FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

NATURAL RESOURCES DEFENSE 

COUNCIL,

Petitioner,

v.

U.S. ENVIRONMENTAL PROTECTION 

AGENCY; MICHAEL S. REGAN, in his 

official capacity as Administrator of 

the United States Environmental 

Protection Agency,

Respondents,

THE HARTZ MOUNTAIN 

CORPORATION,

Respondent-Intervenor.

No. 20-72794

EPA No.

EPA-HQ-OPP2009-0308

OPINION

On Petition for Review of an Order of the

Environmental Protection Agency

Argued and Submitted November 22, 2021

San Francisco, California

Filed April 20, 2022
2 NRDC V. USEPA

Before: Mary H. Murguia, Chief Judge, and R. Guy 

Cole, Jr.* and Ronald M. Gould, Circuit Judges.

Opinion by Judge Gould

SUMMARY**

Pesticides

The panel vacated the U.S. Environmental Protection 

Agency’s (“EPA”) denial of Natural Resources Defense 

Council (“NRDC”)’s petition to cancel the registration of the 

pesticide tetrachlorvinphos (“TCVP”), and remanded for a 

revised EPA response within 120 days.

Under the Federal Insecticide, Fungicide, and 

Rodenticide Act, pesticides sold in the United States 

generally must be registered by the EPA. Private parties can 

petition the EPA to cancel the registration of a pesticide, and 

the EPA is required to resolve those petitions “within a 

reasonable time.” 5 U.S.C. § 555(b). The EPA last approved 

the use of TCVP pesticide in pet products in 2006. In 2009, 

NRDC petitioned the EPA to cancel the registration of 

TCVP for use in household pet products. After repeated 

delays, the EPA eventually denied NRDC’s petition.

* The Honorable R. Guy Cole, Jr., United States Circuit Judge for 

the U.S. Court of Appeals for the Sixth Circuit, sitting by designation.

** This summary constitutes no part of the opinion of the court. It 

has been prepared by court staff for the convenience of the reader.
NRDC V. USEPA 3

The panel held that the EPA’s denial of NRDC’s petition 

was not supported by substantial evidence. The EPA failed 

to provide a reasoned explanation for its denial of NRDC’s 

petition and made several arbitrary calculations. The EPA’s 

errors primarily impacted two calculations central to its 

denial of NRDC’s petition: (1) the amount of TCVP dust 

released by the pet collars, and (2) the assumption that pet 

owners will trim the collars by at least 20%. The EPA 

without any explanation rejected a central finding of a study 

– the Torison Study – that it repeatedly stated was a key to 

its determination. In its brief before this court, the EPA for 

the first time gave a justification for its decision to reject the 

Torison Study’s finding that 97.2% of the dust released from 

the collars comprises TCVP. The panel held that it could 

only uphold agency action based on the reasons the agency 

gave for its decision. The panel held further that the EPA’s 

assumption that only 14.6% of the dust released from the 

collars was TCVP – instead of the Torison Study’s 

measurement of 97.2% – was also troubling on the merits. 

The panel held that it would not defer to the EPA’s highly 

inaccurate calculation that pet owners will trim pet collars 

by 20% when fitting the collar onto a pet’s neck. The panel 

concluded that it was apparent that the EPA’s denial of 

NRDC’s petition was simply not supported by substantial 

evidence when considered on the record as a whole.
4 NRDC V. USEPA

COUNSEL

Peter J. DeMarco (argued) and Aaron Colangelo, Natural 

Resources Defense Council, Washington, D.C.; Ian Fein, 

Natural Resources Defense Council, San Francisco, 

California; for Petitioner.

Gus Maxwell (argued), Attorney; Jean E. Williams, Acting 

Assistant Attorney General; Environment and Natural 

Resources Section, United States Department of Justice, 

Washington, D.C.; Benjamin Wakefield and Erin Koch, 

Attorneys, United States Environmental Protection Agency, 

Washington, D.C.; for Respondents.

Amanda Shafer Berman (argued), Kirsten L. Nathanson, and 

Michael Boucher, Crowell & Moring LLP, Washington, 

D.C., for Respondent-Intervenor.

