                     FOR PUBLICATION

    UNITED STATES COURT OF APPEALS
         FOR THE NINTH CIRCUIT

 NATURAL RESOURCES DEFENSE                         No. 12-70268
 COUNCIL,
                      Petitioner,

                     v.                              OPINION

 UNITED STATES ENVIRONMENTAL
 PROTECTION AGENCY,
                      Respondent,

 HEIQ MATERIALS AG,
            Respondent-Intervenor.


          On Petition for Review of an Order of the
             Environmental Protection Agency

                   Argued and Submitted
        January 16, 2013—San Francisco, California

                    Filed November 7, 2013

  Before: Jerome Farris and Jay S. Bybee, Circuit Judges,
           and Lynn S. Adelman, District Judge.*

                Opinion by Judge Bybee;
Partial Concurrence and Partial Dissent by Judge Adelman

 *
   The Honorable Lynn S. Adelman, District Judge for the U.S. District
Court for the Eastern District of Wisconsin, sitting by designation.
2                          NRDC V. EPA

                           SUMMARY**


    Federal Insecticide, Fungicide, and Rodenticide Act

    The panel granted in part and denied in part a petition for
review of a decision of the Environmental Protection Agency
granting an application for conditional registration of two
pesticides, AGS-20 and AGS-20 U, that applicant HeiQ
Materials sought to apply to manufactured textiles such as
clothing, blankets, and carpet.

    The Federal Insecticide, Fungicide, and Rodenticide Act
prohibits the sale of any pesticide that has not been
“registered” with the Environmental Protection Agency.
AGS-20 and AGS-29U (collectively, “AGS-20) uses
nanosilver to suppress the growth of microbes that cause
odors, stains, discoloration, and degradation. The EPA
conducted a risk assessment of AGS-20 that was published in
its decision granting HeiQ’s application for conditional
registration.

    The panel first held that petitioner Natural Resources
Defense Council, Inc. had Article III standing to challenge
the EPA’s conditional registration of AGS-20. Second, the
panel held that substantial evidence supported the EPA’s
decision to use the characteristics of toddlers rather than
infants in determining whether AGS-20 placed consumers at
risk. Third, the panel vacated the EPA’s decision insofar as
it concluded that there was no risk concern requiring
mitigation for short- and intermediate-term aggregate oral

  **
     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. EPA                          3

and dermal exposure to textiles that are surface-coated with
AGS-20. Fourth, the panel held that substantial evidence
supported the EPA’s decision not to consider additional
sources of exposure to nanosilver other than AGS-20 in
concluding that the product would not have adverse effects on
consumers.

    District Judge Adelman concurred in the judgment insofar
as it granted the petition in part and remanded to the EPA,
and dissented from the judgment insofar as it denies the
petition in part. Judge Adelman would grant the petition for
review in full.


                        COUNSEL

Catherine Marlantes Rahm (argued) and Nancy S. Marks,
Natural Resources Defense Council, New York, New York;
Mae C Wu, Natural Resources Defense Council, Washington,
D.C., for Petitioner.

Matthew B. Henjum (argued), United States Department of
Justice, Environment and Natural Resources Division,
Environmental Defense Section, Washington, D.C.; Ignacia
S. Moreno, Assistant Attorney General, Environment and
Natural Resources Division, Washington, D.C.; Amber
Aranda, Of Counsel, United States Environmental Protection
Agency, Office of General Counsel, Washington, D.C., for
Respondent.
4                     NRDC V. EPA

Benjamin G. Shatz (argued) and Benjamin M. Kleinman,
Manatt, Phelps & Phillips, LLP, Los Angeles, California;
James G. Votaw, Manatt, Phelps & Phillips, LLP,
Washington, D.C., for Intervenor.


                        OPINION

BYBEE, Circuit Judge:

    The Federal Insecticide, Fungicide, and Rodenticide Act
(“FIFRA”) prohibits the sale of any pesticide that has not
been “registered” with the Environmental Protection Agency
(“EPA”). 7 U.S.C. § 136a(a). Through the registration
requirement, EPA ensures that no pesticides that will cause
“unreasonable adverse effects” on human health or the
environment are sold in the United States. See id. This case
involves EPA’s conditional registration of two pesticides,
AGS-20 and AGS-20 U (collectively “AGS-20”), that
applicant-intervenor HeiQ Materials AG seeks to apply to
manufactured textiles such as clothing, blankets, and carpet.
AGS-20 uses nanosilver—that is, extremely small particles of
silver—to suppress the growth of microbes that cause odors,
stains, discoloration, and degradation. After receiving
comments from the public, EPA conducted a risk assessment
of AGS-20 that it published in its decision document, where
it granted HeiQ’s application for conditional registration.
Natural Resources Defense Council, Inc. (“NRDC”) petitions
us to vacate EPA’s decision to conditionally register AGS-20.
We have jurisdiction under 7 U.S.C. § 136n(b).

   This case presents four issues. First, we must determine
whether NRDC has Article III standing. We hold that it does.
Second, NRDC contends that EPA erred in deciding not to
                       NRDC V. EPA                           5

use the body weight and other characteristics of infants in
determining whether AGS-20 places consumers at risk. In its
risk assessment, EPA used the characteristics of a three-year-
old toddler rather than an infant because it concluded that
toddlers are the subpopulation that is most vulnerable to
exposure to AGS-20. We deny NRDC’s petition with respect
to this claim because EPA’s decision is supported by
substantial evidence. Third, EPA’s risk assessment sets out
a rule whereby there is a risk concern requiring mitigation if
the “margin of exposure” to AGS-20 in the short- or
intermediate-term is less than or equal to 1,000. EPA
analyzed the possible level of exposure to AGS-20 under a
number of different circumstances. One of the scenarios
involves a consumer who experiences both dermal and oral
contact with AGS-20 applied to a textile as a surface coating.
In this single scenario, EPA calculated a “margin of
exposure” of 1,000. Based on EPA’s own rule, this finding
presents a risk concern. We vacate EPA’s decision insofar as
it concludes that there is no risk concern requiring mitigation
for short- and intermediate-term aggregate oral and dermal
exposure to textiles that are surface coated with AGS-20.
Fourth, NRDC faults EPA for deciding not to consider
additional sources of exposure to nanosilver other than AGS-
20 in concluding that the product will not have unreasonable
adverse effects on consumers. We deny NRDC’s petition on
this point because EPA’s decision is supported by substantial
evidence.

                               I

    FIFRA permits EPA to conditionally register a pesticide
like AGS-20 that contains a new active ingredient until the
agency receives sufficient data from an applicant such as
HeiQ to decide whether to issue an unconditional
6                         NRDC V. EPA

registration.1 See 7 U.S.C. § 136a(c)(7)(C). Under FIFRA’s
conditional registration provision:

        The Administrator may conditionally register
        a pesticide containing an active ingredient not
        contained in any currently registered pesticide
        for a period reasonably sufficient for the
        generation and submission of required data
        (which are lacking because a period
        reasonably sufficient for generation of the
        data has not elapsed since the Administrator
        first imposed the data requirement) on the
        condition that by the end of such period the
        Administrator receives such data and the data
        do not meet or exceed risk criteria enumerated
        in regulations issued under this subchapter,
        and on such other conditions as the
        Administrator may prescribe. A conditional
        registration under this subparagraph shall be
        granted only if the Administrator determines
        that use of the pesticide during such period
        will not cause any unreasonable adverse effect
        on the environment, and that use of the
        pesticide is in the public interest.

