United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued March 22, 2018 Decided July 20, 2018

No. 16-1413

NATURAL RESOURCES DEFENSE COUNCIL AND SIERRA CLUB,

PETITIONERS

v.

ENVIRONMENTAL PROTECTION AGENCY AND ANDREW 

WHEELER,

RESPONDENTS

AMERICAN PETROLEUM INSTITUTE,

INTERVENOR

On Petition for Review of a Final Agency Action

of the United States Environmental Protection Agency

Margaret T. Hsieh argued the cause for petitioners. With 

her on the briefs were Sanjay Narayan, John Walke, and Emily 

K. Davis. Nancy S. Marks entered an appearance.

Sue Chen, Attorney, U.S. Department of Justice, argued 

the cause for respondents. With her on the brief was Jeffrey H. 

Wood, Acting Assistant Attorney General.

Aaron M. Flynn and Lucinda Minton Langworthy were on 

the brief for intervenor-respondent.
2

Caroline Lobdell was on the brief for amici curiae National 

Cattlemen’s Beef Association, et al. in support of respondents.

Before: GRIFFITH and KATSAS, Circuit Judges, and 

EDWARDS, Senior Circuit Judge.

Opinion for the Court filed by Circuit Judge GRIFFITH.

GRIFFITH, Circuit Judge: The Clean Air Act calls upon the 

Environmental Protection Agency to protect air quality by 

enforcing state and local limits on the amount of pollution. The 

agency need not count against those limits pollution caused by 

“exceptional events.” In this case, Natural Resources Defense 

Council and Sierra Club challenge a rule the agency uses to 

determine whether an event caused by recurring activity is 

“natural,” and thus “exceptional,” or “caused by human 

activity,” and thus not. 42 U.S.C. § 7619(b)(1)(A)(iii). We 

think the agency’s rule is permitted by the Clean Air Act. 

I

To “protect the public health,” 42 U.S.C. § 7409(b)(1), the 

Clean Air Act (the “Act”) established a nationwide policy for 

limiting air pollution on the state and local level, id. § 7410. 

The Act directs the Environmental Protection Agency (EPA)

to set uniform levels of concentrations of various pollutants, 

National Ambient Air Quality Standards (NAAQS), that local 

areas must not exceed. Id. § 7409. Each state must earn EPA’s 

approval of a state implementation plan (SIP), which commits 

the state to recording levels of specified pollutants using a 

network of air-quality monitors. Id. § 7410(a). By recording the 

concentration levels of these pollutants, the monitors identify 

areas that exceed the NAAQS. States report pollutant levels to 

EPA quarterly and receive from the agency “attainment” 

designations when the levels are below the NAAQS and 
3

“nonattainment” designations, accompanied by additional airquality regulations, when the levels exceed the NAAQS. Id. 

§ 7407; see also, e.g., 40 C.F.R. § 50.6 (establishing the 

NAAQS for large particulate matter, setting attainment to be 

exceeding a 24-hour average concentration of 150 μg/m3 no 

more than one day within a calendar year). 

Since 1977, EPA has recognized that “[f]ederal, [s]tate, 

and local air pollution control officials have expressed a great 

deal of concern” that counting emissions caused by 

“exceptional events” inflates reported levels of pollutants, 

which sometimes pushes an area otherwise in attainment to be 

designated as nonattainment. EPA, EPA-450/4-86-007,

Guideline on the Identification and Use of Air Quality Data 

Affected by Exceptional Events 1 (1986). To avoid this, EPA

suggested in a series of informal guidelines that state and

federal agencies need not include in their pollution reports 

those pollutants emitted from exceptional events. See, e.g., 

EPA, OAPQS No. 1.2-008, Guideline for the Interpretation of 

Air Quality Standards (1977). The agency considered events to 

be exceptional if “they are not expected to recur routinely at a 

given location, or they are possibly uncontrollable or 

unrealistic to control through the [SIP] process.” EPA-450/4-

86-007 at 1. In 2005, Congress added this practice to the Act. 

Act of Aug. 10, 2005, Pub. L. No. 109-59, sec. 6013(a), § 319, 

119 Stat 1144, 1882-884 (codified as amended at 42 U.S.C. 

§ 7619(b)) (“Air quality monitoring data influenced by 

exceptional events.”). Since then, EPA has had statutory 

authority to exclude from a state’s reported pollutant levels 

emissions that result from exceptional events. Id. 

The Act sets out several requirements that events must 

satisfy to be exceptional. Id. § 7619(b)(1)(A). However, one of 

those requirements applies only to events “caused by human 

activity” and not “natural event[s].” Id. § 7619(b)(1)(A)(iii)
4

(“[A]n event [must be] caused by human activity that is 

unlikely to recur at a particular location or a natural event.”). 