OPINION

GOULD, Circuit Judge:

The Natural Resources Defense Council (“NRDC”) 

again asks us to intervene in its thirteen-year dispute with the 

Environmental Protection Agency (“EPA”) regarding 

whether EPA is fulfilling its statutory duty to properly assess 

the risks of the pesticide tetrachlorvinphos (“TCVP”). After 

we granted NRDC’s writ of mandamus in April 2020, which 

required EPA to issue a final response to NRDC within 

90 days, EPA denied NRDC’s petition to cancel the 

registration of TCVP. NRDC now argues that EPA erred 

when it denied the petition because EPA failed to provide an 

adequate explanation for its decision and relied on several 

mistaken calculations. Because EPA’s denial of NRDC’s 
NRDC V. USEPA 5

petition lacks substantial evidence, we vacate EPA’s denial 

and remand for a revised EPA response within 120 days.

I. BACKGROUND

Under the Federal Insecticide, Fungicide, and 

Rodenticide Act, pesticides sold in the United States 

generally must be registered by EPA. See 7 U.S.C. 

§ 136a(a). Private parties can petition EPA to cancel the 

registration of a pesticide. 40 C.F.R. § 154.10. EPA is 

required to resolve those petitions “within a reasonable 

time.” 5 U.S.C. § 555(b).

EPA last approved the use of TCVP pesticide in pet 

products in 2006. In 2009, NRDC petitioned EPA to cancel 

the registration of TCVP for use in household pet products. 

TCVP is a type of organophosphate pesticide. 

Organophosphates “were developed from nerve warfare 

agents used during World War II” and “pose recognized 

dangers to the neurodevelopment of children, causing 

reduced cognitive capacity, delays in motor development, 

and behavioral problems.” In re Nat. Res. Def. Council, Inc.

(NRDC 2020), 956 F.3d 1134, 1136 (9th Cir. 2020). Of 

concern here, TCVP is used in pet collars to prevent fleas 

and ticks, and TCVP can be transferred to humans who come 

into contact with pets wearing these collars. TCVP pet 

collars are designed to release TCVP gradually onto pet fur 

in either liquid or dust form. NRDC is concerned that TCVP 

dust, which is more easily transferred to humans through 

contact with pets than TCVP liquid, poses a particular risk 

to young children who are more likely to put their hands in 

their mouths and ingest TCVP after petting an animal 

wearing a TCVP collar. Hartz Mountain Corporation 

(“Hartz”) manufactures pet collars containing TCVP, and 

EPA estimates that collars using alternative pesticides cost 

five to six dollars more a month.
6 NRDC V. USEPA

After filing its petition in 2009, NRDC waited five years 

for a response from EPA until it sought a writ of mandamus 

to compel EPA’s answer in 2014. In November 2014, EPA 

denied NRDC’s petition. NRDC then challenged the denial 

of its petition in this Court as unlawful. A few months later, 

EPA filed a motion for voluntary remand because it was 

completing a new risk assessment that could change its 

position regarding NRDC’s petition. EPA repeatedly 

represented that it “intend[ed] to issue a revised response to 

NRDC’s petition within 90 days after finalizing the [revised] 

risk assessment.” Id. at 1137. Over NRDC’s objections, we 

remanded without a deadline in June 2016. See Order, Nat.

Res. Def. Council v. U.S. EPA, No. 15-70025, ECF No. 30 

(June 9, 2016.

In December 2016, EPA issued a revised risk 

assessment, which found that TCVP exposure could lead to 

health risks for young children (the “2016 Risk 

Assessment”). The 2016 Risk Assessment “recognized that 

children could be exposed to TCVP through contact with 

pets using TCVP products and that such exposure posed 

considerable risks to their health.” NRDC 2020, 956 F.3d 

at 1137. The 2016 Risk Assessment found risks of concern 

for children “regardless of the ratio of liquid/dust assumed.” 

EPA represented that it would reply to NRDC’s petition 

within 90 days and issued a press release stating that it had 

“identified potential risks to people, including children, . . .

which exceed the Agency’s level of concern.” Id. at 1142.