Id. In this case, NRDC contends that EPA erred in
determining that “use of the pesticide . . . will not cause any
unreasonable adverse effect on the environment” while HeiQ


    1
    EPA requested that HeiQ classify AGS-20 as a pesticide with a “new
active ingredient” in its application after it consulted with the FIFRA
Scientific Advisory Panel and concluded that “the nanosilver active
ingredient in AGS-20 differed from the active ingredients in currently
registered silver-based antimicrobial products.”
                          NRDC V. EPA                                7

collects the data requested by EPA. Id. Notably, the phrase
“unreasonable adverse effects on the environment” includes
“any unreasonable risk to man or the environment, taking into
account the economic, social, and environmental costs and
benefits of the use of any pesticide.” 7 U.S.C. § 136(bb)
(emphasis added).

    Pursuant to this statutory framework, EPA conditionally
registered AGS-20. The product is an antimicrobial powder
that includes nanosilver particles measuring between one and
ten nanometers in diameter.2 The particles slowly release
silver ions that suppress the growth of bacteria that cause
odors, stains, and similar damage to textiles such as clothing.
AGS-20 can either be incorporated into textile fibers prior to
the textile manufacturing process or applied as a surface
coating to finished textiles.

    Before conditionally registering AGS-20, EPA assessed
the risks that the product might pose to workers who apply it
to textiles, consumers who use goods treated with AGS-20,
and the environment. NRDC’s challenges focus on the
effects on consumers. EPA’s assessment assumes that the
consumer is a three-year old because it deemed toddlers to be
the subpopulation that is most vulnerable to the possible
harmful effects of the product. Among other things, EPA
examined a hypothetical toddler’s potential (1) dermal, (2)
oral, and (3) aggregate dermal and oral exposure to AGS-20.
The measure of aggregate exposure analyzed the possible
effects of AGS-20 on a three-year old who was both wearing
and mouthing (i.e., chewing or sucking on) clothing or


  2
   As a point of reference, a sheet of paper is approximately 100,000
nanometers thick. Size of the Nanoscale, Nano.gov, http://www.nano.gov/
nanotech-101/what/nano-size (last visited Sept. 23, 2013).
8                       NRDC V. EPA

another item treated with AGS-20. The resolution of this
case turns on the assumptions used and the calculations
performed by EPA in determining that the aggregate exposure
measure does not indicate that AGS-20 poses an unreasonable
risk to consumers.

                               II

    EPA’s conditional registration of AGS-20 “shall be
sustained if it is supported by substantial evidence when
considered on the record as a whole.” 7 U.S.C. § 136n(b).
“Substantial evidence means 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.” Vasquez v. Astrue, 572 F.3d 586, 591 (9th Cir.
2008) (quoting Andrews v. Shalala, 53 F.3d 1035, 1039 (9th
Cir. 1995)) (internal quotation marks omitted). “Under the
substantial evidence standard, we must affirm the
Administrator’s finding ‘where there is “such relevant
evidence as a reasonable mind might accept as adequate to
support a conclusion” even if it is possible to draw two
inconsistent conclusions from the evidence.’” Nw. Food
Processors Ass’n v. Reilly, 886 F.2d 1075, 1079–80 (9th Cir.
1989) (quoting St. Elizabeth Cmty. Hosp. v. Heckler, 745 F.2d
587, 592 (9th Cir. 1984)). When, as in this case, the agency
“is making predictions, within its area of special expertise, at
the frontiers of science . . . a reviewing court must generally
be at its most deferential.” Baltimore Gas & Elec. Co. v.
NRDC, 462 U.S. 87, 103 (1983).

     Although we must give due deference to EPA’s findings,
“It is well-established that an agency’s action must be upheld,
if at all, on the basis articulated by the agency itself.” Motor
Vehicle Mfrs. Ass’n of the U.S., Inc. v. State Farm Mut. Auto.
                        NRDC V. EPA                           9

Ins. Co., 463 U.S. 29, 50 (1983) (citations omitted); see also
Safe Air for Everyone v. EPA, 488 F.3d 1088, 1091 (9th Cir.
2007) (“[O]ur review of an administrative agency’s decision
begins and ends with the reasoning that the agency relied
upon in making that decision.”).

                              III

    As a threshold matter, we hold that NRDC has standing
to challenge the EPA’s conditional registration of AGS-20.
In order to satisfy Article III’s standing requirements, a
petitioner must demonstrate that “(1) it has suffered an ‘injury
in fact’ that is (a) concrete and particularized and (b) actual
or imminent, not conjectural or hypothetical; (2) the injury is
fairly traceable to the challenged action of the defendant; and
(3) it is likely, as opposed to merely speculative, that the
injury will be redressed by a favorable decision.” Friends of
the Earth, Inc. v. Laidlaw Envtl. Servs. (TOC), Inc., 528 U.S.
167, 180–81 (2000) (citation omitted).

     EPA argues that NRDC’s members do not face an injury
that is “actual or imminent” as opposed to “conjectural or
hypothetical.” EPA’s decision to conditionally register AGS-
20 increases the threat of future harm to NRDC’s members.
Absent EPA’s authorization, there is roughly no chance that
the children of NRDC members will be exposed to AGS-20.
Conditional registration of the product increases the odds of
exposure. As with many Article III standing cases, the
threatened harm “is by nature probabilistic.” Cent. Delta
Water Agency v. United States, 306 F.3d 938, 948 (9th Cir.
2002) (internal quotation and citation omitted). Our goal in
these cases is to ensure that the concept of “actual or
imminent” harm is not “stretched beyond its purpose, which
is to ensure that the alleged injury is not too speculative for
10                      NRDC V. EPA

Article III purposes.” Lujan v. Defenders of Wildlife,
504 U.S. 555, 564 n.2 (1992). We have consistently held that
an injury is “actual or imminent” where there is a “credible
threat” that a probabilistic harm will materialize. See
Covington v. Jefferson Cnty., 358 F.3d 626, 641 (9th Cir.
2004) (“A credible threat of risks to [plaintiffs’] home yields
a loss of enjoyment of property. That is enough for injury in
fact. . . .”); Cent. Delta Water Agency, 306 F.3d at 950
(“[W]e agree with those circuits that have recognized that a
credible threat of harm is sufficient to constitute actual injury
for standing purposes. . . .”); Hall v. Norton, 266 F.3d 969,
976 (9th Cir. 2001) (“[E]vidence of a credible threat to the
plaintiff’s physical well-being from airborne pollutants falls
well within the range of injuries to cognizable interests that
may confer standing.”).

    NRDC has carried its burden to demonstrate that there is
a “credible threat” that its members’ children will be exposed
to AGS-20 as a consequence of the EPA’s decision to
conditionally register the product. The ubiquity of textiles
and the lack of public information concerning the chemical
treatments applied to them during the manufacturing process
would combine to make it nearly impossible for NRDC
members to eliminate AGS-20-treated textiles from their
children’s lives, particularly in light of the expansive scope of
permissible applications of AGS-20 acknowledged by EPA.
NRDC’s members cannot reasonably assure that the carpets
at the daycare center, the jackets worn by a caretaker, or the
seats on the school bus have not been treated with AGS-20.

