have
been identified as one of the population subgroups which is most
sensitive to short-term NO2 exposure (CD, 1993, p. 16-1).
Several controlled human exposure studies (Ahmed et al., 1983a,b;
Bylin et al., 1985; Hazucha et al., 1982, 1983; Koenig et al., 1985;
Orehek et al., 1981) of asthmatic individuals showed no significant
effect on responsiveness at very low NO2 concentrations of 0.1 to
0.12 ppm. Folinsbee (1992) analyzed data on asthmatics experimentally-
exposed to NO2 in various studies which used challenges producing
increased airway responsiveness in 96 subjects and decreased airway
responsiveness in 73 subjects. For exposures in the range of 0.2 to 0.3
ppm NO2, he found that the excess increase in airway
responsiveness was attributable to subjects exposed to NO2 at
rest. Because NO2 at these levels does not appear to cause airway
inflammation and the increased airway responsiveness appears fully
reversible, implications of the observed increases in responsiveness
remain unclear. It has been hypothesized that increased nonspecific
airway responsiveness caused by NO2 could lead to increased
responses to a specific antigen; however, there is no plausible
evidence to support this.
(2) Decrease in pulmonary function. Nitrogen dioxide induced
pulmonary function changes in asthmatic individuals have been reported
at low, but not high, NO2 concentrations. For the most part, the
small changes in pulmonary function that have been observed in
asthmatic individuals have occurred at concentrations between 0.2 and
0.5 ppm, but not at much higher concentrations (i.e., up to 4 ppm) (CD,
1993, p. 16-3). In one early study of asthmatics, symptoms of
respiratory discomfort were experienced by 4 of 13 asthmatics exposed
to 0.5 ppm for 2 hours; however, Kerr et al. (1979) concluded that the
symptoms were minimal and did not correlate well with functional
changes. In several other studies of asthmatics, very small changes in
spirometry or plethysmography were reported following acute exposures
in the range of 0.1 (Hazucha et al., 1982, 1983) to 0.6 ppm NO2
(Avol et al., 1988). Hazucha found an 8 percent increase in specific
airway resistance (SRaw) after mild asthmatics were exposed to 0.1
ppm NO2 at rest. However, this finding is not considered
statistically significant. Bauer et al., (1986) reported statistically
significant changes in spirometric response in mild asthmatics exposed
for 20 minutes (with mouthpiece) to 0.3 ppm NO2 and cold air. Avol
et al. (1988) found significant changes in SRaw and 1-second forced
expiratory volume (FEV1) as a function of exposure concentration
and duration for all exposure conditions (i.e., exposure of moderately
exercising asthmatics for 2 hours to 0.3 ppm and 0.6 ppm NO2);
however, it was concluded that there was no significant effect of
NO2 exposure on these measures of pulmonary function (CD, 1993, p.
15-47). Exercising adolescent asthmatics exposed (with mouthpiece) to
air, 0.12 ppm and 0.18 ppm NO2, exhibited small changes in
FEV1, but there were no differences in symptoms between air and
either of the NO2 exposures (Koenig et al., 1987a,b). The absence
of spirometry or plethysmography changes in studies (Avol et al., 1986;
Bylin et al., 1985; Linn et al., 1985b; Linn et al., 1986) conducted at
higher NO2 concentrations makes developing a concentration-
response relationship problematic (CD, 1993, p. 15-62). In assessing
the available data on pulmonary function responses to NO2 in
asthmatic individuals, the CD concludes that the most significant
responses to NO2 that have been observed in asthmatics have
occurred at concentrations between 0.2 and 0.5 ppm (CD, 1993, p. 16-3).
Patients with COPD experience pulmonary function changes with brief
exposure to high concentrations (5 to 8 ppm for 5 minutes) or with more
prolonged exposure to lower concentrations (0.3 ppm for 3.75 hours).
(3) Increased occurrence of respiratory illness among children.
Epidemiological evidence includes a meta-analysis of nine
epidemiological studies of children (5-12 years old) living in homes
with gas stoves. The meta-analysis reported that children (ages 5-12
years) living in homes with gas stoves have an increased risk of about
20 percent for developing respiratory symptoms and disease over
children living in homes without gas stoves. This increase in risk
corresponds to each increase of 0.015 ppm NO2 in estimated 2-week
average NO2 exposure, where mean weekly concentrations in bedrooms
reporting NO2 levels were predominantly between 0.008 and 0.065
ppm NO2 (CD, 1993, p. 14-73). A detailed discussion of the studies
included in the meta-analysis can be found in the 1993 CD as well as in
the 1995 Staff Paper.
[[Page 52879]]
In assessing the potential value of the meta-analysis in developing
the basis for a NAAQS for NO2, the Administrator is mindful of the
limitations of the underlying studies. As discussed in the CD and Staff
Paper, the gas stove studies do not provide sufficient exposure
information, including human activity patterns, to establish whether
the observed health effects are related primarily to peak, repeated
peak, or lower, long-term, average exposures to NO2. Furthermore,
both the staff and CASAC concurred that, absent information on exposure
patterns in the gas stove studies, it is not reasonable to extrapolate
the results of these indoor studies to outdoor exposure regimes (SP,
1995). Indoor exposure patterns to NO2 are quite different
compared to outdoor exposure patterns. With potentially much higher
peaks and average indoor exposures than would be found outdoors, it is
extremely difficult to extrapolate the results of the meta-analysis in
a manner which would provide quantitative estimates of health impacts
for outdoor exposures to NO2 (CD, 1993, p. 16-5).
(4) Biological Plausibility. Animal toxicology studies provide
evidence for possible underlying mechanisms of NO2-induced
respiratory illness. These studies have shown that exposure to NO2
can impair components of the respiratory host defense system and
increase susceptibility to respiratory infection. The increased
respiratory symptoms and illness in children reported in the
epidemiology studies cited above may be a reflection of the increased
susceptibility to respiratory infection caused by the impact of
NO2 on pulmonary defenses. Studies that provide a plausible
biological basis for developing such a hypothesis and that highlight
the potential effects associated with long-term exposures to NO2
are discussed in detail in the 1993 CD and 1995 Staff Paper.
