                        United States Court of Appeals


                     FOR THE DISTRICT OF COLUMBIA CIRCUIT


      Argued September 26, 1997               Decided January 30, 1998 


                                 No. 96-1251


                     American Lung Association, et al., 

                                 Petitioners


                                      v.


                     Environmental Protection Agency and 

                Carol M. Browner, Administrator, United States

                      Environmental Protection Agency, 

                                 Respondents


                     Appalachian Power Company, et al., 

                                 Intervenors


				  ---------


                              Consolidated with


                                 No. 96-1255


				  ---------


                 On Petitions for Review of an Order of the 

                       Environmental Protection Agency


				  ---------




     Robert E. Yuhnke argued the cause for petitioners.  With 
him on the briefs were Christine L. Shaver and Howard I. 
Fox.

     Karen L. Egbert, Attorney, U.S. Department of Justice, 
argued the cause for respondents.  With her on the brief 
were Lois J. Schiffer, Assistant Attorney General, and Mi-
chael L. Goo, Counsel, Environmental Protection Agency.  
Gerald K. Gleason, Counsel, entered an appearance.

     Andrea Bear Field, Henry V. Nickel, Linda C. Trees, and 
James R. Bieke were on the brief for intervenor Appalachian 
Power Company, et al.  Ross S. Antonson entered an appear-
ance.

     Before:  Edwards, Chief Judge, Ginsburg and Tatel, 
Circuit Judges.

     Opinion for the Court filed by Circuit Judge Tatel.

     Tatel, Circuit Judge:  On behalf of the nation's nearly nine 
million asthmatics, the American Lung Association and the 
Environmental Defense Fund challenge the Environmental 
Protection Agency's refusal to revise the primary national 
ambient air quality standards for sulfur dioxide 
(SO2).  Declining to promulgate a more stringent national 
standard, the EPA Administrator concluded that the substan-
tial physical effects experienced by some asthmatics from 
exposure to short-term, high-level SO2 bursts do not amount 
to a public health problem.  Because the Administrator failed 
adequately to explain this conclusion, we remand for further 
elucidation.

                                      I


     Driven by its "deep concern for protection of the health of 
the American people," Sen. Rep. No. 91-1196, at 1 (1970) 
("Senate Report"), Congress enacted the Clean Air Act 
Amendments of 1970, Pub. L. No. 91-604, 84 Stat. 1676 (1970) 
(codified as amended at 42 U.S.C. ss 7401-7671q (1994)), 
mandating a "massive attack on air pollution," Senate Report 
at 1.  As amended, the Clean Air Act erects a comprehensive 
system of national ambient air quality standards ("NAAQS") 



to regulate health-threatening air pollutants.  The statute 
defines primary NAAQS as "ambient air quality standards 
the attainment and maintenance of which in the judgment of 
the Administrator, based on such criteria and allowing an 
adequate margin of safety, are requisite to protect the public 
health."  42 U.S.C. s 7409(b)(1).

     Once the EPA Administrator concludes that a pollutant 
"may reasonably be anticipated to endanger public health or 
welfare" and that it comes from "numerous or diverse mobile 
or stationary sources," id. s 7408(a)(1)(A)-(B), the Act re-
quires the Administrator to produce "criteria," defined as the 
latest scientific data on "all identifiable effects on public 
health" caused by that pollutant.  Id. s 7408(a)(2).  Based on 
these comprehensive criteria and taking account of the "pre-
ventative" and "precautionary" nature of the act, Lead Indus-
tries Ass'n, Inc. v. EPA, 647 F.2d 1130, 1155 (D.C. Cir. 1980), 
the Administrator must then decide what margin of safety 
will protect the public health from the pollutant's adverse 
effects--not just known adverse effects, but those of scientific 
uncertainty or that "research has not yet uncovered."  Id. at 
1153.  Then, and without reference to cost or technological 
feasibility, the Administrator must promulgate national stan-
dards that limit emissions sufficiently to establish that margin 
of safety.  See 42 U.S.C. s 7409(b)(1);  American Petroleum 
Inst. v. Costle, 665 F.2d 1176, 1181-82 (D.C. Cir. 1981) 
(describing NAAQS promulgation procedure);  Lead Indus-
tries, 647 F.2d at 1148-50 (in establishing NAAQS, Congress 
deliberately subordinated economic and technological feasibil-
ity concerns to the achievement of public health goals).  
States bear primary responsibility for attaining, maintaining, 
and enforcing these standards.  See 42 U.S.C. s 7410.

