                     FOR PUBLICATION

    UNITED STATES COURT OF APPEALS
         FOR THE NINTH CIRCUIT

 NATURAL RESOURCES DEFENSE                         No. 15-72308
 COUNCIL,
                      Petitioner,

                      v.

 U.S. ENVIRONMENTAL PROTECTION
 AGENCY,
                      Respondent.


 CENTER FOR FOOD SAFETY;                           No. 15-72312
 INTERNATIONAL CENTER FOR
 TECHNOLOGY ASSESSMENT,
                      Petitioners,                    OPINION

                      v.

 U.S. ENVIRONMENTAL PROTECTION
 AGENCY; SCOTT PRUITT,* in his
 official capacity as Administrator of
 the Environmental Protection
 Agency,
                         Respondents.




      *
         Scott Pruitt, Administrator of the Environmental Protection
Agency, is substituted for his predecessor, Gina McCarthy. Fed. R. App.
P. 43(c)(2).
2                         NRDC V. USEPA

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

           Argued and Submitted November 17, 2016
                   San Francisco, California

                         Filed May 30, 2017

         Before: Michael J. Melloy,** Richard R. Clifton,
               and Paul J. Watford, Circuit Judges.

                     Opinion by Judge Melloy


                            SUMMARY***


                           Pesticides / EPA

    The panel vacated the United States Environmental
Protection Agency’s (“EPA”) conditional registration of the
pesticide NSPW-L30SWS – an antimicrobial materials
preservative that uses nanosilver as its active ingredient –
because the EPA failed to support its requisite finding that
NSPW was in the public interest under 7 U.S.C.
§ 136a(c)(7)(C).




    **
      The Honorable Michael J. Melloy, United States Circuit Judge for
the U.S. Court of Appeals for the Eighth 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 Federal Insecticide, Fungicide, and Rodenticide Act
governs the sale, use, and distribution of pesticides, and the
Act requires that pesticides generally must be registered with
the EPA before being sold or distributed. The EPA may grant
a temporary, conditional registration if it first determines that
use of a pesticide was in the public interest.

  The panel held that substantial evidence supported the
EPA’s findings that NPSW has lower application and
mobility rates than conventional-silver pesticides.

    The panel held, however, that substantial evidence did not
support the EPA’s finding that use of NPSW was in the
public interest because it had the “potential” to reduce the
amount of silver released into the environment. The panel
held that the EPA’s finding was based on two unsubstantiated
assumptions: first, that current users of conventional-silver
pesticides would replace those pesticides with NSPW; and
second, that NSPW would not be incorporated into new
products to the extent that such incorporation would actually
increase the amount of silver released into the environment.
The panel concluded that without evidence in the record to
support the assumptions, it could not find that the EPA’s
public-interest finding was supported by substantial evidence
as required by the Act.


                         COUNSEL

Jaclyn H. Prange (argued), San Francisco, California; Aaron
Colangelo, Washington, D.C.; as and for Petitioner Natural
Resources Defense Council.
4                    NRDC V. USEPA

George A. Kimbrell and Sylvia Wu, Center for Food Safety,
San Francisco, California, for Petitioners Center for Food
Safety and International Center for Technology Assessment.

Sue Chen (argued), Attorney, and John C. Cruden, Assistant
Attorney General, Environmental Defense Section,
Environment & Natural Resources Division, United States
Department of Justice, Washington, D.C.; Amber Aranda, Of
Counsel, Office of General Counsel, United States
Environmental Protection Agency, Washington, D.C.; for
Respondent.