Although the pulmonary immune system has not been adequately
studied to assess the impact of NO2 exposure, there is some
indication that NO2 suppresses some systemic immune responses and
that these responses may be both concentration and time dependent. In
the ambient range of exposures, time may be a more important influence
than concentration. However, there were no data showing clearly the
effect of time on effects of long-term, low-level exposures
representing ambient exposure levels.
In the urban air, the typical pattern of NO2 is a low-level
baseline exposure on which peaks are superimposed. When the
relationship of the peak to baseline exposure and of enhanced
susceptibility to bacterial infection was investigated, the results
indicated that no simplistic concentration times time relationship was
present, and that peaks had a major influence on the outcome (Gardner,
1980; Gardner et al., 1982; Graham et al., 1987). Several other animal
infectivity studies (Miller et al. 1987; Gardner et al., 1982; Graham
et al., 1987) offered evidence which indicated that mice exposed to
baseline plus short-term peaks were more susceptible to respiratory
infection than either those exposed to control or background levels of
NO2. This research also indicated that the pattern of NO2
exposure had a major influence on the response.
The weight of evidence provided by animal toxicology supports the
contention that NO2 impairs the ability of host defense mechanisms
to protect against respiratory infection. Although some of the health
endpoints may not be valid for humans (e.g., increased mortality),
there are many shared mechanisms between animals and humans which
support the hypothesis of association between NO2 exposure and
increases in respiratory symptoms and illness reported in the
epidemiological studies.
Based on the information reviewed in the CD and the Staff Paper, it
is clear that at sufficiently high concentrations of NO2 (i.e., >
8 ppm) for long periods of exposure, NO2 can cause morphologic
lung lesions in animals that meet the criteria for a human model of
emphysema (which requires the presence of alveolar wall destruction in
addition to enlargement of the airspace distal to the terminal
bronchiole). Although current information does not permit
identification of the lowest NO2 levels and exposure periods which
might cause emphysema, it is apparent that levels required to induce
emphysematous lung lesions in animals are far higher than any NO2
levels which have been measured in the ambient air.
c. Air Quality Considerations. One of the factors the Administrator
considered in reaching this proposed decision is the relationship
between short-term exceedances of NO2 concentrations and the
annual NO2 mean. In 1994, McCurdy analyzed air quality data from
the period 1988-1992 to determine the estimated number of exceedances
of various NO2 short-term air quality indicators which would occur
given attainment of a range of annual averages. The annual averages
McCurdy analyzed ranged from 0.02 to 0.06 ppm and included the current
NO2 NAAQS of 0.053 ppm. The 1-hour and daily concentration levels
chosen for analyses were 0.15, 0.20, 0.25, and 0.30 ppm. The results of
this analysis are reported in ``Analysis of High 1 Hr NO2 Values
and Associated Annual Averages Using 1988-1992 Data'' (McCurdy, 1994).
In his report, McCurdy concluded that areas attaining the current
annual NO2 NAAQS reported few, if any, 1 hour or daily exceedances
above 0.15 ppm.
Los Angeles is the only city in the U.S. to record violations of
the annual average NO2 NAAQS during the past decade. However, in
1992, Los Angeles reported air quality measurements which meet the
NO2 NAAQS for the first time. Thus, currently, the entire U.S. is
in attainment of the current NO2 NAAQS.
d. Proposed Decision on the Primary Standard. Based on the
assessment of the health and air quality information presented in the
CD and Staff Paper and discussed above, and taking into account the
advice and recommendations of EPA staff and CASAC, the Administrator
has determined pursuant to section 109(d)(1) of the Act, as amended,
that it is not appropriate to propose any revision of the existing
annual primary standard for NO2 at this time.
In reaching this proposed decision, the Administrator took into
account that the existing standard level is well below those levels
associated with chronic effects observed in animal studies. The current
standard also provides substantial protection against those short-term
peak NO2 concentrations at which clinical studies found
statistically-significant changes in pulmonary function or airway
responsiveness. As part of the review of the primary standard, the
Administrator also considered whether a new short-term standard for
NO2 would be appropriate. Based on the available air quality data,
the Administrator concluded that the existing annual standard provides
adequate protection against potential changes in pulmonary function or
airway responsiveness (which most experts would characterize as mild
responses occurring in the range of 0.2 to 0.5 ppm NO2). The
adequacy of the existing annual standard to protect against potential
pulmonary effects is further supported by the absence of documented
effects in some studies at higher (3 to 4 ppm NO2) concentrations
(SP, 1995, p. 43).
In reviewing the scientific bases for an annual standard, the
Administrator finds that the evidence showing the most serious health
effects associated with long-term exposures (e.g., emphysematous-like
alterations in the lung and increased susceptibility to infection)
comes from animal studies conducted at concentrations well above
[[Page 52880]]
those permitted in the ambient air by the current standard. While
recognizing there is no satisfactory method for quantitatively
extrapolating exposure-response results from these animal studies
directly to humans, the Administrator is concerned that there is some
risk to human health from long-term exposure to elevated NO2
levels given the potential seriousness of the effects in animals.
Other evidence suggesting health effects related to long-term, low-
level exposures, such as the epidemiological studies integrated into
the meta-analysis, provides some qualitative support for concluding
that there is a relationship between long-term human exposure to near-
ambient levels of NO2 and adverse health effects. However, the
various limitations in these studies preclude derivation of
quantitative dose-response relationships for the ambient environment.
The Administrator is mindful that there remains substantial uncertainty
about the actual exposures of subjects in the studies that make up the
meta-analysis. The NO2 levels which were monitored in the gas-
stove studies are only estimates of exposure and do not represent
actual exposures. Because the studies collected 2-week average NO2
measurements, one cannot distinguish between relative contributions to
respiratory symptoms and illness of peak, repeated peak and long-term
average exposure to NO2. In addition, indoor exposure patterns to
NO2 are quite different compared to outdoor exposure patterns.