     In its effort to reduce air pollution, Congress defined public 
health broadly.  NAAQS must protect not only average 
healthy individuals, but also "sensitive citizens"--children, for 
example, or people with asthma, emphysema, or other condi-
tions rendering them particularly vulnerable to air pollution.  
Senate Report at 10;  Lead Industries, 647 F.2d at 1152.  If a 
pollutant adversely affects the health of these sensitive indi-
viduals, EPA must strengthen the entire national standard.  



Lead Industries, 647 F.2d at 1153 (NAAQS "must be set at a 
level at which there is 'an absence of adverse effect' on [ ] 
sensitive individuals") (quoting Senate Report at 10).

                        Sulfur Dioxide and Asthmatics


     A highly reactive colorless gas smelling like rotten eggs, 
sulfur dioxide derives primarily from fossil fuel combustion.  
Best known for causing "acid rain," at elevated concentrations 
in the ambient air, SO2 also directly impairs human health.  
As the Administrator explains in the Final Decision on review 
here, at concentrations above 2.0 parts per million ("ppm"), 
SO2 can affect healthy nonasthmatic individuals;  below 2.0 
ppm, it primarily affects people with asthma.  National Am-
bient Air Quality Standards for Sulfur Oxides (Sulfur Diox-
ide)--Final Decision, 61 Fed. Reg. 25,566, 25,570 (1996).

     Following the passage of the Clean Air Act, EPA promul-
gated the SO2 NAAQS in effect today.  The primary stan-
dards consist of a 24-hour standard (0.14 ppm averaged over 
24 hours not to be exceeded more than once a year) and an 
annual standard (0.03 ppm annual arithmetic mean).  Id. at 
25,568.  EPA also established a "secondary" three-hour stan-
dard (0.50 ppm averaged over three hours not to be exceeded 
more than once a year), designed to protect the "public 
welfare" against non-health-related effects such as visibility 
impairment or environmental degradation, see 42 U.S.C. 
s 7409(b)(2).  Petitioners do not challenge these existing 
standards.

     Approximately four percent of the nation's population suf-
fers from asthma.  Characterized by bronchoconstriction--
shortness of breath, coughing, wheezing, chest tightness, and 
sputum production--asthma is triggered by many different 
stimuli, including cold or dry air, exercise or pollen as well as 
airborne pollutants.  The effects of bronchoconstriction can 
vary from short-term discomfort, such as an hour-long reac-
tion with no lasting after-effects, to asthma attacks requiring 
medication or hospitalization.  Although rare, death can re-
sult.



     Sulfur dioxide induces bronchoconstriction in asthmatics, 
but only under certain conditions.  To experience adverse 
effects from SO2 concentrations below 1.0 ppm, asthmatics 
must be exposed for five minutes or longer while breathing 
quickly and heavily through both nose and mouth, the sort of 
breathing induced by light exercise, shoveling snow, climbing 
several flights of stairs, or jogging to catch a bus.  At 
concentrations above 2.0 ppm, SO2 causes adverse effects 
even if the exposure lasts less than five minutes or the 
asthmatic breathes regularly.  See Second Addendum to Air 
Quality Criteria for Particulate Matter and Sulfur Oxides 
(1982):  Assessment of Newly Available Health Effects Infor-
mation (1986).

                        The Challenged Final Decision


     This case concerns the effect on asthmatics of what are 
known as high-level SO2 bursts, defined as emissions of 0.50 
ppm or more lasting at least five minutes.  Occurring sporad-
ically and from specific sources, SO2 bursts come primarily 
from power utilities;  the rest come from nonutility sources 
such as industrial boilers, petroleum refineries, pulp and 
paper mills, sulfuric acid plants, and aluminum smelters.