                         OPINION

MELLOY, Circuit Judge:

    The Federal Insecticide, Fungicide, and Rodenticide Act
(“FIFRA”), 7 U.S.C. §§ 136–136y, governs the sale, use, and
distribution of pesticides. Under FIFRA, a pesticide
generally must be registered with the Environmental
Protection Agency (“EPA”) before it is sold or distributed.
Id. § 136a(a). In order to obtain pesticide registration, an
applicant must submit sufficient data “concerning the
product’s health, safety, and environmental effects.”
Pollinator Stewardship Council v. EPA, 806 F.3d 520, 523
(9th Cir. 2015). The registration requirement thus enables the
EPA to prohibit pesticides that will cause “unreasonable
adverse effects on the environment.” Id. (quoting 7 U.S.C.
§ 136a(c)(5)). Sometimes, however, the EPA may receive
sufficient data to determine that short-term use of a pesticide
is reasonable, but not enough data regarding its long-term
use. See 7 U.S.C. § 136a(c)(7)(C). If the EPA lacks this data
“because a period reasonably sufficient for generation of the
                      NRDC V. USEPA                            5

data has not elapsed since the [EPA] first imposed the data
requirement,” the EPA may grant a temporary, conditional
registration. Id. But, to issue the conditional registration, the
EPA must first determine “that use of the pesticide is in the
public interest.” Id.

    This case involves the pesticide NSPW-L30SS
(“NSPW”). Manufactured by Nanosilva LLC, NSPW is an
antimicrobial materials preservative that uses nanosilver as its
active ingredient. Petitioners—the Natural Resources
Defense Council, the Center for Food Safety, and the
International Center for Technology Assessment—opposed
the EPA’s conditional registration of NSPW during public
notice and comment. Petitioners argued the EPA failed to
support its findings that (1) use of NPSW is in the public
interest; and (2) Nanosilva LLC had insufficient time to
submit the required data.          The EPA, nonetheless,
conditionally registered NSPW in May 2015. Petitioners
filed a timely petition for review and now renew their
arguments before this Court. We have jurisdiction for direct
review of the agency action pursuant to 7 U.S.C. § 136n(b).

    After reviewing the conditional registration for substantial
evidence, we conclude the EPA failed to support its finding
that NSPW is in the public interest. We therefore vacate the
registration in whole and need not reach Petitioners’
insufficient-time arguments.

                               I

    NSPW is a materials preservative incorporated into
plastic and textile products. When so incorporated, the EPA
explains, NSPW can help “suppress the growth of bacteria,
algae, fungus, mold[,] and mildew, which cause odors,
6                          NRDC V. USEPA

discoloration, stains, and deterioration.”1 NSPW may be used
in products such as trash cans, mops, window blinds,
furniture, baseboards, light switches, plastic decking, carpet,
toilet seats, shower curtains, tubs, cell phones, computers,
plastic components in humidifiers, vacuums, combs, brushes,
electric razors, blow dryers, beds, wall coverings,
wheelchairs, linens, golf bags, exercise equipment, life
preservers, sportswear, nursing uniforms, watch bands,
restaurant uniforms, litter boxes, swimming pool equipment,
ink pens, portable toilets, office supplies, and luggage.
NSPW may not be used, however, in products designed for
food contact, food packaging, or drinking water.

    The active ingredient in NSPW is nanosilver. Simply put,
nanosilver is a version of “conventional” silver that is
engineered to have a much smaller particle size.
Conventional silver has long been recognized as an
antimicrobial agent and is the active ingredient in a number
of currently registered pesticides used as materials
preservatives. Nanosilver, due to its much smaller particle
size, can have significantly different properties than
conventional silver. These different properties provide new
benefits and opportunities to industry.

   But with these new benefits come new risks. After
companies seeking to market nanosilver and other
nanomaterials began approaching the EPA for pesticide


    1
       Under FIFRA, pesticides include “any substance or mixture of
substances intended for preventing, destroying, repelling, or mitigating
any pest.” 7 U.S.C. § 136(u). Pests include, with some exceptions,
“(1) any insect, rodent, nematode, fungus, weed, or (2) any other form of
terrestrial or aquatic plant or animal life or virus, bacteria, or other micro-
organism.” Id. § 136(t).
                     NRDC V. USEPA                        7

registration, the EPA convened a meeting of the FIFRA
Scientific Advisory Panel (“the Panel”) in November 2009 to
discuss potential hazards. In part, the Panel summarized its
meeting as follows:

           The Panel was not aware of any
       information that suggested that silver ions
       released from silver nanomaterials would
       behave differently than silver ions generated
       by any other source. However, the Panel
       believed that the rate of silver ion production,
       as well as the distribution of silver in
       [biological] tissue, may differ substantially
       between silver nanomaterials and other forms
       of silver. Nanomaterials can deliver ions
       directly to specific tissues, cell membranes or
       inside cells. The biological effects of silver
       nanomaterials (including temporal pattern for
       ion delivery), as well as their environmental
       fate, can be affected by other materials
       present in the preparation (e.g., surfactants).
       Nanosilver can also potentially act as a carrier
       for other toxic chemicals. These issues led
       the Panel to suggest that the hazard profile of
       silver nanomaterials may differ from other
       forms of silver.

The Panel thus recommended that the “EPA treat nanosilver
differently from its conventional silver counterpart.” The
Panel also “cautioned about extrapolating from one
nanosilver formulation to another when assessing hazards.”

   After the Panel convened, the EPA evaluated and
conditionally registered two pesticides containing a form of
8                         NRDC V. USEPA

nanosilver: AGS-20 and NSPW. Like NSPW, AGS-20 is a
nanosilver-based antimicrobial pesticide used as a materials
preservative. However, unlike NSPW, which is a liquid
suspension incorporated into plastics and textiles, AGS-20 is
a powder which may be used as a surface coating or by
incorporation with textiles only. The nanosilver in AGS-20
also “has different size range and surface coatings than the
nanosilver in NSPW[ ].” The EPA conditionally registered
AGS-20 in December 2011, approximately three years after
its manufacturer submitted its application for registration.2

     Granting conditional registration to NSPW, however, took
longer. While Nanosilva LLC submitted an application to
register NSPW in 2009, the EPA did not conditionally
register NSPW until May 2015. This conditional registration
is for a four-year period.

    In granting conditional registration, the EPA found, as
required by 7 U.S.C. § 136a(c)(7)(C), that NSPW contained
a new active ingredient and would not cause unreasonable
adverse effects on the environment. The EPA also made two
findings relevant to Petitioners’ arguments in the present
case. The EPA, first, found that granting conditional
registration was in the public interest. Specifically, it found
that NSPW had a lower application rate (i.e., it uses less
silver) and a lower mobility rate (i.e., it is less likely to
release silver into the environment in detectable quantities)
when compared to conventional-silver pesticides. Therefore,
the EPA reasoned, using NSPW has the “potential” to reduce


    2
     This Court partially vacated the conditional registration of AGS-20
because the EPA did not satisfy its own rule for determining particular risk
concerns requiring mitigation. Nat. Res. Def. Council v. EPA (NRDC),
735 F.3d 873, 886–87 (9th Cir. 2013).
                          NRDC V. USEPA                                   9

“environmental loading” and risk caused by silver release.
Second, the EPA found that Nanosilva LLC had insufficient
time to generate the data required for unconditional
registration.

    Petitioners, who had opposed the conditional registration
of NSPW during public notice and comment, filed a timely
petition for review in this Court. They argue the EPA failed
to support its public-interest and insufficient-time findings.3

                                    II

    The following principles guide our review of the EPA’s
conditional registration of NSPW. First, we must sustain the
conditional registration “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.” NRDC, 735 F.3d at 877
(quoting Vasquez v. Astrue, 572 F.3d 586, 591 (9th Cir.
2009)). “Under the substantial evidence standard, we must
affirm the [EPA’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.’” Id. (quoting
Nw. Food Processors Ass’n v. Reilly, 886 F.2d 1075,
1079–80 (9th Cir. 1989)). “When, as in this case, the agency
‘is making predictions, within its area of special expertise, at


    3
      Petitioners do not directly challenge the EPA’s no-adverse-effects
finding (i.e., that NSPW will not have any unreasonable adverse effect on
the environment during the conditional-registration period), but Petitioners
do not concede that NSPW is “safe.”
10                    NRDC V. USEPA

the frontiers of science . . . a reviewing court must generally
be at its most deferential.’” Id. (alteration in original)
(quoting Balt. Gas & Elec. Co. v. NRDC, 462 U.S. 87, 103
(1983)). But, “[a]lthough we must give due deference to
EPA’s findings, ‘[i]t is well-established that an agency’s
action must be upheld, if at all, on the basis articulated by the
agency itself.’” Id. (quoting Motor Vehicle Mfrs. Ass’n of the
U.S., Inc. v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 50
(1983)).