With potentially much higher peaks and average indoor exposures than
would be found outdoors, it is extremely difficult to extrapolate the
results of the meta-analysis in a manner which would provide
quantitative estimates of health impacts for outdoor exposures to
NO2 (CD, 1993, p. 16-5). Given these limitations, the
Administrator concurs with the EPA staff and CASAC that neither the
meta-analysis nor the underlying studies provide a quantitative basis
for standard setting purposes. In her judgement, they do, however,
provide qualitative support for the retention of the existing standard
which provides protection against both peaks and long-term NO2
exposures.
In reaching this proposed decision, the Administrator also took
into account that the available air quality data indicate that if the
existing standard of 0.053 ppm NO2 is attained, the occurrence of
1-hour NO2 values greater than 0.2 ppm would be unlikely in most
areas of the country (McCurdy, 1994). The Administrator also considered
that all areas of the U.S. are in attainment of the current NO2
NAAQS.
After carefully assessing the available health effects and air
quality information, it is the Administrator's judgment that a 0.053
ppm annual standard would keep annual NO2 concentrations
considerably below the long-term levels for which serious chronic
effects have been observed in animals. Retaining the existing standard
would also provide protection against short-term peak NO2
concentrations at the levels associated with mild changes in pulmonary
function and airway responsiveness observed in controlled human
studies. In reaching this judgment, the Administrator fully considered
the 1995 Staff Paper conclusions with respect to the primary standard
and the views of the CASAC (Wolff, 1995). For the above reasons, the
Administrator has determined, under section 109(d)(1) of the Act, as
amended, that it is not appropriate to propose any revision of the
existing primary standard for NO2 of 0.053 ppm annual average at
this time.
B. The Secondary Standard
Nitrogen dioxide and other nitrogen compounds have been associated
with a wide range of effects on public welfare. The effects associated
with nitrogen deposition include acidification and eutrophication of
aquatic systems, potential changes in the composition and competition
of some species of vegetation in wetland and terrestrial systems, and
visibility impairment. The direct effects of NO2 on vegetation and
materials are also considered. The CD and Staff Paper discuss in detail
the major effects categories of concern; the following discussion draws
from these documents.
1. Direct Effects of Nitrogen Dioxides
a. Vegetation. Data evaluated in the 1993 CD indicate that single
exposures to NO2 for less than 24 hours can produce effects on the
growth, development, or reproduction of plants at concentrations that
greatly exceed the ambient levels of NO2 observed in the U.S. In
experiments of 2 weeks or more, with intermittent exposures of several
hours per day, effects on growth or yield start to appear when the
concentration of NO2 reaches the range of 0.1 to 0.5 ppm,
depending on the species of plant and conditions of exposure (CD, 1993,
p. 9-89).
As reported in the 1993 CD (pp. 9-113 to 9-137), several studies
have examined synergistic or additive effects of NO2 and other air
pollutants on plants. These studies report that NO2 in combination
with other pollutants (i.e., sulfur dioxide, ozone) can increase plant
sensitivity, thus lowering concentration and time of exposure required
to produce injury/growth effects. The pollutant concentrations used in
these experimental studies were well above those observed in the
ambient air and at frequency of co-occurrence that are not typically
found in the U.S. (CD, 1993, p. 9-127).
b. Materials. Nitrogen oxides are known to enhance the fading of
dyes; diminish the strength of fabrics, plastics, and rubber products;
assist the corrosion of metals; and reduce the use-life of electronic
components, paints, and masonry. Compared to studies on sulfur oxides,
however, there is only limited information available quantifying the
effects of nitrogen oxides. While NO2 has been qualitatively
associated with materials damage, it is difficult to distinguish a
single causative agent for observed damage to exposed materials because
many agents, together with a number of environmental stresses, act on a
surface throughout its life.
c. Conclusions Concerning Direct Effects on Vegetation and
Materials. Based on the information assessed in the CD and Staff Paper
and taking into account the advice and recommendations of EPA staff and
CASAC, the Administrator has determined that the existing annual
secondary standard appears to be both adequate and necessary to protect
against the direct effects of NO2 on vegetation and materials, and
that it is not appropriate to propose any modifications of the
secondary standard with respect to such effects. In reaching this
proposed decision, the Administrator considered evidence indicating
that attainment of the existing annual secondary standard provides
substantial protection against both long-term and peak NO2
concentrations which may lead to the direct effects described above.
d. Other Related Effects of Nitrogen Dioxide. While NO2 can
contribute to brown haze, the available scientific evidence indicates
that light scattering by particles is generally the primary cause of
degraded visual air quality and that aerosol optical effects alone can
impart a reddish-brown color to a haze layer. Because of this, the
improvement in visual air quality to be gained by reducing NO2
concentrations is highly uncertain at best. In addition, as discussed
in the 1995 Staff Paper, there is no established relationship between
ground level NO2 concentrations at a given point and visibility
impairment due to a plume or regional haze. These considerations led
both the EPA staff
[[Page 52881]]
and CASAC to conclude that establishment of a secondary NO2
standard to protect visibility would not be appropriate. The
Administrator concurs with those judgments.
While concluding that a secondary NO2 standard is not
appropriate to protect visibility, the Administrator is concerned about
visibility impairment in our national parks and wilderness areas. To
address visible plumes that impact the visual quality of Class I areas,
EPA adopted regulations (under section 165(d) of the Act) in 1980. In
addition, EPA is in the process of developing regional haze regulations
under section 169A of the Act.
2. Nitrogen Deposition
As summarized below, the deposition of nitrogen compounds
contributes to a wide range of environmental problems. As discussed in
detail in the 1993 CD and 1995 Staff Paper, nitrogen compounds effect
terrestrial, wetland, and aquatic ecosystems through direct deposition
or by indirectly altering the complex biogeochemical nitrogen cycle. In
assessing the available effects information evaluated in the CD and
Staff Paper, the Administrator is mindful of the scientific complexity
of nitrogen deposition issues and their broad implications for the
environment.