Through notice-and-comment rulemaking, EPA proposed that

“natural events” include events that are caused by both natural 

and human activity, so long as such human activity complies 

with relevant environmental regulations. Treatment of Data 

Influenced by Exceptional Events, 80 Fed. Reg. 72,840, 72,854

(Nov. 20, 2015). Natural Resources Defense Council and 

Sierra Club (together, the “environmental groups”) objected to 

the definition, arguing that an event caused by human activity 

cannot be a natural event. EPA replied that “there is not always 

a bright line” between natural and human-caused events, J.A. 

135, and adopted the definition as a final rule, Treatment of 

Data Influenced by Exceptional Events, 81 Fed. Reg. 68,216 

(Oct. 3, 2016) (“2016 Rule”). 

The environmental groups filed a timely petition for

review in our court,1 and we have jurisdiction to review the 

2016 Rule for compliance with the Act. See 42 U.S.C. 

§ 7607(b)(1). The American Petroleum Institute (API) moved 

to intervene on behalf of EPA but failed to show the required

Article III standing. See Deutsche Bank National Trust Co. v. 

FDIC, 717 F.3d 189, 193 (D.C. Cir. 2013). API claims

representationalstanding,2 API Mot. to Intervene 6 n.2, but that 

 1 Although an EPA rule previously defined “natural event” in 

2007, the 2016 Rule reopened the issue. See Sierra Club v. EPA, 551 

F.3d 1019, 1024 (D.C. Cir. 2008). 2 An association has standing on behalf of its members when:

“(1) ‘its members would otherwise have standing to sue in their own 

right’; (2) ‘the interests it seeks to protect are germane to the 

organization’s purpose’; and (3) ‘neither the claim asserted nor the 

relief requested requires the participation of individual members in 

the lawsuit.’” Ctr. for Sustainable Econ. v. Jewell, 779 F.3d 588, 596 

(D.C. Cir. 2015) (quoting Hunt v. Wash. State Apple Adver. 

Comm’n, 432 U.S. 333, 343 (1977)).
5

requires “specifically identify[ing] members who have 

suffered the requisite harm,” Chamber of Commerce of the U.S. 

v. EPA, 642 F.3d 192, 199 (D.C. Cir. 2011) (citations and 

internal quotation marks omitted). Nowhere in its motion or 

brief does API identify a single member of its organization or 

support with evidence its vague assertion that an adverse result 

in this case will injure any member. Because API failed to 

establish the constitutional standing required to participate as

an intervenor, we instead grant it the status of amicus curiae. 

See Fed. R. App. P. 29(a); see also Old Dominion Elec. Coop. 

v. FERC, 892 F.3d 1223, 1232-34 (D.C. Cir. 2018); Rio 

Grande Pipeline Co. v. FERC, 178 F.3d 533, 539 (D.C. Cir. 

1999). 

We now consider whether the Act’s exceptional-event 

provision permits EPA to attribute emissions to natural causes 

when they were also caused by regulated human activity. 

II

The Act allows areas to keep their attainment designations 

when their pollutant levels exceed the NAAQS so long as those 

emissions resulted from an exceptional event. An exceptional 

event is one that “affects air quality,” is “not reasonably 

controllable or preventable,” and is “caused by . . . activity that 

is unlikely to recur at a particular location.” 42 U.S.C. 

§ 7619(b)(1)(A). But even a recurring event can be 

“exceptional” if it is a “natural event.” Id. § 7619(b)(1)(A)(iii). 

The Act does not give a definition for “natural event,” so 

EPA defined it in the 2016 Rule:

[A natural event is] an event and its resulting emissions, 

which may recur at the same location, in which human 

activity plays little or no direct causal role. For purposes 
6

of the definition of a natural event, anthropogenic sources 

that are reasonably controlled shall be considered to not 

play a direct role in causing emissions.

81 Fed. Reg. at 68,277 (codified at 40 C.F.R. § 50.1(k)). In 

other words, to determine whether a recurring event is natural,

and thus exceptional, EPA looks at the activities that caused the 

emissions. See id. at 68,232 (explaining that EPA classifies 

events based on the underlying sources of their emissions).

When making this decision, EPA will disregard contributions

to the emissions made by human activities, or “anthropogenic 

sources,” that “are reasonably controlled” by complying with 

emissions regulations.3 As a consequence, an event is natural

if it resulted from at leastsome natural activity and any amount, 

no matter how significant, of reasonably controlled human 

activity. See id. at 68,231 (explaining that an event cannot be 

“natural if all of the event-related emissions originated from 

anthropogenic sources”).