    Second, “[w]hen interpreting a statute, we are guided by
the fundamental canons of statutory construction and begin
with the statutory text.” United States v. Neal, 776 F.3d 645,
652 (9th Cir. 2015). “The words of a statute should be
accorded their plain meaning, as considered in light of ‘the
particular statutory language at issue, as well as the language
and design of the statute as a whole.’” Preap v. Johnson,
831 F.3d 1193, 1200 (9th Cir. 2016) (quoting K Mart Corp.
v. Cartier, Inc., 486 U.S. 281, 291 (1988)). And, where a
statute is ambiguous, “we may turn to legislative history for
guidance.” United States v. Thomsen, 830 F.3d 1049, 1058
(9th Cir. 2016).

                              III

    We are unaware of any prior decision considering the
public-interest requirement under 7 U.S.C. § 136a(c)(7)(C).
Accordingly, to illuminate the requirement’s role in FIFRA’s
regulatory scheme, we begin with the statutory background.
We then turn to whether the EPA supported its public-interest
finding in the present case with substantial evidence.
                      NRDC V. USEPA                          11

                               A

    Generally, “no person . . . may distribute or sell to any
person any pesticide that is not registered” with the EPA.
7 U.S.C. § 136a(a). In order for the EPA to evaluate an
application for pesticide registration, an application must
“describ[e] how the pesticide will be used, the claims made
of its benefits, the ingredients, and a description of all tests
and studies done and the results thereof, concerning the
product’s health, safety, and environmental effects.”
Pollinator Stewardship Council, 806 F.3d at 523. The EPA
“shall publish guidelines specifying the kinds of information
which will be required to support the registration of a
pesticide and shall revise such guidelines from time to time.”
7 U.S.C. § 136a(c)(2)(A).

    After an applicant submits sufficient data for pesticide
registration, the EPA may grant “unconditional registration”
under § 136a(c)(5). “Unconditional registration necessarily
requires sufficient data to evaluate the environmental risks.”
Pollinator Stewardship Council, 806 F.3d at 523; see also
7 U.S.C. § 136a(c)(5) (listing the findings required for
unconditional registration). If an applicant has not submitted
sufficient data to support unconditional registration, however,
the EPA may conditionally register the pesticide under
limited circumstances. See 7 U.S.C. § 136a(c)(7). As
relevant here,

       [t]he [EPA] 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
12                   NRDC V. USEPA

       reasonably sufficient for generation of the
       data has not elapsed since the [EPA] first
       imposed the data requirement) on the
       condition that by the end of such period the
       [EPA] 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 [EPA] may
       prescribe. A conditional registration under
       this subparagraph shall be granted only if the
       [EPA] 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. § 136a(c)(7)(C) (emphasis added).

    The public-interest requirement reflects an important
distinction between conditional registration and unconditional
registration. Conditional registration under § 136a(c)(7)(C)
does not require all the risk data that unconditional
registration requires. Cf. 7 U.S.C. § 136a(c)(5). Instead,
§ 136a(c)(7)(C) allows the EPA to temporarily register a
pesticide with less-than-complete risk data so long as the
EPA, among other additional requirements, determines “that
use of the pesticide is in the public interest.”

    The statute does not further define the public-interest
requirement, but the legislative history of conditional
registration provides some insight. For example, a report by
the Senate Committee on Agriculture, Nutrition, and Forestry,
stated:
                       NRDC V. USEPA                            13

        No new-chemical conditional registration
        [under § 136a(c)(7)(C)] could be issued unless
        the [EPA] determined that use of the pesticide
        during the period required to complete the
        testing would not cause any unreasonable
        adverse effect on the environment, and that
        use of the pesticide was required in the public
        interest, as for example, in a situation in
        which there is a significant pest control
        problem which cannot satisfactorily be
        handled by use of products which have been
        fully registered.