Nitrogen moves through the biosphere via a complex series of
biologically and non-biologically mediated transformations. The
processes that make up the nitrogen cycle and transform nitrogen as it
moves through an ecosystem include: assimilation, nitrification,
denitrification, nitrogen fixation, and mineralization. Similar types
of transformations can be found in diverse habitats, but the organisms
responsible for the transformations and the rates of the
transformations themselves can vary greatly.
Atmospheric deposition of nitrogen can disturb the nitrogen cycle
and result in the acidification of soils, lakes, and streams. It can
also lead to the eutrophication of sensitive estuarine ecosystems by
changing vegetation composition and affecting nutrient balance. Because
a great degree of diversity exists among ecosystem types, as well as in
the mechanisms by which these systems assimilate nitrogen inputs, the
time to nitrogen saturation (i.e., nitrogen input in excess of total
combined plant and microbial nutritional demands) will vary from one
system or site to another. As a consequence, the relationship between
nitrogen deposition rates and their potential environmental impact is
to a large degree site or regionally-specific and may vary considerably
over broader geographical areas or from one system to another because
of the amount, form, and timing of nitrogen deposition, forest type and
status, soil types and status, the character of the receiving
waterbodies, the history of land management and disturbances across the
watersheds and regions, and exposure to other pollutants. Absent better
quantification of these factors, it is difficult to link specific
nitrogen deposition rates with observed environmental effects,
particularly at the national level.
a. Terrestrial/Wetland. The principal effects on soils and
vegetation associated with excess nitrogen inputs include: (1) Soil
acidification and mobilization of aluminum, (2) increase in plant
susceptibility to natural stresses, and (3) modification of inter-plant
competition. Atmospheric deposition of nitrogen can accelerate the
acidification of soils and increase aluminum mobilization if the total
supply of nitrogen to the system (including deposition and internal
supply) exceeds plant and microbial demand. However, the levels of
nitrogen input necessary to produce measurable soil acidification are
quite high. As reported in the Criteria Document (Tamm and Popovic,
1974; Van Miegroet and Cole, 1984), it is estimated that nitrogen
inputs ranging from 50 to 3,900 kilograms per hectare (kg/ha) for 50
and 10 years respectively, would be required to affect a change in soil
potential for hydrogen (pH) of 0.5 pH units. At present, nitrogen
deposition has not been directly associated with the acidification of
soils in the U.S. The potential exists, however, if additions are high
enough for sufficiently long periods of time, particularly in areas
where soils have low buffering capacity. Mobilization of aluminum can
be toxic to plants and, if transported to waterways, can be toxic to
various aquatic species (SP, 1995, pp. 64,65).
Several studies evaluated in the CD and Staff Paper examined the
effects of nitrogen deposition on forest species sensitivity to
drought, cold, or insect attack. While some studies (Margolis and
Waring, 1986; De Temmerman et al., 1988; Waring and Pitman, 1985;
White, 1984) report that increased nitrogen deposition can alter tree
susceptibility to frost damage, insect and disease attack, and plant
community structure, other studies (Klein and Perkins, 1987; Van Dijk
et al., 1990) did not. For example, Margolis and Waring showed that
fertilization of Douglas fir with nitrogen could lengthen the growing
season to the point where frost damage became a problem. However, Klein
and Perkins presented other evidence that showed no additional winter
injury of high elevation conifer forests when fertilized with 40
kilogram total nitrogen/ha/year. On the other hand, De Temmerman et al.
provided data showing increased fungal outbreaks and frost damage on
several pine species exposed to very high ammonia deposition rates (>
350 kg/ha/year). Numbers of species and fruiting bodies of fungi have
also increased concomitantly with nitrogen deposition in Dutch forests
(Van Breeman and Van Dijk, 1988). The CD evaluated a number of other
studies which also gave mixed results as to the impact of excessive
inputs of nitrogen into forest ecosystems (CD, 1993, pp. 10-92,93).
Climate is thought to play a major role in the severe red spruce
decline in the Northeastern U.S., perhaps with some additional
exacerbation due to the direct effects of acid mist on foliage (Johnson
et al., 1992). There is also some evidence that suggests that indirect
effects of nitrogen saturation, namely nitrate and aluminum leaching,
may be contributing factors to red spruce decline in the Southern
Appalachians (CD, 1993, p. 10-74).
In wetland ecosystems, primary biomass production is most commonly
limited by the availability of nitrogen. Several fertilization studies
have reported that nitrogen application can result in changes in
species composition or dominance in wetland systems. Vermeer (1986)
found that in fen and wet grassland communities, grasses tended to
increase in dominance over other species. Jefferies and Perkins (1977)
also found a species-specific change in stem density at a Norfolk,
England, salt marsh after fertilizing monthly with 610 kg NO3
nitrogen/ha/year or 680 kg NH4+ nitrogen/ha/year over a period of
3 to 4 years.
Long-term studies (greater than 3 years) of increased nitrogen
loadings to wetland systems have reported that increases in primary
production can result in changes in species composition and succession
(U.S. EPA, 1993, pp. 10-120-121). Changes in species composition may
occur from increased evapotranspiration (Howes et al., 1986; Logofet
and Alexander, 1984) leading to a changed water regime that favors
different species or from increased nutrient loss from the system
through incorporation into or leaching from aboveground vegetation. In
parts of Europe, historical data seem to implicate pollutant nitrogen
in altering the competitive relationships among plants and threatening
wetland species adapted to habitats of low fertility
[[Page 52882]]
(Tallis, 1964; Ferguson et al., 1984; Lee et al., 1986).