    The potentially extensive applications of AGS-20 and the
inability of NRDC’s members to fully control their children’s
exposure to the product distinguish this case from the two
cases relied on most heavily by EPA: City of Los Angeles v.
                         NRDC V. EPA                            11

Lyons, 461 U.S. 95 (1983), and Coalition for Mercury-Free
Drugs v. Sebelius, 671 F.3d 1275 (D.C. Cir. 2012). In Lyons,
the probability that the plaintiff would be exposed to the risk
of harm at issue was low because he was unlikely to have
another encounter with a member of the Los Angeles Police
Department that would lead to an officer administering an
allegedly injurious chokehold. Lyons, 461 U.S. at 105–06.
The contingency in this case—whether the children of NRDC
members will sustain dermal and oral contact with AGS-20-
treated textiles—is far more likely to materialize given the
wide range of proposed uses of the product. In Coalition for
Mercury-Free Drugs, the plaintiffs asked the Food and Drug
Administration (“FDA”) to ban the use of thimerosal—a
mercury-based preservative—in vaccines.                Coal. for
Mercury-Free Drugs, 671 F.3d at 1276–77. FDA denied the
plaintiffs’ petition, and they filed suit in district court. Id. at
1277. The district court dismissed the suit because the
plaintiffs lacked standing, and the court of appeals affirmed.
Id. The plaintiffs themselves did not intend to receive
vaccines with thimerosal, which is a realistic objective
because mercury-free versions of all essential vaccines are
readily available. Id. at 1280. Thus, the plaintiffs could not
show “that they face a ‘certainly impending,’ or even likely,
risk of future physical injury from thimerosal in vaccines.”
Id. By contrast, it is far more difficult for parents to ensure
that their children are not exposed to textiles treated with a
particular coating than to verify the type of vaccines that their
children receive. Exposure to common textiles is a far greater
risk for the public than exposure to particular vaccines. In
short, this case differs from Lyons because the probability of
exposure to the risk of harm is quite high, and it differs from
Coalition for Mercury-Free Drugs because the probability
that NRDC’s members will be able to avoid exposing their
children to the risk of harm is quite low.
12                          NRDC V. EPA

    We conclude that NRDC has shown a “credible threat”
that its members’ children will be exposed to AGS-20,
meaning that the purported injury is “actual or imminent.”
Neither Supreme Court precedent nor the out-of-circuit cases
relied on by EPA dictate that we hold otherwise. NRDC has
standing to sue under Article III and so we must proceed to
consider the merits of its petition.

                                    IV

A. EPA’s Decision to Use the Characteristics of Toddlers
   Rather Than Infants in its Risk Assessment of AGS-20

    We begin with NRDC’s challenge to EPA’s decision to
use the characteristics of three-year olds rather than infants in
its risk assessment on the grounds that they are the
subpopulation that is most vulnerable to AGS-20 exposure.3

 3
    NRDC did not forfeit its argument that EPA should have analyzed the
effects of AGS-20 on infants. In its comments to EPA, NRDC indicated
its position that the EPA must analyze both toddlers and infants. Its
September 10, 2010 comment states that “some mouthing of the material
is highly likely to occur with infants and young children who come into
contact with the clothing. . . . The special considerations of the impact of
these exposures on children and infants must be incorporated into EPA’s
assessment of these unique materials.” NRDC’s reference to infants—as
distinct from older children—was sufficient to notify EPA of the possible
need to consider the effects of AGS-20 on infants. See Lands Council v.
McNair, 629 F.3d 1070, 1076 (9th Cir. 2010) (“[A] claimant need not
raise an issue using precise legal formulations, as long as enough clarity
is provided that the decision maker understands the issue raised.”);
Portland Gen. Elec. Co. v. Bonneville Power Admin., 501 F.3d 1009, 1024
(9th Cir. 2007) (“[W]e will not invoke the waiver rule in our review of a
notice-and-comment proceeding if an agency has had an opportunity to
consider the issue.”). NRDC’s admittedly brief reference to the fact that
mouthing of material is especially likely to occur with infants—and not
just young children—also “show[ed] why the mistake was of possible
                          NRDC V. EPA                              13

We hold that EPA’s decision is supported by substantial
evidence.

    As part of its risk assessment, EPA must determine which
subpopulation is most vulnerable to the pesticide at issue. If
the pesticide does not pose risk concerns for the most
vulnerable subpopulation, then it is presumably safe for less
vulnerable groups as well. Here, EPA used the body weight
of an average three-year old in calculating the dermal and oral
exposure to AGS-20 of a child who wears and mouths
clothing treated with the product. NRDC asserts that EPA
should have used the body weight of an infant. EPA’s
decision to use the weight of a three-year old—thereby
implying that toddlers rather than infants are the most
vulnerable subpopulation—is supported by substantial
evidence because it is consistent with EPA’s practices and the
specific behaviors most likely to cause exposure to AGS-20.

    The agency’s guidelines state that past assessments of the
effects of dermal and non-dietary oral exposure to a pesticide
have found that toddlers between three and five years old are
the most vulnerable subpopulation. By using the body weight
of an average three-year old, the agency selected the
lightest—and hence most vulnerable—member of the
subpopulation specified in its guidelines. This is wholly
consistent with the agency’s general approach: it routinely
uses the body weight of three-year olds to evaluate the risk of
dermal and oral exposure to pesticides such as AGS-20 that
are applied to textiles.



significance in the results.” Vermont Yankee Nuclear Power Corp. v.
NRDC, 435 U.S. 519, 553 (1978) (internal quotation marks and citations
omitted).
14                      NRDC V. EPA

    In evaluating the specific risks posed by AGS-20, EPA
cited a study that considered the extent to which surface-
treated textiles shed small particulates when laundered. The
study provided a useful baseline for EPA’s analysis of
whether nanosilver ions might be released when a child
chews on a textile that has been surface-treated with AGS-20.
The agency explained that while the study “does not exactly
simulate wearing or chewing on textiles, because it involved
aggressive conditions potentially resulting in greater release
than might otherwise occur during chewing on or wearing
textiles, it does provide a reasonable first estimate for the
amount [of] silver transferred to the mouth while chewing on
and transferred to skin while wearing textiles treated with
AGS-20.” Importantly, EPA observed that “aggressive
conditions” potentially cause the release of more nanosilver
ions. From this, it is reasonable to conclude that aggressively
chewing on clothing treated with AGS-20—as a three-year
old might—causes more exposure than sucking on or
mouthing the same textile—as an infant would be inclined to
do.

    EPA’s decision to use the characteristics of three-year
olds rather than infants in its risk assessment created tradeoffs
in the elements that comprise the formula that the agency
used to calculate oral and dermal exposure to AGS-20.
Infants are more vulnerable because they weigh less, but
toddlers are more vulnerable because they can chew fabric
aggressively. Infants spend more time resting on a single
piece of cloth, such as a parent’s shirt, but toddlers move
around and make contact with more textiles. NRDC has
merely demonstrated that there is a reasonable basis for
disagreement over whether infants are a somewhat more
vulnerable subpopulation than toddlers. Under these
circumstances, the agency’s findings are “supported by
                            NRDC V. EPA                                 15

substantial evidence when considered on the record as a
whole,” and we deny the petition.4 7 U.S.C. § 136n(b); see
Am. Textile Mfrs. Inst., Inc. v. Donovan, 452 U.S. 490,
522–23 (1981) (“[W]e have defined substantial evidence as
such relevant evidence as a reasonable mind might accept as
adequate to support a conclusion. The reviewing court must
take into account contradictory evidence in the record, but the
possibility of drawing two inconsistent conclusions from the
evidence does not prevent an administrative agency’s finding
from being supported by substantial evidence.”) (internal
quotations and citations omitted); Envtl. Def. Ctr. Inc. v. EPA,
344 F.3d 832, 872 n.56 (9th Cir. 2003); Nw. Food
Processors Ass’n, 886 F.2d at 1079–80; ASARCO, Inc. v.
OSHA, 746 F.2d 483, 490 (9th Cir. 1984) (“Where the agency
presents scientifically respectable evidence which the
petitioner can continually dispute with rival, and we will
assume, equally respectable evidence, the court must not
second-guess the particular way the agency chooses to weigh
the conflicting evidence or resolve the dispute.”) (internal
quotations and citation omitted).