S. Rep. No. 95-334, at 21 (1977).

     On the Senate floor, Senator Leahy, who sponsored the
bill that created the conditional registration provision, stated
that the Senate committee carefully considered the statutory
requirements so conditional registration “would be reserved
to the truly exceptional case.” 123 Cong. Rec. 25,706 (1977).
The public-interest requirement, he stated, was “a more
stringent test” than that required for unconditional
registration. Id. (“[A]s with all other registrations, the [EPA]
has to find the pesticide would not have an unreasonable
adverse effect on the environment. Finally, a more stringent
test also applies. The [EPA] must be shown evidence
sufficient to find that this confidential [sic] registration is ‘in
the public interest.’” (emphasis added)). Senator Leahy
continued:

        Suppose we have a serious pest problem for
        which this new pesticide would be an
        alternative to other chemicals. The [EPA] is
14                    NRDC V. USEPA

        sure that this pesticide is as safe as any
        pesticide already registered for that use. . . .

        . . . However, in the best case there may be
        gaps in the safety data which supports the
        registration. . . . Clearly the objectives of the
        Act are best served with the added flexibility
        to permit a new product to be used which will
        meet the stringent test of being “in the public
        interest.”

Id.

    The EPA Administrator, while testifying before the
Senate committee, made statements to similar effect. See S.
Rep. No. 95-334, at 74 (Statement of Douglas Costle, Adm’r,
Environmental Protection Agency) (“[T]here may be a real
need for use of the pesticide to avoid pest outbreaks. It is our
opinion that in some of these cases it would be proper to
allow conditional registration . . . if the public interest would
be served by issuance of a conditional registration, bearing in
mind the benefits as well as the likely scope of the risk.
Although we think that the exercise of this conditional
registration authority for new chemicals would be rare, we
feel that it should be available in appropriate cases.”).

                               B

    In the present case, the EPA found that use of NSPW is in
the public interest because it has the “potential” to reduce the
amount of silver released into the environment. Petitioners
do not dispute that reducing the amount of silver in the
environment is in the public interest. Rather, Petitioners
dispute the factual premises underlying the EPA’s public-
                         NRDC V. USEPA                                15

interest finding: (1) that NSPW has a lower application rate
(i.e., it uses less silver) than conventional-silver pesticides;
(2) that NSPW has a lower mobility rate (i.e., it is less likely
to release silver into the environment in detectable
quantities); and (3) that current users of conventional-silver
pesticides will switch to NSPW and/or that NSPW will not be
incorporated into new products. Although we conclude that
the first two premises are supported by substantial evidence,
we hold that the third premise impermissibly relies on
unsubstantiated assumptions.4

                                    1

    The EPA found that NSPW has a lower application rate
than conventional-silver pesticides. Petitioners make several
arguments as to why this finding is not supported by
substantial evidence. First, Petitioners argue that the
increased toxicity of nanosilver5 outweighs any benefits that
may be gained from its lower application rate. We conclude,
however, that the EPA presented substantial evidence to
support its conclusion regarding these toxicity risks. In its
response to public comments, the EPA agreed that the
evidence suggests “nano-scale silver can potentially be more
toxic than ionic silver alone.” But the EPA also concluded
that the evidence suggests nanosilver would not be


    4
      The EPA argues that Petitioners waived their arguments regarding
EPA’s assumptions. After reviewing Petitioners’ comments, however, we
conclude Petitioners more than adequately raised the issue during the
public notice-and-comment period. See Portland Gen. Elec. Co. v.
Bonneville Power Admin., 501 F.3d 1009, 1023–24 (9th Cir. 2007).
    5
      For ease of reference and at the risk of conflating different
formulations of nanosilver, our references to “nanosilver” from this point
forward pertain only to the formulation in NSPW.
16                    NRDC V. USEPA

“sufficiently more toxic to raise risk concerns when taking
into account leaching rates for the proposed uses.” For
NSPW incorporated into plastics, the EPA specifically
concluded that nanosilver would have to be “on the order of
5 to 53 times more toxic” than conventional silver to raise
concerns. And for textiles, the EPA concluded that
nanosilver would need to be “on the order of 19 to 190 times
more toxic.”