Potential changes in species composition and succession in wetlands
is of particular concern because wetlands are habitats to many rare and
threatened plant species. Some of these plants have adapted to systems
low in nitrogen or with low nutrient levels. For some species, these
conditions can be normal for growth. Therefore, excess nitrogen
deposition can alter these conditions and thus alter species density
and diversity. In the contiguous U.S., wetlands harbor 14 percent (18
species) of the total number of plant species that are formally listed
as endangered. Several species on this list, such as the insectivorous
plants, are widely recognized to be adapted to nitrogen-poor
environments. While changes in species composition and succession are
of concern, such changes have not been associated with nitrogen
deposition in the U.S.
b. Aquatic. Some aquatic systems are potentially at risk from
atmospheric nitrogen additions through the processes of eutrophication
and acidification. Both processes can sufficiently reduce water quality
making it unfit as a habitat for most aquatic organisms and/or human
consumption. Acidification of lakes from nitrogen deposition may also
increase leaching and methylation of mercury in aquatic systems.
Atmospheric nitrogen can enter aquatic systems either as direct
deposition to water surfaces or as nitrogen deposition to the
watershed. In northern climates, nitrate may be temporarily stored in
snow packs and released in a more concentrated form during snow melt.
Nitrogen deposited to the watershed is then routed (e.g., through plant
biomass and soil microorganisms) and transformed (e.g., into other
inorganic or organic nitrogen species) by watershed processes, and may
eventually run off into aquatic systems in forms that are only
indirectly related to the original deposition. The contributions of
direct and indirect atmospheric loadings have received increased
attention. While the available evidence indicates that the impact of
nitrogen deposition on sensitive aquatic systems can be significant, it
is difficult to quantify the relationship between atmospheric
deposition of nitrogen, its appearance in receiving waters, and
observed effects.
(1) Acidification. In the U.S., the most comprehensive assessment
of chronic acidification of lakes and streams comes from the National
Surface Water Survey (NSWS) conducted as part of the National Acid
Precipitation Assessment Program (NAPAP). A detailed discussion of the
findings in the NSWS can be found in both the 1993 CD and the 1995
Staff Paper. The studies highlighted in these documents reported mixed
observations as to the relative contribution of nitrogen compounds to
chronic acidification in North American lakes. However, the National
Stream Survey (NSS) data do suggest that the Catskills, Northern
Appalachians, Valley and Ridge Province, and Southern Appalachians all
show some potential for chronic acidification due to nitrate ions
(NO3). Two studies (Kaufmann et al., 1991; Driscoll et al., 1989)
have examined whether atmospheric deposition is the source of the
NO3 leaking out of these watersheds. Data from the NSS (Kaufmann
et al., 1991) suggest a strong correlation between concentrations of
stream water and levels of wet nitrogen deposition in each of the NSS
regions. Secondly, Driscoll et al. (1989) collected input/output budget
data for a large number of watersheds in the U.S. and Canada and
summarized the relationship between nitrogen export and nitrogen
deposition at all the sites. Though the relationships discovered should
not be over-interpreted or construed as an illustration of cause and
effect, they do show that watersheds in many regions of North America
are retaining less than 75 percent of the nitrogen that enters them,
and that the amount of nitrogen being leaked from these watersheds is
higher in areas where nitrogen deposition is highest.
On a chronic basis in the U.S., especially in the eastern part of
the country, nitrogen deposition does play a role in surface water
acidification. However, there are significant uncertainties with regard
to the long-term role of nitrogen deposition in surface water acidity
and with regard to the quantification of the magnitude and timing of
the relationship between atmospheric deposition and the appearance of
nitrogen in surface waters.
Episodic acidification in surface waters is a concern in the
Northeast, Mid-Atlantic, Mid-Atlantic Coastal Plain, Southeast, Upper
Midwest, and West regions (Wigington et al., 1990). In the Mid-Atlantic
Coastal Plain and Southeast regions, all of the episodes reported to
date have been associated with rainfall. In contrast, most of the
episodes in the other regions are related to snowmelt, although rain-
driven episodes apparently can occur in all regions of the country. It
is important to stress that even within a given area, such as the
Northeast, major differences can be evident in the occurrence, nature,
location (lakes or streams), and timing of episodes at different sites.
The 1995 Staff Paper provides a detailed description of the processes
which may contribute to the timing and severity of acidic episodes.
Some broad geographic patterns in the frequency of episodes in the
U.S. are now evident. Episodes driven by NO3 are common in the
Adirondacks and Catskill Mountains of New York, especially during
snowmelt, and also occur in at least some streams in other portions of
the Northeast (e.g., Hubbard Brook). Nitrate contributes on a smaller
scale to episodes in Ontario and may play some role in episodic
acidification in the Western U.S. There is little current evidence that
NO3 episodes are important in the acid-sensitive portions of the
Southeastern U.S. outside the Great Smoky Mountains. There is no
information on the relative contribution of NO3 to episodes in
many of the subregions covered by the NSS, including those that
exhibited elevated NO3 concentrations at spring base flow (e.g.,
the Appalachian Plateau, the Valley and Ridge Province and Mid-Atlantic
Coastal Plain), because temporally-intensive studies have not been
published for these areas.
While the available data suggest that NO3 episodes are more
severe now than they were in the past, it is important to emphasize
that only the data reported for the Catskills can be considered truly
long-term (up to 65 years of record). Data for the Adirondacks
(Driscoll and Van Dreason, 1993) and other areas of the U.S. (Smith et
al., 1987) span only 1 to 2 decades and should be interpreted with
caution.
Because surface water nitrogen increases have occurred at a time
when nitrogen deposition has been relatively unchanged in the
Northeastern U.S. (Husar, 1986; Simpson and Olsen, 1990), it is
suggestive that nitrogen saturation of watersheds is progressing and
that current levels of nitrogen deposition are too high for the long-
term stability of aquatic systems in the Adirondacks, the Catskills,
and possibly elsewhere in the Northeast. It is important to note that
this supposition is dependent on our acceptance of NO3 episodes as
evidence of nitrogen saturation. While there is some support for this,
there are significant uncertainties with respect to the quantification
of the linkage and the timing of the relationship between the
atmospheric deposition of nitrogen and its episodic or chronic
appearance in surface waters.