According to the environmental groups, this approach 

stretches the meaning of “natural event” beyond what the text 

of the Act can bear. They concede that, in some circumstances, 

the Act permits EPA to classify an event and its resulting 

emissions as natural even though human activity played a small

role. However, they contend that EPA must count, for the 

purposes of characterizing an event as natural, the role played 

by both types of human activity—that which complies with 

environmental regulations and that which does not.

We review EPA’s definition of natural event under 

Chevron U.S.A. Inc. v. Natural Resources Defense Council, 

Inc., 467 U.S. 837 (1984): “If the Act unambiguously 

 3 In general, “reasonably controlled” means that the human 

activity satisfied the relevant SIP standards. 40 C.F.R. 

§ 50.14(b)(8)(v).
7

authorizes or forecloses EPA’s . . . rule, step one of the 

Chevron analysis requires that we follow Congress’s express 

policy choice. If the Act is unclear on the matter, step two of 

Chevron requires that we defer to EPA’s reasonable 

interpretation.” Sierra Club v. EPA, 536 F.3d 673, 677 (D.C. 

Cir. 2008) (citing Chevron, 467 U.S. at 842-43). 

At step one, we consider whether the Act issued 

unambiguous instructions for distinguishing natural events

from events caused by human activity. Outside its statutory 

context, “natural” ordinarily means something unaffected by 

human activity. See, e.g., Merriam-Webster’s Collegiate 

Dictionary 774 (10th ed. 1997) (defining natural as “growing 

without human care . . . existing in or produced by nature: not 

artificial”); Oxford English Dictionary Online, 

http://www.oed.com/view/Entry/125333 (3d ed. 2003) 

(defining natural as “[f]ormed by nature; not subject to human 

intervention, not artificial”); Black’s Law Dictionary 1048 (7th 

ed. 1999) (defining natural as “[b]rought about by nature as 

opposed to artificial means”). And an ordinary reading of 

“natural event” summons images of natural disasters such as

tornados and volcanic eruptions; cosmic episodes, such as

comets and harvest moons; and organic processes, such as viral 

epidemics and seasonal changes. These examples leave little 

room for human causation.

But what “natural event” means in the Act does not depend 

entirely on its ordinary reading because “the words of a statute 

must be read in their context and with a view to their place in 

the overall statutory scheme.” Nat’l Ass’n of Home Builders v. 

Defenders of Wildlife, 551 U.S. 644, 666 (2007) (internal 

quotation marks omitted); see also Pereira v. Sessions, 138 S. 

Ct. 2105, 2110 (2018) (interpreting the statute in light of its 

statutory context). By pairing “natural event” alongside “an 

event caused by human activity,” the Act uses the phrase as a 
8

tool to separate events into the two categories, requiring it to 

carry a special meaning. Perhaps if EPA had to separate only 

those events caused by either solely natural or solely human 

activity, the ordinary understanding of “natural event” might 

do the trick. But many events are caused by a combination of 

the two. For example, consider a windstorm that sweeps dust 

into the air so that it is emitted as small particulate matter, 

which is subject to the NAAQS. At first blush, the emissions 

appear to be the result of the windstorm and, therefore, a natural 

event. But this is less obvious if the storm swept up the dust 

only because the ground’s surface had been loosened by recent 

construction. See 80 Fed. Reg. at 72,854 n.34. In that case, the 

natural event caused the emissions only because human activity 

changed the landscape. The point at which human 

contributions convert a natural event into one caused by human 

activity is blurry at best.

But EPA must draw that line, and the Act provides little 

guidance beyond establishing that the distinction exists.4 See 

42 U.S.C. § 7619(b)(1)(A)(iii) (allowing an event to be 

exceptional only if it is “natural” or “caused by human activity 

that is unlikely to recur at a particular location”). Many 

possible rules for sorting events may be permissible under the 

statute. Some may be easier to administer than others, but the 

Act leaves the choice to EPA. The statutory language is far 

from unambiguous and is, instead, a classic example of 

Congress leaving a gap for EPA to fill with reasonable 

regulations. See Nat’l Cable & Telecomm. Ass’n v. Brand X 

Internet Servs., 545 U.S. 967, 980 (2005) (“[A]mbiguities in 

statutes within an agency’s jurisdiction to administer are 

delegations of authority to the agency to fill the statutory gap 

in reasonable fashion.”); Chevron, 467 U.S. at 843-44.

 4 The Act also provides broad governing principles, such as the 

supremacy of public health, that could place some limits on EPA’s 

choice of rules. See 42 U.S.C. § 7619(b)(3).
9

It is at Chevron step two that we determine if the 2016 Rule 

“fill[s] the statutory gap in reasonable fashion.” Brand X, 545 

U.S. at 980. The environmental groups think the rule upends 

the statutory distinction between natural events and those 

caused by human activity. They contend it is unreasonable for 

EPA to assume that human activity did not cause an event 

simply because that activity complied with environmental 

regulations. As a consequence, they worry, EPA will treat 

emissions from recurring human activity as emissions from an

exceptional event.