    Petitioners appear to attack these conclusions as too
probabilistic. Nevertheless, on substantial evidence review,
the EPA need not present evidence to support an outcome
with certainty; it only needs to present “such relevant
evidence as a reasonable mind might accept as adequate to
support a conclusion.” NRDC, 735 F.3d. at 877 (quoting
Vasquez, 572 F.3d at 591). We defer to the EPA on this
matter as it “is making predictions, within its area of special
expertise, at the frontiers of science.” Id. (quoting Balt. Gas
& Elec. Co., 462 U.S. at 103).

    Second, Petitioners argue the EPA failed to explain why,
given the different risks that nanosilver poses, total mass of
silver applied (i.e., the application rate) is a relevant point of
comparison. We disagree. The EPA acknowledged in its
response to public comments that there was an “ongoing
debate within the scientific community that metrics other than
mass (such as particle number or surface area) may be more
suitable for assessing nanosilver risks.” But the EPA also
considered its use of mass in light of NSPW’s “low release[ ]
[levels], the available toxicity data, use of multiple risk
uncertainty factors . . . [,] and required risk-mitigation
measures on the product label.” Under these circumstances,
we conclude that the EPA’s qualified use of application rate
                     NRDC V. USEPA                        17

is supported by substantial evidence and defer to the EPA’s
expertise.

    Finally, Petitioners argue that the EPA failed to support
its finding that conventional-silver pesticides have higher
application rates than NSPW. While the EPA did not list
every conventional-silver pesticide’s application rate, the
EPA listed 16 different conventional-silver pesticides and
their application rates in its response to public comments.
These rates are many times higher than the permitted
application rate of NSPW. As a result, we cannot say the
EPA’s lower-application rate finding is unsupported by
substantial evidence.

                             2

    The EPA next found that the nanosilver in NSPW has a
lower mobility rate than conventional-silver pesticides.
Petitioners argue this finding relied on a plastic-leaching
study the EPA had previously found deficient. But the EPA’s
finding does not rely solely on the plastic-leaching study.
The EPA also relied on an undisputed textile-leaching study
which also demonstrated low release levels. Further, the EPA
noted, the nanosilver in NSPW “is complexed to a larger
particle that is then embedded into a polymer. The submitted
leaching studies show that nanosilver is unlikely to be
released in quantities above the detection limit from this
polymer. Absent release from the polymer, the mobility of
nanosilver in [NSPW] is greatly reduced.” The EPA then
compared this structure to “silver salts,” which are in “most
silver-based pesticide products” and          “immediate[ly]
dissol[ve].” We therefore cannot say the EPA’s lower-
mobility finding is unsupported by substantial evidence.
18                       NRDC V. USEPA

                                   3

    Our inquiry cannot end with the EPA’s findings that
NSPW has lower application and mobility rates than
conventional-silver pesticides. The EPA ultimately found
that, based on these comparatively lower rates, use of NSPW
is in the public interest because it has the “potential” to
reduce the amount of silver released into the environment.
This finding, however, also requires two interrelated and
unstated assumptions. It assumes, first, that current users of
conventional-silver pesticides will replace those pesticides
with NSPW (“the substitution assumption”). It also assumes
that NSPW will not be incorporated into new products to the
extent that such incorporation would actually increase the
amount of silver released into the environment (“the no-new-
products assumption”).6 Because the substitution and no-
new-products assumptions are unsubstantiated, Petitioners
argue that the EPA’s public-interest finding is not supported
by substantial evidence. We agree.