  4
    NRDC argues that observations about how toddlers can chew more
aggressively and cover more of the surface area of a textile because they
have larger mouths, more teeth, and stronger jaws are post hoc
rationalizations that lack support in the record. To the contrary, they are
commonsense statements about the apparent distinctions between children
of different ages. NRDC relies on the same type of commonsense
observations. See Petitioner’s Opening Br. at 26 (“It is common sense
that infants and babies are held by parents and caretakers more often than
other children, giving them more frequent access to shoulders and sleeves,
and that teething infants (not toddlers) are most likely to chew on clothing
and other fabrics within reach.”). The agency need not state every
patently obvious observation underlying its reasoning.
16                          NRDC V. EPA

B. EPA’s Conclusion That Toddlers’ Potential Aggregate
   Dermal and Oral Exposure to AGS-20 Applied as a
   Surface Treatment Does Not Pose a Risk Concern
   Requiring Mitigation

    Next, we address whether EPA’s determination that there
is no risk concern for toddlers exposed to AGS-20-treated
textiles is supported by substantial evidence. EPA’s own rule
of decision states that there is a risk concern if the aggregate
dermal and oral exposure to AGS-20 is less than or equal to
1,000. In one instance, EPA calculated an aggregate
exposure of 1,000, which is obviously equal to 1,000. Yet
EPA erroneously concluded that there was no risk concern on
the basis that all of its calculations exceeded 1,000.5 We
vacate this portion of EPA’s decision and remand to the
agency because it did not satisfy its own rule for determining
when there is a risk concern requiring mitigation.

     Central to EPA’s analysis of the risks posed by a pesticide
is its calculation of the margin of exposure (“MOE”), which
is used to determine whether exposure to a pesticide might
cause an adverse effect. To discern whether a risk concern
exists, EPA first calculates a target MOE by multiplying a
variety of uncertainty factors. Here, EPA calculated a target
MOE of 1,000 for short-term (less than 30 days) and




 5
    NRDC did not raise this issue in its opening brief. The court raised it
sua sponte and the parties submitted supplemental briefing on the question
at our request.
                             NRDC V. EPA                                  17

intermediate-term (between one and six months) exposure to
AGS-20.6

    After setting the target MOE, the agency calculated the
actual (or “calculated”) MOE under a number of scenarios.
The actual MOE equals the toxicological point of departure
divided by the daily dose. The point of departure is the
amount of the pesticide that a person must be exposed to
before there is a possible adverse effect. Here, EPA used 0.5
mg/kg/day as the point of departure for both dermal and oral
exposure to AGS-20. This figure was based on a prior study
that found no adverse effects in mice who had ingested 0.5
mg/kg/day of nanosilver for 28 days.7 The daily dose is the
amount of nanosilver that a person might be exposed to under
various scenarios, such as wearing a shirt treated with AGS-
20 or chewing on a blanket coated with the product.

    All else equal, a higher actual MOE means there is less
cause for concern. For example, if studies showed that
humans exposed to 500 units per day of a particular pesticide
experienced no adverse health effects, we would likely
tolerate being exposed to 50 units of the pesticide per day. In
this hypothetical, the actual MOE would be 10 (500 / 50 =


  6
    The short- and intermediate-term MOEs were calculated using a ten-
fold interspecies uncertainty factor to account for the fact that the data was
derived from experiments on animals rather than humans, a ten-fold
intraspecies uncertainty factor to account for variable sensitivity to
pesticides among different humans, and a ten-fold database uncertainty
factor to account for the incomplete data on nanosilver toxicity. The three
uncertainty factors of ten are multiplied to produce the target MOE of
1,000 (10 x 10 x 10 = 1,000).
  7
    The risk that nanosilver might affect humans differently than mice is
accounted for by the ten-fold interspecies uncertainty factor.
18                      NRDC V. EPA

10). But we would presumably be even more comfortable if
we were exposed to only 5 units of the pesticide per day, in
which case the actual MOE would be 100 (500 / 5 = 100). In
reality, the EPA cannot be absolutely certain that it is safe for
humans to be exposed to, say, 500 units per day of the
pesticide where, as here, the product is new and has not been
extensively studied. For this reason, the actual MOE must
exceed the target MOE, which, as noted, accounts for various
uncertainty factors.

   The EPA’s decision document offers the following rule of
decision for determining whether there is a risk concern:

        •   If calculated MOE > target MOE: risk is
            not of concern and mitigation is not
            required

        •   If calculated MOE # target MOE: risk is
            of concern and mitigation is required

In Table 4 of the Decision Document, the EPA lists the target
MOE’s for AGS-20 based on the interspecies, intraspecies,
and database uncertainty factors that it identified:
                           NRDC V. EPA                                 19

           Table 4 — Target Margins of Exposure

 Continuous and Daily                        Target MOE
  Exposure Duration
                                  Inhalation       Oral       Dermal
 Short-Term (< 30 days)              1,000         1,000       1,000
     Intermediate-Term               1,000         1,000       1,000
       (1 to 6 months)
         Long-Term                   3,000         3,000       3,000
        (> 6 months)

There is no risk concern if the actual MOE via inhalation,
oral, or dermal exposure is greater than 1,000 for short- or
intermediate-term exposure, or greater than 3,000 for long-
term exposure. If, on the other hand, the calculated MOE is
less than or equal to 1,000 for short-term or intermediate-term
exposure, or less than or equal to 3,000 for long-term
exposure, there is a risk concern and mitigation is required.

    In order to determine the potential risks posed by AGS-20
to consumers, EPA evaluated the actual MOE for inhalation,
dermal contact (i.e., wearing a shirt treated with AGS-20),
and oral contact (i.e., chewing on a blanket treated with AGS-
20).8 EPA also computed the actual MOE for “aggregate”



 8
   In calculating the dermal and oral MOE, EPA assumed that the textile
contains 20 parts per million of nanosilver applied as a surface coating,
which is the maximum amount permitted by EPA. AGS-20 can also be
incorporated into textile fibers, and EPA conducted the same analysis of
both incorporated and surface-coated textiles. But only the surface-coated
treatment type is subject to the issue raised here because the actual MOE
20                            NRDC V. EPA

dermal and oral contact, which estimates the exposure faced
by a toddler who, for example, chews on an AGS-20-treated
shirt while wearing the shirt. Table 12 of the decision
document, which is excerpted in part below, shows the actual
aggregate MOE.