To illustrate their concern, they describe how the 2016 

Rule would apply to a windstorm that blew pollutants emitted 

from a reasonably controlled power plant to another 

jurisdiction’s air-quality monitor, which then registered a much 

higher pollutant concentration than would be typical for that 

area. The environmental groups argue that EPA would consider 

the emissions to be the result of a natural event. Although

human activity produced the emissions, they believe EPA 

would not consider the role played by the power plant because 

it complied with the relevant air-quality regulations. Instead, 

EPA would look to only the windstorm and conclude that the 

event was natural. 

But such an outcome wouldn’t be possible under the 

statutory and regulatory safeguards in place. The 

environmental groups do not challenge EPA’s understanding 

that an “event” must be an occurrence that “deviat[es] from 

normal or expected conditions.” 81 Fed. Reg. at 68,228.

Emissions that result from only “routine” activity cannot be 

treated as exceptional-event emissions. Id.

The environmental groups seem to suggest that emissions 

from “routine” activities could become emissions from an 
10

“event” if they are later affected by unexpected activity. 

However, this misreads the 2016 Rule. “Natural event means 

an event and its resulting emissions,” 40 C.F.R. § 50.1(k) 

(second emphasis added), and EPA has made clear that 

emissions only result from the events that generate them, 81 

Fed. Reg. at 68,226.5 Activities that cause emissions to behave 

in a certain way, such as migrating to new areas or 

concentrating in dangerous amounts, are not events that cause 

the emissions. Id. In the windstorm example, it was the power 

plant and not the windstorm that generated the emissions. 

Although the pollutants only reached the monitor because they 

were carried by the windstorm, they were emitted into the air 

before the wind arrived at the scene. Under the 2016 Rule, 

those emissions would be attributed to the power plant, not the 

natural activity of the windstorm.6

The agency would give a different answer for emissions 

generated by a windstorm that swept up particulate matter from 

a dirt road. In that case, no pollutants would be emitted until 

the wind struck the road and swept particles into the air. If the 

road were reasonably controlled, EPA would discount the 

 5 To the extent the environmental groups are challenging EPA’s 

interpretation of its own regulations, we defer to the agency’s 

understanding. See Auer v. Robbins, 519 U.S. 452, 461 (1997) 

(holding that an agency’s interpretation of its own regulation, unless 

“plainly erroneous or inconsistent with the regulation,” controls 

(internal quotation marks omitted)). Even if the regulation could be 

read to include emissions that have already been generated, EPA’s 

reading is certainly consistent with the 2016 Rule. 

6 This does not mean that emissions produced by human activity 

and transported by wind to a new area can never be the result of an 

exceptional event. It simply means that EPA would treat them as a 

result of human activity and consequently subject to the recurrence 

condition for exceptional events. See, e.g., 81 Fed. Reg. at 68,280 

(discussing “emissions-generating activity that occurs outside of the 

State’s jurisdictional boundaries”). 
11

road’s role and look only to the windstorm. But if the road had 

been improperly maintained, EPA would consider both the 

road’s and the windstorm’s contributions to the emissions. 

We think the 2016 Rule preserves the Act’s distinct 

treatment of natural events. Although we recognize the 

possibility raised, but not demonstrated, by the environmental 

groups that extreme and unforeseen applications of the rule 

might have problematic results, the 2016 Rule still passes 

muster under Chevron step two. The “possibility that the rule, 

in uncommon particular applications, might exceed EPA’s 

statutory authority does not warrant judicial condemnation of 

the rule in its entirety.” EPA v. EME Homer City Generation, 

L.P., 134 S. Ct. 1584, 1609 (2014); see also Babbitt v. Sweet 

Home Chapter of Cmtys. for a Great Or., 515 U.S. 687, 699-

700 (1995) (upholding a rule as reasonable despite possible 

applications that would be impermissible under the statute). If 

EPA applies the rule in a way that the Act would not permit, an 

injured party can petition us to review the agency’s action at 

that time. See EME Homer City, 134 S. Ct. at 1609 (explaining 

that as-applied challenges remain available if EPA were to 

apply a rule, which the Court had upheld under Chevron, in a 

way that was impermissible under the statute); Util. Air 

Regulatory Grp. v. EPA, 885 F.3d 714, 723 (D.C. Cir. 2018). 

For now, we uphold the definition of natural event against the 

environmental group’s facial challenge to the 2016 Rule.

III

We deny the petition for review because the 2016 Rule’s

definition of natural event is permissible under the Act.

So ordered.