    The EPA cites no evidence in the record to support its
substitution assumption. In briefing before this Court, the
EPA contends that substitution will occur as a “logical
matter.” To support this proposition, the EPA cites a 1983
entry in the Federal Register, stating that “the pesticide
market in general is finite, relatively ‘saturated’ and inelastic”
and that increased competition may “shift the user’s purchase


     6
      New “products” should not be confused with new “use pattern.”
When referring to new products, we refer to new incorporations of NSPW
within current registered use patterns. In other words, we base our
analysis on the understanding that NSPW is not registered for use in a
greater range of products than conventional-silver pesticides. We provide
an example below of how NSPW may be used in new products.
                     NRDC V. USEPA                          19

from one company to another.” See Regulations for the
Enforcement of FIFRA; Conditional Registration, 48 Fed.
Reg. 34000, 34003 (July 26, 1983). We cannot, however,
“accept appellate counsel’s post-hoc rationalizations for
agency action.” Hernandez-Cruz v. Holder, 651 F.3d 1094,
1109 (9th Cir. 2011) (quoting Nw. Envtl. Def. Ctr. v.
Bonneville Power Admin., 477 F.3d 668, 688 (9th Cir. 2007)).
And even if we did consider the general description of the
pesticide market as it existed in 1983, we have no way of
knowing from the record whether that description is
applicable to the current and narrower markets for silver-
based pesticides and materials preservatives.

    The lack of evidence supporting the substitution
assumption may not be inherently problematic: if no
manufacturer adopts NSPW, then NSPW will not present any
environmental risk through those manufacturers’ products.
The substitution assumption is certainly problematic,
however, in light of the no-new-products assumption, which
the EPA also fails to support. In response, the EPA argues
that there is no evidence to suggest that NSPW will be
incorporated into new products. It also contends that it need
not prove a negative proposition. But, as a logical matter, the
EPA’s no-new-products assumption must be weighed against
its substitution assumption. On the one hand, the EPA
assumes that current users of conventional-silver pesticides
will switch to NSPW. On the other hand, however, it
assumes that the benefits of NSPW will not also invite
manufacturers to incorporate NSPW into new products. The
EPA assumes, moreover, that the costs of switching to NSPW
are not prohibitive, while it also assumes that the costs of
adding NSPW will discourage new incorporations during the
conditional-registration period.
20                    NRDC V. USEPA

    We do not foreclose the possibility that the EPA could
have proved these assumptions. Nevertheless, without
evidence in the record to support the assumptions, we cannot
find that the EPA’s public-interest finding is supported by
substantial evidence as required by FIFRA. Requiring
substantial evidence for public-interest findings, moreover, is
not just a statutory technicality as this case demonstrates.
Here, if the EPA’s substitution and no-new-products
assumptions are incorrect, NSPW may increase the amount
of silver released into the environment and contravene the
identified public interest. For example, the EPA permits
NSPW and conventional-silver pesticides to be incorporated
into certain plastic furniture under their current registrations.
That does not mean, however, that every plastic furniture
product actually incorporates a silver-based pesticide.
Furniture A may use a conventional-silver pesticide as a
materials preservative, but Furniture B may not. If NSPW is
added to Furniture B, but Furniture A continues to use the
conventional-silver pesticide, there would be a net increase in
the amount of silver incorporated into Furniture A and B.

    The EPA argues that its public-interest finding allows for
the possibility that NSPW could be added to new products.
It points out, again, that the maximum application rate for the
nanosilver in NSPW “is a fraction of what is permitted in
pesticides containing conventional silver.” The EPA also
states that it used “worst-case scenarios” when assessing how
much nanosilver from NSPW will leach from products and
end up in the environment. The maximum-application-rate
argument, however, still requires the assumption that a new
product would have used a conventional-silver pesticide if
NSPW was not available or that there will be an offsetting
switch from conventional-silver pesticides to NSPW in other
products. Regardless of whether the NSPW in new products
                         NRDC V. USEPA                               21

only releases a minimal amount of silver in the “worst-case
scenario,” the public interest that the EPA identified—
reducing silver in the environment—will not be met unless
one of these two conditions occurs. In short, these arguments
do nothing to address the core flaw in the EPA’s public-
interest finding: it assumed, without citing evidence, that
NSPW would be used enough by some, but not too much by
others, to reduce the amount of silver released into the
environment.