                          Table 12 (excerpt)

 Application       Incidental      Dermal       Aggregate       Aggregate
   Rate            Oral Dose        Dose          Dose            MOE
  (mg/kg)           (mg/kg/        (mg/kg/       (mg/kg/
 Treatment            day)          day)          day)
   Type

       20           0.00020       0.000011       0.00021           2,400
     Surface
     Coated         0.00047       0.000027       0.00050          1,000

      100           0.00010       0.000057       0.00016          3,100
 Incorporated
   in Fibers       0.000087       0.000049       0.00014          3,600


    The notable finding is that the aggregate MOE for a
surface-coated textile is 1,000. Under the EPA’s own rule of
decision, there is a risk concern requiring mitigation when the
short- or intermediate-term MOE is less than or equal to
1,000. EPA seemingly overlooked its rule in describing its
findings. The agency wrote that “[a]ll of these MOE’s are
greater than the target MOE of 1,000 indicating that the risk
for short- and intermediate-term exposure to toddlers who
wear and chew on AGS-20 treated textiles is not of concern.”
One page later, EPA noted that “[a]ggregating the dermal and
oral daily doses for children yielded MOEs ranging from


for the incorporated treatment type was consistently higher (i.e., less likely
to pose risk of an adverse effect).
                       NRDC V. EPA                         21

1,000 to 3,600, which is not of concern because they exceed
the target MOE of 1,000.” These statements are not
supported by substantial evidence because a calculated MOE
of 1,000 poses a risk concern given that it is less than or
equal to the target MOE of 1,000.

    In supplemental briefing addressing this issue, EPA raises
two arguments. First, EPA contends that the precise
aggregate MOE is 1,006, not 1,000. As noted above, the
MOE is the point of departure divided by the daily dose, and
EPA used 0.5 mg/kg/day as the point of departure. As Table
12 indicates, the aggregate daily dose is the sum of the oral
dose and the dermal dose. In the relevant line of Table 12,
the oral dose is 0.00047 and the dermal dose is 0.000027.
These two figures are summed for an aggregate dose of
0.00050. The point of departure (0.5 mg/kg/day) is then
divided by this aggregate daily dose (0.00050 mg/kg/day) to
produce the aggregate MOE (1,000).

    But EPA points out that 0.00047 + 0.000027 is not
actually 0.00050; rather it is 0.000497. If we use 0.000497 as
the aggregate daily dose, then the aggregate MOE is 1,006
because 0.5 / 0.000497 = 1,006. Since 1,006 is greater than
1,000, EPA argues there is no risk concern.

    There is a good reason, however, why EPA used 0.00050
as the aggregate daily dose in Table 12 instead of 0.000497.
In several places throughout the decision document, EPA
noted that it was rounding the daily dose to two significant
digits. In footnotes accompanying both Table 10, which
provides the oral daily dose, and Table 11, which provides the
dermal daily dose, EPA stated that the numbers were rounded
to two significant digits. The oral and dermal daily doses
22                     NRDC V. EPA

reprinted in Table 12—which is at issue here—are therefore
rounded to two significant digits as well.

    Importantly, the oral dose (0.00047) has five digits after
the decimal point and the dermal dose (0.000027) has six
digits after the decimal point. The sum of these two
numbers—which is the aggregate daily dose—can have no
more than five digits after the decimal point because of the
rule that “[w]hen measured quantities are added or subtracted,
the number of decimal places in the result is the same as that
in the quantity with the greatest uncertainty and hence the
smallest number of decimal places.” William L. Masterton,
Cecile N. Hurley & Edward J. Neth, Chemistry: Principles
and Reactions 12 (7th ed. 2012) (emphasis removed). In
Table 12, EPA correctly offered an aggregate daily dose with
five digits after the decimal point (0.00050). EPA’s data is
not precise enough to allow it to “unround” this number to
0.000497, as it attempts to do in its supplemental brief. The
data available in the decision document only permit us to
conclude that the aggregate dose is 0.00050, which means
that the aggregate MOE is 1,000, and not 1,006.

     EPA’s second argument is that its MOE calculations are
based on very conservative assumptions so an actual MOE in
the neighborhood of 1,000 does not mean that consumers are
at risk. For example, using a target MOE of 1,000 means that
a toddler simultaneously wearing and chewing a textile
treated with AGS-20 can be exposed to no more than
1/1000th of the amount of nanosilver that has been shown to
produce no harmful effects in mice in laboratory studies.
Although EPA’s point is well taken as a practical matter, it is
irrelevant as a legal matter. This is EPA’s rule, not ours.
EPA articulated the assumptions that led it to set the target
MOE at 1,000. And it stated the rule that there is a risk
                       NRDC V. EPA                          23

concern if the MOE is less than or equal to 1,000. Its
assessment then produced an aggregate MOE of 1,000, which
means, by its own assumptions, that there is a risk concern
requiring mitigation. Having established a rule of decision of
less than or equal to 1,000, EPA cannot unmake it because its
actual MOE is in the neighborhood. Nor can we revise
EPA’s assumptions, alter its rule of decision, or perform our
own risk assessment. See State Farm, 463 U.S. at 50 (“It is
well-established that an agency’s action must be upheld, if at
all, on the basis articulated by the agency itself.” (citations
omitted)); Safe Air for Everyone, 488 F.3d at 1091 (“[O]ur
review of an administrative agency’s decision begins and
ends with the reasoning that the agency relied upon in making
that decision.”). EPA may wish to revisit its standards in the
future, but it cannot ignore them.

   EPA’s conclusion that short- and intermediate- term
aggregate dermal and oral exposure to textiles surface-coated
with AGS-20 poses no risk concern is not supported by
substantial evidence. We grant the petition and vacate EPA’s
decision to the extent that it states otherwise.

C. EPA’s Decision Not to Account for Potential Sources of
   Exposure to Nanosilver Other Than AGS-20

    Finally, NRDC takes issue with EPA’s decision against
conducting an aggregate risk assessment that accounts for
sources of potential exposure to nanosilver other than AGS-
20. Both parties agree that nanosilver presently exists in
other products. EPA explains in its decision document and its
brief that there is no data available about whether other
nanosilver in the marketplace is chemically similar to AGS-
20 or how consumers might be exposed to other sources of
nanosilver. NRDC does not dispute this finding, but insists
24                     NRDC V. EPA

that it is unreasonable for EPA to effectively assume that
consumers will never be exposed to nanosilver that is
chemically similar to AGS-20.

    As an initial matter, EPA is not obligated by statute to
conduct an aggregate risk assessment in conditionally
registering a non-food-use pesticide such as AGS-20. In
1996, Congress amended the Federal Food, Drug, and
Cosmetic Act to require EPA to complete an aggregate risk
assessment for pesticides found in foods. See 21 U.S.C.
§ 346a(b)(2)(A)(ii) (providing that EPA must “determine[]
that there is a reasonable certainty that no harm will result
from aggregate exposure to the pesticide chemical residue,
including all anticipated dietary exposures and all other
exposures for which there is reliable information”) (emphasis
added); see also 21 U.S.C. § 346a(b)(1)(A) (stating that the
section applies to “a pesticide chemical residue in or on a
food.”). At the same time, Congress amended FIFRA to
require EPA to assess “human dietary risk from residues that
result from a use of a pesticide in or on any food inconsistent
with the standard under” the Federal Food, Drug, and
Cosmetic Act. 7 U.S.C. § 136(bb) (emphasis added). In
short, Congress expressly required aggregate risk assessment
for food-use pesticides, but did not impose a similar
requirement on non-food-use pesticides such as AGS-20.