EPA, however, did not issue a response to NRDC’s 

petition within 90 days. Instead, two years after EPA’s selfimposed deadline, NRDC sought a second writ of mandamus 

from us to compel EPA to issue a final response to its 

petition filed a decade earlier. EPA defended its delay by 

stating that it needed additional data from Hartz, specifically 
NRDC V. USEPA 7

the results of a torsion study, which EPA had originally 

requested in 2017. Five days after NRDC filed its second 

request for a writ of mandamus, EPA for the first time 

compelled Hartz to perform the torsion study (the “Torsion 

Study”), which was completed in August 2019.

In April 2020, we granted NRDC’s second petition for a 

writ of mandamus. We reprimanded EPA, stating

[T]he EPA’s years-long delay on this critical 

matter of public health has been nothing short 

of egregious. For more than a decade, the 

EPA has frustrated NRDC’s ability to seek 

judicial review by withholding final agency 

action, all the while endangering the 

wellbeing of millions of children and 

ignoring its core mission of protecting human 

health and the environment.

Id. at 1142–43 (internal citation omitted). We “order[ed] the 

EPA to issue a full and final response to” NRDC within 

90 days. Id. at 1143. Exactly 90 days after our ruling, EPA 

again denied NRDC’s petition.

EPA based its denial on a revised risk assessment (the 

“2020 Risk Assessment”) dated one day before it denied 

NRDC’s petition. The 2020 Risk Assessment concluded 

that all seven TCVP pet collars of Hartz then on the market 

were unacceptably dangerous to human health because they 

had a margin of exposure surpassing the threshold of 

unacceptable risk, known as the Level of Concern (“LOC”). 

In response, Hartz requested voluntary cancellation of one 

of its feline collars and mitigated the exposure of TCVP on 

its six remaining collars by amending their instructions, 

reducing the amount of TCVP in the collars, and 

implementing other design changes. With these changes, 
8 NRDC V. USEPA

EPA determined that Hartz’s remaining and modified pet 

collars did not exceed the LOC and therefore denied 

NRDC’s petition. After Hartz’s modifications, TCVP 

comprised roughly 14.6% of the remaining collars’ weight.

EPA’s denial of NRDC’s petition relied heavily on two 

studies completed by Hartz. First, it relied on the Torsion 

Study, which in part determined that: (1) 0.38% of the pet 

collars’ total mass would be released as dust; 

(2) approximately 97.2% of the dust released from the 

collars consisted of TCVP; and (3) an average of 1.1% of the 

TCVP in the collars was released as dust before any twisting. 

Second, EPA relied on Hartz’s Normal Wear Study, which 

measured the amount of TCVP and non-pesticides released 

by the collars after they were worn by dogs for between one 

and twenty-one days. Researchers wiped the collars after 

each period of use, and then measured the amount of 

released TCVP dust. In the Normal Wear Study, 4% of the 

collars’ total observed weight loss was measured as TCVP 

after three days.

NRDC timely challenged EPA’s denial of its petition.

II. STANDARD OF REVIEW

We review the denial of NRDC’s petition for 

“substantial evidence when considered on the record as a 

whole.” 7 U.S.C. § 136n(b). Substantial evidence is “more

than a mere scintilla but less than a preponderance; it is such

relevant evidence as a reasonable mind might accept as

adequate to support a conclusion.” Nat. Res. Def. Council v.

U.S. EPA (NRDC 2013), 735 F.3d 873, 877 (9th Cir. 2013) 

(quoting Vasquez v. Astrue, 572 F.3d 586, 591 (9th Cir. 

1989)).
NRDC V. USEPA 9

III. DISCUSSION

a. Legal Standard

“It is well-established that an agency’s action must be 

upheld, if at all, on the basis articulated by the agency itself.” 

Id. at 877 (quoting Motor Vehicle Mfrs. Ass’n of the U.S.,

Inc. v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 50 

(1983)). Courts do not “accept appellate counsel’s post-hoc 

rationalizations for agency action.” Nat. Res. Def. Council

v. U.S. EPA (NRDC 2017), 857 F.3d 1030, 1040 (9th Cir. 