    The EPA is capable of evaluating the kind of evidence
that would have substantiated these assumptions. In its own
guidance document, the EPA listed several factors it may
consider in determining whether a pesticide is in the public
interest. See Pesticide Programs; Conditional Registration of
New Pesticides, 51 Fed. Reg. 7628-02, 7632–33 (Mar. 5,
1986).7 These include economic factors, such as the
“[c]omparative estimated costs and savings if the new
pesticide is used instead of equivalent registered pesticides or
alternative non-pesticide methods.” Id. at 7633. NSPW,
moreover, is not the first registered nanosilver-based
materials preservative on the market—it is the second. While
there are important differences between AGS-20 and NSPW,
we have to imagine that at least some data about AGS-20’s
use and adoption were ascertainable and would have been
useful in evaluating the EPA’s assumptions.




    7
      We acknowledge that the EPA is not bound to follow this guidance
document and do not base our decision on the EPA’s failure to follow this
guidance. The guidance is only relevant to the extent it illustrates the
EPA’s capability to evaluate and consider the kind of evidence that would
have supported its public-interest finding in the present case.
22                    NRDC V. USEPA

    To be clear, the EPA need not “ferret out every possible
alternative” scenario that may occur when determining that a
conditional registration is in the public interest. Vermont
Yankee Nuclear Power Corp. v. Nat. Res. Def. Council, 435
U.S. 519, 551 (1978). Nor is the EPA required to support a
public-interest finding with “scientific certainty.” ASARCO,
Inc. v. Occupational Safety & Health Admin., 746 F.2d 483,
490 (9th Cir. 1984) (citation omitted). But the EPA must
support its predicted public-interest scenario with “substantial
evidence when considered on the record as a whole.” See
7 U.S.C. § 136n(b). Further, § 136a(c)(7)(C) does not
presume, as the EPA argues, that the EPA may collect
missing data related to its public-interest finding after
granting a conditional registration. To the contrary,
§ 136a(c)(7)(C) only presumes the EPA needs more data to
make a risk determination regarding use of the pesticide
beyond the conditional-registration period. The statute
instead requires the EPA to find, before granting conditional
registration, “that use of the pesticide is in the public
interest.” 7 U.S.C. § 136a(c)(7)(C) (emphasis added).

    In sum, the public-interest requirement of § 136a(c)(7)(C)
is an additional, “more stringent test” that distinguishes
conditional registration from unconditional registration. See
123 Cong. Rec. at 25,706; see also Preap, 831 F.3d at 1200
(stating that a statute’s words must be considered in light of
the “design of the statute as a whole” (citation omitted)). To
pass this test, the EPA must support its finding with
substantial evidence. The EPA may not satisfy the
requirement by simply finding that a pesticide has the
“potential” to be in the public interest—especially where the
pesticide also has the “potential” to contravene the public
interest. Accordingly, where an essential premise of a public-
interest finding is only supported by bare assumptions, as in
                     NRDC V. USEPA                          23

the present case, we will find substantial evidence lacking.
See Pollinator Stewardship Council, 806 F.3d at 538 (N.R.
Smith, J., concurring in judgment) (“Although the EPA
certainly has authority to rely on its well-founded beliefs,
scientifically-derived knowledge, and experience-driven
professional judgment, it must support the beliefs,
knowledge, and judgment with evidence.”).

                              IV

    The public-interest finding is an essential prerequisite to
conditional registration under § 136a(c)(7)(C). With respect
to NSPW, the EPA failed to support that finding with
substantial evidence. We therefore vacate the conditional
registration in whole, see 7 U.S.C. § 136n(b), and need not
reach the remaining grounds for vacatur raised by Petitioners,
see Pollinator Stewardship Council, 806 F.3d at 532–33.

   VACATED and REMANDED.

   Costs are awarded to the Petitioners.