This relationship between deposition and effect becomes more
complex because the capacity to retain nitrogen
[[Page 52883]]
differs from one watershed to another and from one region to another as
watershed and regional features differ. The differing features that may
contribute to these differences include, the amount, form and timing of
nitrogen deposition, forest type and status (including soil type and
status), the character of the receiving waterbodies, the history of
land management and disturbances across watersheds and regions and
exposure to other pollutants. For example, the Northeast, because of
the presence of aggrading forests and deeper soils in comparison to
those of the West, may be able to absorb higher rates of deposition
without serious effects than areas of the mountainous West, where soils
are thin in comparison and forests are often absent at the highest
elevations (CD, p. 10-179). The data of Silsbee and Larson (1982)
suggest strongly that forest maturation is also linked to the process
of NO3 leakage from Great Smoky Mountain watersheds.
In summary, the available data indicate that nitrogen contributes
to episodic acidification of sensitive streams and lakes in the
Northeast. The data also suggest that some watersheds of the Northeast
and the mid-Appalachians may be nearing nitrogen saturation. If, and
when, this occurs, nitrogen deposition will become a more direct cause
of chronic surface water acidification. At present, however, it is
difficult to establish quantitative relationships between nitrogen
deposition and the appearance of nitrogen in receiving waters, given
the uncertainties in determining time to nitrogen saturation for
varying systems and sites. The complexity of the scientific issues
involved led the CASAC to conclude that available scientific
information assessed in the Criteria Document and Staff Paper did not
provide an adequate basis for standard setting purposes at this time
(see Wolff, 1995). In its review of the Acid Deposition Standard
Feasibility Study: Report to Congress (U.S. EPA, 1995), the Acid
Deposition Effects Subcommittee of the Ecological Processes and Effects
Committee of the EPA's Science Advisory Board also concluded that there
was not an adequate scientific basis for establishing an acidic
deposition standard (see ``An SAB Report: Review of the Acid Deposition
Standard Feasibility Study Report to Congress,'' U.S. EPA, 1995).
(2) Eutrophication. Eutrophication is the process by which aquatic
systems are enriched with the nutrient(s) that are presently limiting
for primary production in that system. Eutrophication may produce
conditions of increased algal biomass and productivity, nuisance algal
populations, and decreases in oxygen availability for heterotrophic
organisms. Another effect of chronic eutrophication is increased algal
biomass shading out ecologically-valuable estuarine seagrass beds.
Eutrophy can lead to fish kills and the permanent loss of some
sensitive species caused by suffocation or rarely because of some kind
of toxic algal bloom. Though this process often occurs naturally over
the long-term evolution of lakes, it can be significantly accelerated
by the additional input of the limiting nutrients from anthropogenic
sources. In order to establish a link between nitrogen deposition and
the eutrophication of aquatic systems, one must first demonstrate that
the increase in biomass within the system is limited by nitrogen
availability, and second, that nitrogen deposition is a major source of
nitrogen to the system.
In most freshwater systems, phosphorus, not nitrogen, is the
limiting nutrient. Therefore, eutrophication by nitrogen inputs will
only be a concern in lakes that are chronically nitrogen limited and
have a substantial total phosphorous concentration. This condition is
common only in lakes that have received excessive inputs of
anthropogenic phosphorous, or in rare cases, have high concentrations
of natural phosphorus. In the former case, the primary dysfunction of
the lakes is an excess supply of phosphorous, and controlling nitrogen
deposition would be an ineffective method of gaining water quality
improvement. In the latter case, lakes with substantial total
phosphorous concentrations would experience measurable increases in
biomass from increases in nitrogen deposition.
In contrast to freshwater systems, the productivity of estuarine
waters of the U.S. correlates more closely with supply rates of
nitrogen than of other nutrients (Nixon and Pilson, 1983). Because
estuaries and coastal waters receive substantial amounts of weathered
material from terrestrial ecosystems and from exchange with sea water,
acidification is not a concern. However, this same load of weathered
material and anthropogenic inputs makes these same areas prone to the
effects of eutrophication.
Considerable research has focused on whether estuarine and coastal
ecosystems are limited by nitrogen, phosphorus, or some other factor.
Numerous geochemical and experimental studies have suggested that
nitrogen limitation is much more common in estuarine and coastal waters
than in freshwater systems (CD, 1993, pp. 10-189 to 197). However,
specific instances of phosphorus limitation (Smith, 1984) and of
seasonal switching between nitrogen and phosphorus limitation (D'Elia
et al., 1986; McComb et al., 1981) have been observed.
Estimation of the contribution of nitrogen deposition to the
eutrophication of estuarine and coastal waters is made difficult by the
multiple direct anthropogenic sources (e.g., from agriculture and
sewage) of nitrogen. In the U.S., only a few systems have been studied
with enough intensity to develop predictions about the contribution of
atmospheric nitrogen to total nitrogen inputs. One example is the
Chesapeake Bay, where a large effort has been made to establish the
relative importance of different sources of nitrogen to the total
nitrogen load entering the bay (e.g., D'Elia et al., 1982; Smullen et
al., 1982; Fisher et al., 1988a; Tyler, 1988). The signatories to the
Chesapeake Bay Agreement (i.e., Maryland, Virginia, Pennsylvania, the
District of Columbia, and EPA, through their Baywide Nutrient Reduction
Strategy and individual tributary watershed nutrient reduction
strategies) have committed to reduce nitrogen and phosphorus loadings
to the bay by 40 percent (from 1985 baseline) by the year 2000.
Enhanced modeling is being used to better assess source
responsibility for the transport and deposition of nitrogen from the
350,000 square miles Chesapeake Bay airshed. This enhanced modeling
will assist EPA in deciding: (1) Whether to include reductions in
atmospheric NOX and resultant decreased loadings via atmospheric
deposition in the reductions of total nitrogen loading necessary to
achieve the planned 40 percent reduction goal by the year 2000, and (2)
the role implementation of the Act will play in ensuring nitrogen
loadings are capped at the 40 percent reduction goal beyond the year
2000 in the face of significant projected population increases within
the Chesapeake Bay watershed (and surrounding airshed). This
integration of modeling, watershed, and airshed management will serve
as a case study and a prototype method for other geographic areas.