2017) (quoting Hernandez-Cruz v. Holder, 651 F.3d 1094,

1109 (9th Cir. 2011)). “If the agency did not meet its burden, 

we ‘should not attempt . . . to make up for such deficiencies’ 

and ‘may not supply a reasoned basis for the agency’s action 

that the agency itself has not given.’” Ctr. for Biological

Diversity v. Haaland, 998 F.3d 1061, 1067 (9th Cir. 2021) 

(quoting State Farm, 463 U.S. at 43).

Administrative law does not require agency perfection. 

The agency’s decision, even if of “less than ideal clarity,” 

will be upheld “if the agency’s path may reasonably be 

discerned.” State Farm, 463 U.S. at 43. It is fundamental 

that the agency must have “considered the relevant factors 

and articulated a rational connection between the facts found 

and the choices made.” Ctr. for Biological Diversity, 

998 F.3d at 1067 (quoting Alaska Oil & Gas Ass’n v.

Pritzker, 840 F.3d 671, 675 (9th Cir. 2016)). 

“Unsubstantiated” or “bare assumptions” will not be 

credited. NRDC 2017, 857 F.3d at 1038, 1042; see also 

Nat’l Parks Conservation Ass’n v. U.S. EPA, 788 F.3d 1134, 

1143 (9th Cir. 2015) (“[U]nexplained assertions . . . 

unsupported by any explained reasoning” were “arbitrary 

and capricious”); Greater Yellowstone Coal., Inc. v.

Servheen, 665 F.3d 1015, 1020 (9th Cir. 2011) (requiring 

agency to “articulate a rational connection between the data 
10 NRDC V. USEPA

in the record and its determination”). Conversely, a “mere[] 

. . . reasonable basis for disagreement” about the evidence 

will not disturb an agency’s otherwise reasonable findings. 

NRDC 2013, 735 F.3d at 880; see also ASARCO, Inc. v.

Occupational Safety & Health Admin., 746 F.2d 483, 490 

(9th Cir. 1984).

Courts give special deference to agency interpretations 

of scientific issues. NRDC 2017, 857 F.3d at 1036 (internal

citation omitted) (“When, as in this case, the agency is

making predictions, within its area ofspecial expertise, at the

frontiers of science . . . a reviewing court must generally be

at its most deferential.”); United States v. Alpine Land &

Reservoir Co., 887 F.2d 207, 213 (9th Cir. 1989). Even so, 

we do not credit “arbitrary and highly inaccurate 

calculations.” Trs. of Cal. State Univ. v. Riley, 74 F.3d 960, 

967 (9th Cir. 1996), as amended (Feb. 20, 1996).

b. Substantial evidence does not support EPA’s denial

of NRDC’s Petition

For the following reasons, we conclude that EPA’s 

denial of NRDC’s petition is not supported by substantial 

evidence. EPA failed to provide a reasoned explanation for 

its denial of NRDC’s petition and made several arbitrary 

calculations. EPA’s errors primarily impact two calculations 

central to its denial of NRDC’s petition: (1) the amount of 

TCVP dust released by the collars, and (2) the assumption 

that pet owners will trim the collars by at least 20%. Further, 

we cannot consider EPA’s post-hoc rationalizations, which 

themselves suffer from numerous flaws.

i. The amount of TCVP dust released by the collars

When calculating the amount of TCVP dust released by 

Hartz’s pet collars, EPA relied on only some of the Torsion 
NRDC V. USEPA 11

Study’s findings. In particular, EPA accepted the Torsion 

Study’s finding that 0.38% of the collars’ total weight was 

released as dust (and the rest released as liquid). However, 

EPA did not use one of the Torsion Study’s other main 

findings: that about 97.2% of the dust released from the 

collars consisted of TCVP. Instead, EPA assumed that the 

dust lost by the collars contained only 14.6% TCVP—an 

amount equivalent to the percent of TCVP in the collars as a 

whole. This second assumption, which directly contradicted 

the results of the Torsion Study and was unexplained, had 

the effect of significantly lowering EPA’s estimate of how 

much TCVP dust was released by the collars.