     Citing the health concerns of asthmatics and relying on a 
1977 amendment to the Clean Air Act, in which Congress 
ordered the Agency to review and revise all criteria and 
NAAQS by 1980 and at five-year intervals thereafter, 42 
U.S.C. s 7409(d), petitioners urged EPA to issue a new 
NAAQS limiting short-term SO2 bursts.  Not until 1996, after 
petitioners sued twice to compel a decision, see Environmen-
tal Defense Fund v. Thomas, 870 F.2d 892 (2d Cir. 1989);  
American Lung Ass'n v. Browner, Civil Action No. 92-5316 
(E.D.N.Y. Nov. 12, 1992), and after two rounds of public 
notice and comment, did EPA issue its final decision regard-
ing SO2 NAAQS.  See NAAQS for Sulfur Oxides (Sulfur 
Dioxide)--Reproposal, 59 Fed. Reg. 58,958 (1994);  Proposed 
Decision Not To Revise the National Ambient Air Quality 
Standards for Sulfur Oxides (Sulfur Dioxide), 53 Fed. Reg. 
14,926 (1988).  Rejecting petitioners' arguments, EPA con-



cluded not only that the annual and 24-hour primary stan-
dards needed no revision, but also that an additional five-
minute standard was unnecessary to protect asthmatics.  See 
Final Decision at 25,575-76.

     In arriving at her final decision, the Administrator re-
viewed a decade of data on the extent of high-level short-term 
SO2 bursts and their effects on public health.  See Review of 
the National Ambient Air Quality Standards for Sulfur Ox-
ides:  Assessment of Scientific and Technical Information:  
Supplement to the 1986 OAQPS Staff Paper Addendum (Sept. 
1994);  Supplement to the Second Addendum (1986) to Air 
Quality Criteria for Particulate Matter and Sulfur Oxides 
(1982):  Assessment of New Findings on Sulfur Dioxide Acute 
Exposure Health Effects in Asthmatic Individuals (Aug. 
1994).  Based on clinical studies of mild to moderate asthmat-
ics, she found that when such individuals breathe rapidly 
while exposed to SO2 concentrations of 0.60 ppm for five 
minutes, "substantial percentages (>= 25 percent)" experience 
effects "distinctly exceeding ... [the] typical daily variation in 
lung function" that asthmatics routinely experience.  Final 
Decision at 25,572.  The severity of these atypical effects, she 
found, "is likely to be of sufficient concern to cause disruption 
of ongoing activities, use of bronchodilator medication, and/or 
possible seeking of medical attention."  Id. 

     The scientific community disagreed about the medical sig-
nificance of these effects and whether they should be consid-
ered "adverse."  Some experts took the position that such 
symptoms usually have no lasting impact, amounting at worst 
to a brief period of reversible discomfort;  others argued that 
even a one-hour disruption of activity can amount to a worri-
some adverse health effect.  The Administrator left this 
dispute unresolved.  Instead, she discerned in the medical 
debate a consensus, which she adopted, that "repeated occur-
rences of such effects should be regarded as significant from 
a public health standpoint."  Id. at 25,573 (emphasis added).

     The Administrator then discussed the three exposure anal-
yses on which the 1994 version of the proposed rule rested.  



These studies estimated that from 180,000 to 395,000 "expo-
sure events"--defined as a heavily breathing asthmatic ex-
posed to an SO2 burst--occur annually, affecting from 68,000 
to 166,000 asthmatic individuals.  Id. at 25,574.  In view of 
the Administrator's previous finding, reiterated by agency 
counsel at oral argument, that at least 25 percent of asthmat-
ics experience atypical effects from exposure events, these 
data suggest that as many as 41,500 (>= 25 percent of 166,000) 
asthmatics experience atypical effects from repeated SO2 
bursts each year.  At the same time, the Administrator 
acknowledged that subsequent industry studies of four nonu-
tility sources suggest that the 1994 studies may have overesti-
mated exposure for certain SO2 sources, id., meaning that the 
number of affected asthmatics could be lower.  The Adminis-
trator did not resolve the conflict between the studies.