Though estimates for each individual source are very uncertain,
studies undertaken to determine the proportion of the total NO3
load to the bay, which was attributable to nitrogen deposition,
produced estimates in the range of 18 to 39 percent. These estimates,
which reflect the current status of the area, suggest that supplies of
nitrogen from deposition exceed supplies from all
[[Page 52884]]
other non-point sources (i.e., farm runoff) to the bay and only point-
source inputs (i.e., discharges to water, emissions from industrial
facilities) represent a greater input than deposition.
Based on the available data, it is clear that atmospheric nitrogen
inputs to estuarine and coastal ecosystems are of concern. The
importance of atmospheric inputs will vary, however, from site to site
and will depend on the availability of other growth nutrients, the
flushing rate through the system, the sensitivity of resident species
to added nitrogen, the types and chemical forms of nitrogen inputs from
other sources, as well as other factors. Given these complexities,
site-specific investigations, such as the Chesapeake Bay Study, are
needed to ascertain the most effective mitigation strategy. Similar
place-based studies are already under way in the Tampa Bay and other
coastal areas.
3. Direct Toxic Effects of Ammonia Deposition to Aquatic Systems
Nitrogen deposition could potentially contribute directly to toxic
effects in surface waters. High ammonia concentrations are associated
with lesions in gill tissue, reduced growth rates of trout fry, reduced
fecundity (number of eggs), increased egg mortality, and increased
susceptibility of fish to other diseases, as well as a variety of
pathological effects in invertebrates and aquatic plants. Given current
maximal concentrations of ammonium ions (NH4+) in wet deposition
and reasonable maximum rates of dry deposition, even if all nitrogen
species were ammonified, the maximum potential NH4+ concentrations
attributable to deposition would be approximately 280 nmol/L and would
be unlikely to be toxic except in unusual circumstances. Therefore, it
appears that the potential for toxic effects directly attributable to
nitrogen deposition in the U.S. is very limited. In addition, EPA has
established water quality standards for ammonia to protect against
these effects (50 FR 30784, July 29, 1984; also see guidance document
EPA-440/5-85-001).
4. Proposed Decision on the Secondary Standard
As discussed above, after carefully considering the information on
the direct effects of NO2, the Administrator has determined that
the existing annual secondary standard is both necessary and adequate
to protect vegetation and materials from the direct effects of
NO2. The Administrator has also determined that establishment of a
secondary NO2 standard to protect visibility is not appropriate.
In reaching these provisional conclusions, the Administrator has
assessed the evidence provided in the CD and the Staff Paper as well as
the advice and recommendations of the EPA staff and CASAC.
With respect to nitrogen deposition, the Administrator is concerned
about the growing body of scientific information, assessed in the CD
and Staff Paper and discussed above, that associates nitrogen
deposition with a wide range of environmental effects. Of particular
concern is the available data that indicate nitrogen deposition plays a
significant role in the episodic acidification of certain sensitive
streams and lakes and could cause long-term chronic acidification of
such surface waters. The Administrator notes, as did CASAC, that
because of the variations in the actual rate of nitrogen uptake,
immobilization, denitrification, and leaching, it is very difficult,
given current quantification of these processes, to link specific
nitrogen deposition rates with observed environmental effects.
In considering the available data, the Administrator is also
mindful, given the complex processes involved, that the time to
nitrogen saturation will vary from one system to another. As a
consequence, the relationship between nitrogen deposition rates and
their potential environmental impact is to a large degree site- or
regionally-specific and may vary considerably over broader geographical
areas. These complexities led both the EPA and CASAC to conclude that
there is currently insufficient information to set a national secondary
NO2 standard which would protect against the acidification effects
of nitrogen deposition. Because of the site- and regional-specific
nature of the problem, the staff also questioned whether adoption of a
national secondary NO2 standard would be an effective tool to
address such effects.
In considering the staff's latter view, the Administrator also
recognizes that Congress reserved judgment regarding the possible need
for further action to control acid deposition beyond the provisions of
title IV of the 1990 Amendments and what form any such action might
take (Pub. L. 101-549, sec. 404, 104 Stat. 2399, 2632 (1990)). For a
more complete discussion of the congressional deliberation on the
acidic deposition issue, see 58 FR 21356-21357, April 21, 1993. Among
other things, Congress directed EPA to conduct a study of the
feasibility and effectiveness of an acid deposition standard(s), to
report to Congress on the role that a deposition standard(s) might play
in supplementing the acidic deposition program adopted in title IV, and
to determine what measures would be needed to integrate it with that
program. The resulting document entitled, ``Acid Deposition Standard
Feasibility Study: Report to Congress'' (U.S. EPA, 1995), concluded, as
did the CD and staff paper, that nitrogen deposition plays a
significant role in the acidification of certain sensitive streams and
lakes and that the time to nitrogen saturation varies significantly
from one system or region to another. The complexities of watershed
nitrogen dynamics (e.g., the biological processes) and the
uncertainties in modeling results that project future effects of
nitrogen deposition under alternative emission scenarios, however, led
EPA staff (as well as the Acid Deposition Effects Subcommittee of the
Ecological Processes and Effects Committee of the EPA's Science
Advisory Board that reviewed the report) to conclude that current
scientific uncertainties associated with determining the level(s) of an
acid deposition standard(s) are significant (see ``An SAB Report:
Review of the Acid Deposition Standard Feasibility Study Report to
Congress,'' U.S. EPA, 1995). The study does not advocate setting an
acid deposition standard at this time. The study does, however, set
forth a range of regionally-specific goals to help guide the policy
maker when assessing NOX control strategies and their potential
for reducing nitrogen deposition effects.