EPA’s selective use of the Torsion Study is problematic 

for several reasons. Before its denial of NRDC’s petition—

including in its briefing to us—EPA repeatedly emphasized 

the importance of the Torsion Study, variously referring to it 

as the “best means,” the “optimal method,” and “most 

promising solution” for determining the physical form of the 

TCVP released from the collars.1

 In assessing the study, 

EPA further noted that “the purpose of the [Torsion Study] 

was to measure the amount of solids (dust/powder) released 

from a flea and tick collar and the active ingredient content 

1 See EPA’s Opp. to NRDC Pet. for a Writ of Mandamus, NRDC v. 

U.S. EPA, No. 19-71324, ECF No. 13 (Sept. 9, 2019), 15 (“EPA has 

concluded that the best means of determining the form of TCVP released 

from the collars is through a mechanical torsion study where the collar 

is twisted and stretched repeatedly to exaggerate the movement of the 

collar against the animal.”), 16 (“[T]he Agency continues to believe that 

a mechanical torsion study is needed.”), 20 (“[T]he Agency has 

determined that the optimal method for reducing the uncertainty relating 

to the physical form of TCVP is to require the registrant to conduct a 

composition study in the form of a mechanical torsion test.”), Reaves 

Decl. ¶ 23 (“EPA has determined that the most promising solution for 

identifying the physical form of TCVP released from each pet collar is 

to require the registrant to conduct . . . a mechanical torsion study.”).
12 NRDC V. USEPA

of the released solids when the collar is exposed to 

mechanical torsion and stress.” However, in sharp contrast 

to its earlier statements, in its denial of NRDC’s petition and 

in the 2020 Risk Assessment, EPA provides no explanation 

for why it rejected the Torsion Study’s calculation of TCVP 

dust released from the collars. This is despite its earlier 

statements that the Torsion Study was the best method for 

measuring the form of TCVP released by the collars. 

Without any explanation, EPA’s rejection of a central 

finding of a study it repeatedly stated was key to its 

determination is troubling. See NRDC 2013, 735 F.3d 

at 877.

In its brief, EPA for the first time gives a justification for 

its decision to reject the Torsion Study’s finding that 97.2% 

of the dust released from the collars comprises TCVP. EPA 

now claims that it rejected this measurement because the 

Torsion Study used “exaggerated twisting conditions . . .

[which] likely overrepresented the amount of [TCVP] in dust 

extruded from the collars in normal use on pets.” Even 

assuming EPA’s current rationale withstands scrutiny, it is 

simply not contained in its denial of NRDC’s petition or in 

its 2020 Risk Assessment. The law is clear that we can only 

uphold agency action based on the reasons the agency gave 

for its decision. See SEC v. Chenery Corp., 332 U.S. 194, 

196 (1947) (“[A] reviewing court, in dealing with a 

determination or judgment which an administrative agency 

alone is authorized to make, must judge the propriety of such 

action solely by the grounds invoked by the agency.”); Fed.

Power Comm’n v. Texaco Inc., 417 U.S. 380, 397 (1974).

Further casting doubt on its position, EPA’s brief 

repeatedly argues that the Normal Wear Study provided 

more reliable information about the amount of TCVP dust 

that would be shed by the collars, citing this study 
NRDC V. USEPA 13

approximately 20 times. Again, though, EPA’s reliance on 

the Normal Wear Study is new to its briefing: EPA only cited 

the Normal Wear Study once (as part of a string cite in a 

footnote with no explanation) in its petition denial and 2020 

Risk Assessment. EPA’s newly-discovered explanations 

appear to be “post-hoc rationalizations” that cannot support 

a finding of substantial evidence. See NRDC 2017, 857 F.3d 

at 1040 (quoting Hernandez-Cruz, 651 F.3d at 1109).