    EPA’s decision not to conduct an aggregate risk
assessment in this instance is consistent with its regulations.
“The Agency has discretion to review applications under
either the unconditional registration criteria of FIFRA
sec. 3(c)(5) or the conditional registration criteria of FIFRA
sec. 3(c)(7).” 40 C.F.R. § 152.111. The regulation further
provides that “[t]he type of review chosen depends primarily
on the extent to which the relevant data base has been
                       NRDC V. EPA                          25

reviewed for completeness and scientific validity. EPA
conducts data reviews needed to support unconditional
registrations on a chemical-by-chemical basis, according to
an established priority list.” Id. An aggregate risk
assessment is thus potentially part of the unconditional
registration process, which proceeds on a “chemical-by-
chemical basis” that might sweep in all types of nanosilver,
rather than only AGS-20. But in considering whether to
proceed first with conditional registration—which, as noted
above, does not require an aggregate risk assessment—EPA
considers the completeness of the available data. Here, there
is no data whatsoever on whether other types of nanosilver
are chemically similar to AGS-20. In urging EPA to conduct
an aggregate risk assessment, NRDC is effectively attempting
to short circuit the statutory scheme providing for conditional
registration by forcing AGS-20 into the unconditional
registration process where EPA proceeds on a “chemical-by-
chemical basis.” It is true that EPA might conduct a full data
review sufficient to support an unconditional registration
where, as here, it receives “applications for registration of a
new active ingredient.” Id. But unconditional registration is
not required, as FIFRA expressly provides that EPA “may
conditionally register a pesticide containing an active
ingredient not contained in any currently registered
pesticide.” 7 U.S.C. § 136a(c)(7)(C) (emphasis added).

    Within the confines of EPA’s conditional review of AGS-
20, its decision not to conduct an aggregate risk assessment
covering other sources of nanosilver exposure is supported by
substantial evidence. As previously noted, after consulting
with the FIFRA Scientific Advisory Panel, EPA requested
that HeiQ classify its application as a “new active ingredient”
application because it “determined that the nanosilver active
ingredient in AGS-20 differed from the active ingredients in
26                          NRDC V. EPA

currently registered silver-based antimicrobial products.”
The Scientific Advisory Panel also “cautioned about
extrapolating from one nanosilver formulation to another
when assessing hazards because differences in particle
formulation (e.g., coating and inert ingredients) are likely to
affect biological activity, among other things.” In light of
these perceived differences between types of nanosilver, the
Panel “recommended a case-by-case basis approach to hazard
and exposure assessment (i.e., product-by-product).”

    The Scientific Advisory Panel’s statements are
meaningful in the absence of any data indicating that other
nanosilver is chemically similar to AGS-20. EPA could have
blindly speculated that (1) other types of nanosilver are
chemically similar to AGS-20 and (2) consumers might be
exposed to them in the same way that they are potentially
exposed to AGS-20 (e.g., through simultaneous dermal and
oral contact). But this possibility does not mean that it was
improper for EPA to instead assume that (1) other types of
nanosilver might not be chemically similar to AGS-20 in light
of the Scientific Advisory Panel’s statements or (2)
consumers are unlikely to be exposed to them in the same
way in meaningful quantities.9 This is yet another scenario
where there is “the possibility of drawing two inconsistent
conclusions from the evidence[, which] does not prevent an


  9
    It is worth noting that the only calculated MOEs that are particularly
close to the target MOEs involve short- and intermediate-term oral and
aggregate oral and dermal contact with surface-coated textiles. All of the
other calculated MOEs are at least three times greater than the target
MOE, meaning it is even less likely that consumers will be exposed to risk
from contact with AGS-20 alone or in conjunction with chemically similar
nanosilver, if such products even exist. Exposure via dermal contact
alone, or simultaneous oral and dermal contact with AGS-20 incorporated
into fibers, does not even approach the level where there is a risk concern.
                       NRDC V. EPA                         27

administrative agency’s finding from being supported by
substantial evidence.” Am. Textile Mfrs. Inst., 452 U.S. at
523; see also Nw. Food Processors Ass’n, 886 F.2d at
1079–80. We deny the portion of NRDC’s petition that faults
EPA for deciding not to account for sources of nanosilver
other than AGS-20 in conducting its risk assessment.

                              V

     We have considered three distinct issues with EPA’s
decision to conditionally register AGS-20. First, we deny
NRDC’s petition with respect to its claim that EPA should
have used the body weight and other characteristics of infants
in assessing the risks posed to consumers by AGS-20. EPA’s
decision to consider three-year-old toddlers rather than
infants the most vulnerable subpopulation is supported by
substantial evidence. Second, we grant the petition and
vacate EPA’s decision to the extent that it concludes that
there is no risk concern requiring mitigation for short- and
intermediate-term aggregate oral and dermal exposure to
textiles that are surface coated with AGS-20. Our decision to
grant the petition and vacate the decision in part is based
solely on the fact that EPA’s own rule states that there is a
risk concern requiring mitigation when the calculated MOE
is less than or equal to 1,000 and, under these circumstances,
28                           NRDC V. EPA

the actual MOE equals 1,000. This holding does not affect
any portion of EPA’s decision where the calculated MOE is
greater than 1,000.10 Third, we deny NRDC’s petition with


  10
     The dissent would hold that we need not reach the two arguments
raised by NRDC once we conclude that one of EPA’s findings is not
supported by substantial evidence. See Dissent at 29–32. But our
decision to evaluate all of NRDC’s contentions is consistent with the
statute and our case law.

      The statute that vests this court with jurisdiction provides that “the
court shall have exclusive jurisdiction to affirm or set aside the order
complained of in whole or in part.” 7 U.S.C. § 136n(b) (emphasis added).
The dissent’s suggestion—that we should remand without resolving the
issues that brought the parties before this court so that they may
potentially relitigate them in future proceedings—is at odds with the
statute. Cf. Envtl. Def. Fund, Inc. v. EPA, 485 F.2d 780, 783 (D.C. Cir.
1973) (“FIFRA provides ‘[u]pon the filing of [a] petition the court [of
appeals] shall have exclusive jurisdiction to affirm or set aside the order
complained of in whole or in part.’ When the Congress required that
courts of appeals exercise exclusive jurisdiction over petitions to review
a FIFRA order, it was to ensure speedy resolution of the validity of EPA
determinations. . . . This policy would be defeated if we were to allow the
[ ] order to be litigated in several proceedings.”) (first three alterations in
original) (citations omitted).

     In similar contexts, we have resolved all of the arguments presented
by the party petitioning for review of the agency’s action even after
determining that one of the claims is meritorious. See Hall v. EPA,
273 F.3d 1146, 1152, 1154–55, 1162–64 (9th Cir. 2001) (rejecting the
arguments raised in the petitioner’s opening brief after deciding to vacate
EPA’s decision and remand to the agency for an unrelated reason
discussed in supplemental briefing); Ober v. EPA, 84 F.3d 304, 308,
311–13, 316 (9th Cir. 1996) (resolving the second and third arguments
raised by the petitioner in favor of EPA after determining that the
petitioner’s first argument was meritorious and justified remanding to the
agency for further proceedings). In both of these cases, the court granted
in part and denied in part the petition for review. See Hall, 273 F.3d 1164;
Ober, 84 F.3d at 316.
                            NRDC V. EPA                                29

respect to its claim that EPA should have considered sources
of potential consumer exposure to nanosilver other than AGS-
20. EPA’s decision not to conduct an aggregate risk
assessment of other possible sources of nanosilver exposure
is supported by substantial evidence. Each party shall bear its
own costs for this petition for review.