     Armed with all these data, the Administrator concluded 
that "the likelihood that asthmatic individuals will be exposed 
... is very low when viewed from a national perspective," 
that "5-minute peak SO2 levels do not pose a broad public 
health problem when viewed from a national perspective," 
and that "short-term peak concentrations of SO2 do not 
constitute the type of ubiquitous public health problem for 
which establishing a NAAQS would be appropriate."  Id. at 
25,575.  Describing SO2 bursts as "localized, infrequent and 
site-specific," she concluded that a new national standard was 
unnecessary.   Id.  The Administrator nevertheless decided 
to encourage individual states to address short-term high-
level SO2 emissions, initiating a rulemaking to provide appro-
priate guidance.  Proposed Implementation Requirements for 
Reduction of Sulfur Oxide (Sulfur Dioxide) Emissions, 62 
Fed. Reg. 210 (Jan. 2, 1997) ("Proposed State Guidelines 
Rulemaking") (soliciting public comment on proposed guide-
lines for state monitoring and regulation of five-minute peaks 
of SO2).

     Petitioners now challenge the Administrator's decision de-
clining to promulgate a new NAAQS.  They assert that by 
failing to establish a five-minute NAAQS capping SO2 emis-
sions at 0.60 ppm, EPA has violated its statutory responsibili-
ty to protect the public health.  We review the Administra-
tor's decision pursuant to 42 U.S.C. s 7607(d)(9)(A)-(C) 



("[C]ourt may reverse any such [agency] action found to be 
... arbitrary, capricious, an abuse of discretion, or otherwise 
not in accordance with law;  ... [or] in excess of statutory ... 
authority, or limitations....").

                                      II


     Petitioners challenge much of the data the Administrator 
relied on, as well as the conclusions she drew.  Generally 
speaking, we will not second-guess EPA in its area of special 
expertise.  See Natural Resources Defense Council v. United 
States EPA, 824 F.2d 1146, 1163 (D.C. Cir. 1987) (en banc);  
American Petroleum Institute, 665 F.2d at 1184.  Applying 
this deferential standard of review, we accept the Administra-
tor's analysis of the exposure studies in the record, as well as 
the implication of her analysis--that thousands of asthmatics 
can be expected to react atypically to SO2 bursts each year.

     Petitioners contend that the Administrator's analysis 
amounts to a conclusive finding that SO2 bursts adversely 
affect asthmatics' health, thus triggering her duty to promul-
gate a new NAAQS.   See Lead Industries, 647 F.2d at 1153.  
At oral argument, counsel for EPA vigorously disputed peti-
tioners' contention that the Administrator "found" an adverse 
health effect.  As we read the record, agency counsel appears 
to be correct:  The Administrator did not decide whether 
asthmatic reaction to SO2 bursts--"disruption of ongoing 
activities, use of bronchodilator medication, and/or possible 
seeking of medical attention"--amounts to an adverse health 
effect or merely, as some medical experts argued, run-of-the-
mill asthma symptoms indistinguishable from bronchodilation 
due to cold air or exercise.  Final Decision at 25,572-73.  
Skipping this disputed question, the Administrator concluded 
that, regardless of the impact of single occurrences, "repeated 
occurrences of such effects should be regarded as significant 
from a public health standpoint."  Id. at 25,573.

     Disagreeing with this approach, petitioners argue that the 
Administrator had to answer the subsidiary "adverse effects" 
question, pointing to her warning to all states in the subse-



quent rulemaking that "[a]lthough these episodes are few, it 
is clear that 5-minute SO2 ambient concentration peaks pose 
a health threat to sensitive exposed populations," Proposed 
State Guidelines Rulemaking at 211.  We need not decide 
that issue at this time, however, because we think the Admin-
istrator has failed to explain the answer she did give, i.e., that 
SO2 bursts do not amount to a "public health" problem within 
the meaning of the Act.  The link between this conclusion and 
the factual record as interpreted by EPA--that "repeated" 
exposure is "significant" and that thousands of asthmatics are 
exposed more than once a year--is missing.  Why is the fact 
that thousands of asthmatics can be expected to suffer atypi-
cal physical effects from repeated five-minute bursts of high-
level sulfur dioxide not a public health problem?  Why are 
from 180,000 to 395,000 annual "exposure events" (the range 
indicated by the 1994 studies) or some fewer number (as 
suggested by the industry studies) so "infrequent" as to 
warrant no regulatory action?  Why are disruptions of ongo-
ing activities, use of medication, and hospitalization not "ad-
verse health effects" for asthmatics?  Answers to these ques-
tions appear nowhere in the administrative record.