EPA’s assumption that only 14.6% of the dust released 

from the collars is TCVP—instead of the Torsion Study’s 

measurement of 97.2%—is also troubling on the merits for 

several reasons. First, EPA ignored obvious evidence 

suggesting its assumption is an underestimate. The TCVP 

in the collar is specifically designed to be released; other 

parts of the collar, like the plastic buckle, are unlikely to 

release dust. So, it seems reasonable to expect that TCVP 

will make up a disproportionate amount of the dust released 

by collars. Further, the Normal Wear Study which EPA now 

praises, showed that after only three weeks of use the percent 

of TCVP remaining in the collars had decreased by nearly 

half, while the total weight of the collars had declined by 

much less than half. This again strongly supports the 

common-sense conclusion that the collars would likely 

release a disproportionate amount of TCVP.

Second, EPA does not explain why, if it believed that the 

Torsion Study exaggerated the amount of TCVP released by 

the collars, it still relied on the Torsion Study’s other finding 

regarding the total amount of dust released from the collars. 

If we were to accept EPA’s reasoning, both measurements 

should be suspect: they should be equally affected by the 

Torsion Study’s supposedly exaggerated twisting.

Third, in its brief EPA seeks to buttress its assumption 

that only 14.6% of the dust released from collars is TCVP by 
14 NRDC V. USEPA

pointing to the Normal Wear Study’s finding that only 4% 

of the collars’ total observed weight loss was measured as 

TCVP after three days. According to EPA, this result 

“validated” its assumption that the Torsion Study 

exaggerated the amount of TCVP in the dust. Once again, 

EPA’s rationale is not contained in its petition denial and is 

at best an improper post-hoc rationalization. See NRDC

2017, 857 F.3d at 1040. Further, EPA’s reasoning suffers 

from the obvious flaw that the Normal Wear Study only 

measured the composition of fifteen percent of the total 

observed weight loss; thus, EPA assumes that the remaining 

uncollected 85% of the observed weight loss would not 

change its analysis.

Another display of irrationality shown by EPA’s use of 

the Torsion Study is its assumption that only 0.38% of the 

TCVP in the collars will be released as dust. However, the 

Torsion Study undermines this assumption because it found 

that an average of 1.1% of the TCVP on the collars—roughly 

three times as high as EPA’s estimate—was released as dust 

before any twisting occurred.

ii. Assumption that owners will trim pet collars by 

20%

EPA also assumed that pet owners will always trim the 

collars by at least 20% when fitting the collar onto a pet’s 

neck. This assumption reduces the expected amount of 

TCVP to which individuals would be exposed: all else equal, 

less collar means less TCVP.2

 EPA’s assumption that 

2 In its brief, EPA argues that because longer collars are more likely 

to be used on larger pets, longer collars disperse collars’ TCVP across a 

greater surface area of fur and actually reduce TCVP exposure in 

humans. However, even if larger pets disperse collars’ TCVP across a 
NRDC V. USEPA 15

owners will trim 20% of collars is drawn from a Hartz 

laboratory study of a different type of collar which does not 

contain TCVP. In this study (the “Efficacy Study”), 

researchers trimmed 20–43% of collars when fitting them to 

dogs’ necks to test the collars’ efficacy at protecting pets 

from ticks, fleas, and mosquitos. EPA assumed that this 

study was applicable to consumers using Hartz’s pet collars. 

And, “to provide a conservative assumption of how much 

collar might be removed during use,” used the lowest figure 

that the researchers trimmed from collars in the Efficacy 

Study (i.e. 20%).

EPA’s assumption that pet owners will remove 20% of 

collars reversed its earlier assumption: when preparing its 

2016 Risk Assessment, EPA noted that because it could not 

determine the amount of collars owners would trim, it 

assumed that owners would not trim any of the collar. EPA’s 

assumption that owners would trim 20% of the collars also 

conflicts with its 2012 Standard Operating Procedures for 

Residential Pesticide Exposure Assessment, which states 

that “[b]ecause the trimmed length and corresponding active 

ingredient loss cannot be determined, the maximum 

application rate of the [pet] collar as labeled should be 

assumed for assessment of post-application risk.” Far from 

providing a sound justification for departing from its 

previous assumption and standard operating procedure, 

though, EPA’s denial of NRDC’s petition blandly stated that 

“[a]ccounting for the percentage of the pet collar removed is 

believed to better represent typical usage of the product as it 

is fit to the treated animal.” EPA provides no explanation 

for its reliance on the Efficacy Study, which did not use 

TCVP pet collars and did not mimic consumer behavior. 