    We GRANT in part the petition, DENY in part the
petition, and REMAND.



ADELMAN, District Judge, concurring in part and dissenting
in part:

    I concur in the judgment insofar as it grants the petition
in part and remands to the EPA. I agree that the NRDC has


     In the specific context of a registration decision by EPA pursuant to
FIFRA, the D.C. Circuit has dispensed with all of the petitioner’s non-
meritorious arguments even after determining that EPA erred with respect
to part of the registration decision. See Envtl. Def. Fund, Inc. v. EPA,
548 F.2d 998, 1002, 1011–12 (D.C. Cir. 1976) (overruled on other
grounds by Director, Office of Workers’ Comp. Program, Dep’t of Labor
v. Greenwich Collieries, 512 U.S. 267 (1994)) (affirming several
components of EPA’s order to suspended the registration of a pesticide
despite concluding that EPA erred by acting arbitrarily when deciding one
portion of the order and directing EPA to reconsider that portion on
remand); Envtl. Def. Fund, Inc. v. EPA, 510 F.2d 1292, 1295–96, 1306
(D.C. Cir. 1975) (same).

     This salutary rule is particularly appropriate here where the issue on
which we remand was not briefed or argued by the parties, but was raised
sua sponte by the court. The remaining issues were well briefed by both
parties and ripe for review. We can see no reason for delaying ruling on
these matters.
30                      NRDC V. EPA

standing to challenge the EPA’s conditional registration of
AGS-20 and join Part III of the majority opinion. I also agree
that because the EPA inexplicably stated that all calculated
MOEs are greater than the target MOE of 1,000, when in fact
the calculated MOE for aggregate dermal and oral contact for
toddlers exposed to AGS-20 treated textiles is equal to 1,000,
the EPA’s order conditionally registering AGS-20 is not
supported by substantial evidence. I dissent from the
judgment insofar as it purports to deny the petition in part.

    First, I think that under the majority’s reasoning in Part
IV.B of its opinion, the petition must be granted in full. As
the majority explains, the EPA’s conclusion that aggregate
dermal and oral exposure to textiles surface-coated with
AGS-20 poses no risk concern is not supported by substantial
evidence. That being so, it follows that the EPA has failed to
demonstrate that its determination that AGS-20 will not have
an unreasonable adverse effect on the environment is
supported by substantial evidence. Accordingly, the EPA’s
conditional registration of AGS-20 cannot stand. 7 U.S.C.
§ 136(a)(c)(7)(C). Because the only relief requested in the
NRDC’s petition is setting aside the conditional registration,
see Pet. at 2, it follows that the NRDC’s petition should be
granted in full.

    It is likely that the majority thinks it appropriate to deny
the petition in part because it has ruled against the NRDC on
the other issues raised in its briefs. But the other issues raised
in the NRDC’s briefs are not alternative claims for relief;
they are alternative reasons for granting the only relief
requested in the petition—vacating the conditional
registration of AGS-20. Thus, unless the majority thinks that
some part of the conditional registration of AGS-20 has
survived its conclusion in Part IV.B, the NRDC’s petition
                        NRDC V. EPA                           31

should be granted in full. And if the majority does think that
some part of the conditional registration has survived, I would
ask it to identify that part. It seems to me that the conditional
registration does not contain any “parts” at all—either AGS-
20 is conditionally registered or it is not—and that therefore
nothing could have survived the majority’s conclusion in Part
IV.B.

    Another way of looking at this is that the majority seems
to be affirming part of the EPA’s reasoning in support of its
decision to conditionally register AGS-20 rather than part of
the order granting conditional registration. But appellate
courts review decisions, judgments, orders, and decrees—not
opinions, factual findings, reasoning, or explanations.
Weissman v. Quail Lodge, Inc., 179 F.3d 1194, 1200 (9th Cir.
1999) (quoting In re Williams, 156 F.3d 86, 90 (1st Cir.
1998)); see also 7 U.S.C. § 136n(b) (court has jurisdiction to
“affirm or set aside the order complained of in whole or in
part” (emphasis added)). Here, because of the majority’s
conclusion in Part IV.B, the conditional-registration order
should be vacated in full, even if the majority believes that
some parts of the EPA’s reasoning in support of its decision
to issue that order are sound.

     I also disagree with the majority’s decision to comment
on the other issues raised by the NRDC. As noted, those
issues are alternative grounds for setting aside the EPA’s
conditional registration of AGS-20. Because the majority has
already concluded in Part IV.B that the conditional
registration must be set aside, it is unnecessary to comment
on the alternative grounds. The majority likely chooses to
address the issues because it believes that doing so will
provide guidance for the EPA on remand and prevent
relitigation. However, it is not clear to me that the majority’s
32                     NRDC V. EPA

unnecessarily addressing those issues will serve as guidance
or prevent relitigation. Because it was not necessary for the
majority to address the issues, its comments on them are
dicta, and thus it is questionable whether the majority’s
resolution of the issues will be binding in a subsequent case.
Thus, the NRDC or some other injured party might be
entitled to relitigate them. For the same reason, the EPA
cannot accept the majority’s comments as guidance—the
EPA cannot be sure that those comments will be binding in
a subsequent case. Moreover, the majority’s choosing to
comment on the issues might lead to unnecessary further
litigation. Even though the NRDC has won its case, it might
feel the need to seek further review of the majority’s
unnecessary comments either in this court or in the Supreme
Court. Yet, if the case were simply remanded to the EPA on
the basis identified by the majority in Part IV.B, the
additional issues raised by the NRDC might be rendered
moot: the EPA might decide to deny conditional registration
of AGS-20 because of the risk concern to toddlers identified
by the majority or because of some new information about
AGS-20 that has come to light since the EPA last considered
the matter. For these reasons, I think the most prudent course
is to simply remand to the agency on the ground stated by the
majority in Part IV.B. Should the EPA reissue a conditional-
registration order and stand by its current reasoning on the
other issues raised by the NRDC, those issues can be
addressed at that time.

   I also disagree with the majority’s resolution of the
NRDC’s other issues. The first of these involves the EPA’s
decision to use toddlers rather than infants as the most
vulnerable subpopulation. The NRDC contends that this
                           NRDC V. EPA                               33

decision is not supported by substantial evidence.1 The
majority finds that the decision is supported by substantial
evidence for two reasons: (1) the EPA has in the past used
toddlers as the most vulnerable subpopulation, and (2)
toddlers behave in ways that make them more vulnerable to
AGS-20 exposure than infants.

    It is important to emphasize that the EPA does not explain
in the decision document why it decided that toddlers are the
most vulnerable subpopulation. The EPA does not say that it
made this decision in order to remain consistent with its past
practices or because toddlers behave in ways that make them
more vulnerable to AGS-20 exposure than infants. The lack
of an explanation for this key assumption should be enough
to require that the EPA’s decision be set aside. As the
majority recognizes, Maj. Op. at 8, “[i]t is well-established
that an agency’s action must be upheld, if at all, on the basis
articulated by the agency itself.” Motor Vehicle Mfrs. Ass’n
v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 50 (1983).
Here, the EPA has articulated no basis for selecting toddlers
as the most vulnerable subpopulation. The basis that the
majority adopts is one articulated by the agency’s appellate
counsel in his briefs. See Resp. Answering Br. at 48–51. But
the Supreme Court has expressly stated that “the courts may
not accept appellate counsel’s post hoc rationalizations for
agency action.” Motor Vehicle Mfrs., 463 U.S. at 50.