     In her only statement resembling an explanation for her 
conclusion that peak SO2 bursts present no public health 
hazard, the Administrator characterizes the bursts as "local-
ized, infrequent and site-specific."  Final Decision at 25,575.  
But nothing in the Final Decision explains why "localized," 
"site-specific" or even "infrequent" events might nevertheless 
create a public health problem, particularly since, in some 
sense, all pollution is local and site-specific, whether spewing 
from the tailpipes of millions of cars or a few offending smoke 
stacks.  From the record, we know that at least six communi-
ties experience "repeated high 5-minute peaks greater than 
0.60 ppm SO2," id., and agency counsel told us at oral 
argument that these so-called "hot spots" are not the only 
places where repeated exposure occurs.  Nowhere, however, 
does the Administrator explain why these data amount to no 
more than a "local" problem.



     Without answers to these questions, the Administrator 
cannot fulfill her responsibility under the Clean Air Act to 
establish NAAQS "requisite to protect the public health," 42 
U.S.C. s 7409(b)(1), nor can we review her decision.  Judicial 
deference to decisions of administrative agencies like EPA 
rests on the fundamental premise that agencies engage in 
reasoned decision-making.  See Vermont Yankee Nuclear 
Power Corp. v. Natural Resources Defense Council, 435 U.S. 
519, 524-25, 544-45, 558 (1978);  SEC v. Chenery Corp., 332 
U.S. 194, 209 (1947) (agency's experience, appreciation of 
complexities and policies, and responsible treatment of the 
facts "justifies the use of the administrative process").  With 
its delicate balance of thorough record scrutiny and deference 
to agency expertise, judicial review can occur only when 
agencies explain their decisions with precision, for "[i]t will 
not do for a court to be compelled to guess at the theory 
underlying the agency's action...."  SEC v. Chenery Corp., 
332 U.S. at 196-97.  Where, as here, Congress has delegated 
to an administrative agency the critical task of assessing the 
public health and the power to make decisions of national 
import in which individuals' lives and welfare hang in the 
balance, that agency has the heaviest of obligations to explain 
and expose every step of its reasoning.  For these compelling 
reasons, we have always required the Administrator to "co-
gently explain why [she] has exercised [her] discretion in a 
given manner."  Motor Vehicle Mfrs. Ass'n v. State Farm 
Mut. Auto. Ins., 463 U.S. 29, 48 (1983).

     In this case, the Administrator may well be within her 
authority to decide that 41,500 or some smaller number of 
exposed asthmatics do not amount to a public health problem 
warranting national protective regulation, or that three or six 
or twelve annual exposures present no cause for medical 
concern.  But unless she describes the standard under which 
she has arrived at this conclusion, supported by a "[ ]plausi-
ble" explanation, id. at 43, we have no basis for exercising our 
responsibility to determine whether her decision is "arbitrary, 
capricious, an abuse of discretion, or otherwise not in accor-
dance with law;  ... [or] in excess of statutory ... authority, 
or limitations...."  42 U.S.C. s 7607(d)(9)(A)-(C).



     Given the gaps in the Final Decision's reasoning, we must 
remand this case to permit the Administrator to explain her 
conclusions more fully.  We therefore need not resolve the 
debate between the parties over whether the Clean Air Act 
authorizes the Administrator to decline to protect an identifi-
able group of asthmatics from a known adverse health effect.  
Although our cases make clear that the Administrator has 
broad discretion to establish an "adequate margin of safety" 
above and beyond what scientific certainty prescribes and to 
craft regulations that protect against unknown harms, see 
Lead Industries, 647 F.2d at 1153-55 (Administrator must 
"err on the side of caution" when establishing the margin of 
safety, even where the "medical significance [of the effects] is 
a matter of disagreement"), they do not necessarily establish 
the converse proposition--that the Administrator may decline 
to establish a margin of safety in the face of documented 
adverse health effects.  Since in this case the Administrator 
has failed adequately to explain her conclusion that no public 
health threat exists, we can leave the issue of the scope of her 
authority for another day.

     We remand this case to the agency for further proceedings 
consistent with this opinion.

									So ordered.


                                                            