greater surface area of fur, an untrimmed collar would disperse more 

TCVP on a pet than a collar trimmed by 20% on that same pet.
16 NRDC V. USEPA

Nor did EPA explain how the results of the Efficacy Study, 

which only examined medium-sized dogs, could inform how 

pet owners are likely to use TCVP pet collars for dogs of all 

sizes and cats. EPA’s reliance on this assumption is further 

undermined by Hartz’s own representations. Hartz 

advertises that at least one of its collars fits dogs with necks 

that measure up to 26 inches, but the longest collar it sells is 

only 27 inches. In other words, it would be impossible for 

the owner of a dog with a 26-inch neck to remove 20% of 

the collar. We do not defer to such a “highly inaccurate” 

calculation. See Riley, 74 F.3d at 967.

iii. Lack of substantial evidence

EPA’s denial of NRDC’s petition suffers from numerous 

flaws. EPA does not explain its selective use of the Torsion 

Study, instead relying now on post-hoc rationalizations 

which we do not consider and which suffer from obvious 

flaws. See Ctr. for Biological Diversity, 998 F.3d at 1068

(“[A]n agency must provide its ‘reasoned explanation’ in a 

form that can adequately be examined on judicial review, not 

simply present arguments in its briefing how the decision 

might have been reached.”). EPA’s selective use of the 

Torsion Study reveals further inconsistences and 

weaknesses in its decision to deny NRDC’s petition. 

Further, and very importantly, many justifications now 

urged by EPA are not given as a part of its basis for its 

regulatory decision. We cannot consider contentions raised 

in EPA’s briefs that were not given previously as the basis 

for EPA’s decision. Finally, EPA’s decision to abandon its 

own guidance regarding how much pet owners will remove 

from a collar, without a discernable rationale, and instead 

rely on an assumption from a different study which is clearly 

inapplicable to at least some of Hartz’s pet collars, is not a 

viable basis for EPA’s decision. See Riley, 74 F.3d at 967. 
NRDC V. USEPA 17

In combination, it is apparent to us that EPA’s denial of 

NRDC’s petition is simply not supported by “substantial 

evidence when considered on the record as a whole.” See

7 U.S.C. § 136n(b).3, 4

IV. CONCLUSION

At times, NRDC’s efforts to receive a reasoned response 

from EPA have seemed Sisyphean as the agency consistently 

delayed its decision. After NRDC had doggedly pursued this 

matter for more than a dozen years, when EPA finally did 

reach a decision under pressure of a mandamus order of our 

court, NRDC was justified in expecting a rational, 

supported, and reasoned response from EPA. EPA, though, 

did not provide a well-reasoned or reasonable decision. 

Instead, its stated reasons were cursory and often at odds 

with EPA’s own prior assumptions and statements. Then, in 

response to this lawsuit by NRDC, EPA has provided many 

arguments in its briefing that were never given in the record 

as the basis for the administrative decision. Because EPA’s 

denial of NRDC’s petition is not supported by substantial 

evidence, we VACATE EPA’s denial of NRDC’s petition 

3 Because we grant NRDC’s requested relief without considering the 

Declaration of Miriam Rotkin-Ellman (which was attached to NRDC’s 

Opening Brief), we do not consider whether this declaration was 

properly before us.

4 Hartz and EPA have demonstrated compelling reasons to maintain 

under seal only the Efficacy Study and the Petting Study; conversely, 

they have not met their burden to overcome the presumption in favor of 

open access to court records for the Torsion Study and the Normal Wear

Study. See Ctr. for Auto Safety v. Chrysler Grp., LLC, 809 F.3d 1092, 

1096 (9th Cir. 2016). We order the unsealing of the Torsion Study and 

the Normal Wear Study.
18 NRDC V. USEPA

and REMAND to EPA to issue a revised response to 

NRDC’s petition within 120 days.

VACATED AND REMANDED.