    Moreover, I find it puzzling that the majority thinks that
the EPA’s decision to select toddlers as the most vulnerable


 1
    I agree with the majority that the NRDC did not forfeit the argument
that the EPA’s decision to use toddlers rather than infants as the most
vulnerable subpopulation is not supported by substantial evidence. See
Maj. Op. at 12 n.3.
34                      NRDC V. EPA

subpopulation is supported by substantial evidence, in part,
because that decision is “consistent with EPA’s practices.”
Maj. Op. at 13. Why should it matter that the decision is
consistent with EPA’s practices? Maybe it would matter if
the EPA could point to some agency document indicating that
the agency had studied the issue and determined that the
agency should use toddlers as the most vulnerable
subpopulation when assessing the risk associated with
pesticide exposure, but the agency has pointed to no such
document. Instead, the EPA points to a document indicating
that the agency has drafted a standard operating procedure
providing that, in pesticide assessments, the effects of dermal
and non-dietary oral exposures on toddlers should be
considered. But this standard operating procedure does not
state that the agency should always select toddlers as the most
vulnerable subpopulation in pesticide assessments. Rather,
it states that toddlers may be used to represent the one-to-six-
year-old age group and that they may be assumed to weigh
fifteen kilograms. See Standard Operating Procedures (SOPs)
for Residential Exposure Assessments at 109 (Dec. 19 1997).
There is no analysis in this document supporting the
conclusion that the one-to-six-year-old age group should be
assumed to be the subpopulation most vulnerable to pesticide
exposure. Thus, to the extent the agency has a “practice” of
using toddlers as the most vulnerable subpopulation in
pesticide assessments, that practice appears to be nothing
more than a naked assumption unsupported by substantial
evidence.

    The other reason the majority cites in upholding the
agency’s decision is that toddlers are likely to receive more
exposure to AGS-20 than infants because toddlers are likely
to chew on clothing treated with AGS-20 more aggressively
than infants. Maj. Op. at 14. Again, however, the EPA never
                       NRDC V. EPA                          35

states in the decision document that it decided to use toddlers
as the most vulnerable subpopulation for this reason.
Moreover, as the majority recognizes, certain characteristics
of infants might cause them to be more vulnerable than
toddlers despite their inability to chew as aggressively:
infants spend more time resting on a single piece of cloth than
toddlers, and because infants weigh less than toddlers, they
are more vulnerable to smaller exposures. Maj. Op. at 14.
Thus, there are “tradeoffs” to consider when selecting the
most vulnerable subpopulation. Maj. Op. at 14. If the EPA
had considered these tradeoffs and concluded that toddlers are
more vulnerable than infants, then I would agree with the
majority that the EPA’s decision was supported by substantial
evidence—the EPA would have applied its expertise, to
which we must defer. But once again, I must emphasize that
the agency did no such thing. It is the majority, not the EPA,
that has evaluated the tradeoffs and concluded that toddlers
are the most vulnerable subpopulation. Maj. Op. at 14–15.

    The remaining issue involves the EPA’s decision to
ignore potential sources of nanosilver other than AGS-20
when determining whether use of AGS-20 would cause
unreasonable adverse effects. The majority upholds this
decision for two reasons: (1) the EPA is “not obligated by
statute” to conduct an aggregate risk assessment when
considering whether to conditionally register a pesticide, Maj.
Op. at 24, and (2) the EPA’s decision to not conduct an
aggregate risk assessment involving sources of nanosilver
exposure other than AGS-20 is supported by substantial
evidence, Maj. Op. at 25. Here, I must confess that I find the
majority’s approach confusing. If the EPA is “not obligated
by statute” to conduct an aggregate risk assessment, why does
the majority also consider whether the EPA’s decision to not
conduct one was supported by substantial evidence? In any
36                      NRDC V. EPA

event, as explained below, I would hold that the EPA is
obligated by statute to consider whether the pesticide under
review will combine with other similar substances in the
environment to cause an unreasonable adverse effect. I would
also hold that the EPA’s failure to consider the effects of
aggregate exposure to AGS-20 and other sources of
nanosilver is inexcusable.

     The majority’s holding that the EPA is not obligated by
statute to conduct an aggregate risk assessment involves
statutory interpretation. Thus, I examine the statute’s plain
meaning. See, e.g., United States v. Flores, 729 F.3d 910,
914 (9th Cir. 2013) (“The interpretation of a statutory
provision must begin with the plain meaning of its language”
(internal quotation marks and citation omitted)). FIFRA
states that the EPA may conditionally register a pesticide
“only if the Administrator determines that use of the pesticide
during [the conditional-registration] period will not cause any
unreasonable adverse effect on the environment and that use
of the pesticide is in the public interest.” 7 U.S.C.
§ 136a(c)(7)(C). The term “unreasonable adverse effect”
means, among other things, “any unreasonable risk to man or
the environment” 7 U.S.C. § 136(bb). It is plain to me that
if use of a pesticide will cause aggregate exposure to a certain
substance or family of related substances to reach dangerous
levels, then use of that pesticide has the potential to cause an
unreasonable risk to man or the environment. Thus, the plain
text of the FIFRA requires the EPA to study whether
aggregate exposure will result in an unreasonable adverse
effect.

   The majority reasons that because Congress did not
expressly state in the FIFRA that the EPA must consider risks
caused by aggregate exposure, like it did in a different statute,
                        NRDC V. EPA                           37

it must have intended for the EPA to ignore that risk when
evaluating pesticides for conditional registration. The
majority is essentially telling us that Congress would have
wanted the EPA to approve the use of a pesticide even if its
use could, when combined with uses of other substances,
result in unreasonably harmful effects on the environment. I
think it is obvious that Congress, in instructing the EPA to
approve the use of a pesticide “only if” it determines that
such use will not cause any unreasonable adverse effect on
the environment, had no such intent.

    The majority also reasons that the EPA’s decision to
ignore the effects of aggregate exposure to nanosilvers was
supported by substantial evidence. The majority seems to
think that because the EPA did not have sufficient
information to form any firm conclusions about the effects of
aggregate exposure to AGS-20 and other nanosilvers, it was
permitted to ignore the risk that such aggregate exposure
could result in unreasonable adverse effects. Maj. Op. at
26–27. I think the majority has things backwards. The
FIFRA states that the EPA may conditionally register a
pesticide “only if” it first determines that use of the pesticide
“will not cause” any unreasonable adverse effect. 7 U.S.C.
§ 136a(c)(7)(C). It follows from this that if the EPA lacks
information to determine whether use of a pesticide will
cause an unreasonable adverse effect, then the EPA’s only
option is to deny conditional registration. The majority’s
approach would be sound only if the FIFRA stated that the
EPA may grant conditional registration “unless” it determines
that use of the pesticide “will cause” an unreasonable adverse
effect. As the FIFRA does not so state, I would hold that the
EPA may not conditionally register a pesticide when it has no
38                    NRDC V. EPA

idea whether use of that pesticide will, when aggregated with
other substances in the environment, cause unreasonable
harm.