The Administrator has also examined the available information that
indicates atmospheric nitrogen deposition can play a significant role
in the eutrophication of estuarine and coastal waters. However,
estimation of the contribution of nitrogen deposition to the
eutrophication of estuarine and coastal waters is made difficult by
multiple direct anthropogenic sources of nitrogen. Thus, the importance
of atmospheric inputs will vary from site to site and will depend on
the availability of other growth nutrients, the flushing rate through
the system, the sensitivity of resident plant species to added
nitrogen, as well as the types of chemical forms of nitrogen inputs
from other sources. Given the complexities of these factors and the
limited data currently available, the Administrator concurs with the
EPA staff and CASAC conclusion that there is not sufficient
quantitative information to establish a national secondary standard to
protect sensitive ecosystems from the eutrophication effects caused by
[[Page 52885]]
nitrogen deposition. Rather, additional site-specific investigations
(such as the Chesapeake Bay Study) are needed to ascertain the most
effective mitigation strategies.
For the above reasons, the Administrator has determined pursuant to
section 109(d)(1) of the Act, as amended, that it is not appropriate to
propose any revision of the current secondary standard for NO2 to
protect against welfare effects at this time. As provided for under the
Act, the EPA will continue to assess the scientific information on
nitrogen-related effects as it emerges from ongoing research and will
update the air quality criteria accordingly. These revised criteria
should provide a more informed basis for reaching a decision on whether
a revised NAAQS or other regulatory measures are needed in the future.
In the interim, the 1990 Clean Air Act Amendments (Pub. L. 101-549,
104 Stat. 2399 (1990)) require EPA to promulgate a number of control
measures to reduce NOX emissions from both mobile and stationary
sources. These reductions are in addition to those required under title
IV of the 1990 Amendments (Pub. L. 101-549, secs. 401-413, 104 Stat.
2399, 2584-2634 (1990)). Title IV, in conjunction with other titles of
the Act, requires EPA to reduce nitrogen oxide emissions by
approximately two million tons from 1980 emission levels. The
reductions achieved through these EPA initiatives will provide
additional protection against the potential acute and chronic effects
associated with exposure to NOX while EPA continues to generate
and review additional information on the effects of oxides of nitrogen
on public welfare and the environment. The EPA believes it is important
to continue to recognize the benefit to the environment that can be
achieved by further reducing NOX emissions. Therefore, as part of
this process, the EPA will integrate, to the extent appropriate,
nitrogen deposition considerations when assessing new NOX control
strategies.
III. Miscellaneous
A. Executive Order 12866
Under Executive Order 12866, the Agency must determine whether a
regulatory action is ``significant'' and, therefore, subject to Office
of Management and Budget (OMB) review and the requirements of the
Executive Order. The order defines ``significant regulatory action'' as
one that may:
(1) Have an annual effect on the economy of $100 million or more or
adversely affect in a material way the economy, a sector of the
economy, productivity, competition, jobs, the environment, public
health or safety, or State, local, or tribal governments or
communities;
(2) create a serious inconsistency or otherwise interfere with an
action taken or planned by another Agency;
(3) materially alter the budgetary impact of entitlements, grants,
user fees, or loan programs or the rights and obligations or recipients
thereof; or
(4) raise novel legal or policy issues arising out of legal
mandates, the President's priorities, or the principles set forth in
the Executive Order.
Although the EPA is not proposing any modification of the existing
NO2 NAAQS, the OMB has advised the EPA that this proposal should
be construed as a ``significant regulatory action'' within the meaning
of the Executive Order. Accordingly, this action was submitted to the
OMB for review. Any changes made in response to OMB suggestions or
recommendations will be documented in the public record.
B. Regulatory Flexibility Analysis
The Regulatory Flexibility Act (RFA) requires that all Federal
agencies consider the impacts of final regulations on small entities,
which are defined to be small businesses, small organizations, and
small governmental jurisdictions (5 U.S.C. 601 et seq.). These
requirements are inapplicable to rules or other administrative actions
for which the EPA is not required by the Administrative Procedure Act
(APA), 5 U.S.C. 551 et seq., or other law to publish a notice of
proposed rulemaking (5 U.S.C. 603(a), 604(a)). The EPA has elected to
use notice and comment procedures in deciding whether to revise the
NO2 standards based on its assessment of the importance of the
issues. Under section 307(d) of the Act, as the EPA interprets it,
neither the APA nor the Act requires rulemaking procedures where the
Agency decides to retain existing NAAQS without change. Accordingly,
the EPA has determined that the impact assessment requirements of the
RFA are inapplicable to the decision proposed in this notice.
C. Impact on Reporting Requirements
There are no reporting requirements directly associated with an
ambient air quality standard promulgated under section 109 of the Act
(42 U.S.C. 7400). There are, however, reporting requirements associated
with related sections of the Act, particularly sections 107, 110, 160,
and 317 (42 U.S.C. 7407, 7410, 7460, and 7617). This proposal will not
result in any changes in these reporting requirements since it would
retain the existing level and averaging times for both the primary and
secondary standards.
D. Unfunded Mandates Reform Act
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), P.L.
104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local and tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
1 year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law.
Before EPA establishes any regulatory requirements that may
significantly or uniquely affect small governments, including tribal
governments, it must have developed, under section 203 of the UMRA, a
small government agency plan. The plan must provide for notifying
potentially affected small governments, enabling officials of affected
small governments to have meaningful and significant Federal
intergovernmental mandates, and informing, educating, and advising
small governments on compliance with the regulatory requirements.
A decision by the Administrator pursuant to section 109(d) of the
Act not to propose any revision of the existing national primary and
secondary standards for NO2 does not require rulemaking
procedures, and EPA has elected to provide notice and an opportunity
for comment concerning this proposed decision in view of the importance
of the issues. If the Administrator makes a final decision not to
modify the existing NAAQS for NO2, this will not impose any new
expenditures on governments or on the private sector, or establish any
new regulatory requirements affecting small governments. Accordingly,
the EPA has determined that the provisions of sections 202, 203, and
205 of the UMRA do not apply to this proposed decision.
List of Subjects in 40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
[[Page 52886]]
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
Dated: October 2, 1995.
Carol M. Browner,
Administrator.
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