The Environmental Protection Agency on June 30 approved three new pesticides that under certain definitions — (but not the EPA’s) — would constitute PFAS chemicals.

The agency approved the pesticides’ registration under section 3(c)(5) of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). The three approved active ingredients are:

1. Epyrifenacil:  for use on canola, field corn, soybean, wheat, and certain fallow lands and to maintain bare ground in non-crop areas. Epyrifenacil (S-3100), ethyl 2-[[3-[2-chloro-5-[3,6-dihydro-3-methyl-2,6-dioxo-4 (trifluoromethyl)-1(2H)-pyrimidinyl]-4-fluorophenoxy]-2-pyridinyl]oxy]acetate, is a new active ingredient that is proposed for use as a pre-plant burndown herbicide for agricultural use in canola, field corn, soybean, wheat, and fallow land (corn, soybean, and wheat), and for non-agricultural use on non-crop areas (including guard rails, above-ground pipelines, railroads and surrounding areas, parking and storage areas, airports, industrial areas, around farm buildings, fence rows, windbreaks, shelterbelts, road surfaces, and shoulders). EPA’s memorandum in support of its final decision to approve can be found here.
2. Diflufenican, (N-(2,4-difluorophenyl)-2-[3-(trifluoromethyl)phenoxy]-3-pyridinecarboxamide), is a new active ingredient herbicide for preplant and preemergence control of waterhemp, Palmer amaranth, and other pigweed species in corn and soybean. Diflufenican is a selective contact herbicide with a pesticidal mode-of-action that acts via inhibition of phytoene desaturase (PDS) enzyme. This interferes with carotenoid biosynthesis which leads to bleaching and death in susceptible plants. The Weed Science Society of America (WSSA) classifies diflufenican as a Group 12 herbicide, which makes diflufenican the first effective PDS inhibitor herbicide for control of waterhemp, Palmer amaranth, and other pigweed species in corn and soybean. This is a novel mechanism of action (MOA) that could be a valuable rotation tool for pest resistance management in the registered crops. EPA’s memorandum in support of its final decision to approve can be found here.
3. Trifludimoxazin (1,5-dimethyl-6-thioxo-3-[2,2,7-trifluoro-3-oxo-4-(prop-2-yn-1-yl)-3,4-dihydro 2-1,4-benzoazin-6-yl]-1,3,5-triazinane-2,4-dione; CAS Number: 1258836-72-4), is a new oxazine contact herbicide to control broadleaf and grass weed species. Both the Herbicide Resistance Action Committee (HRAC) and WSSA classify trifludimoxazin as a Group 14 (PPO inhibition) herbicide. The herbicidal action of trifludimoxazin is as a light-dependent peroxidizing herbicide (LDPH) that acts through inhibition of the protoporphyrinogen-IX-oxidase (PPO) enzyme, which is an important component of biosynthesis of chlorophyll in plants. PPO inhibition ultimately causes the cell membranes to leak, rapidly dry, and disintegrate. Trifludimoxazin is primarily a contact herbicide and is rapidly absorbed by roots and foliage. Susceptible emerging weed seedlings treated with trifludimoxazin absorb the chemical through roots and foliage and will usually die as they reach the soil surface or shortly after emergence. Trifludimoxazin will be an additional broad-spectrum weed control option in the labeled crops to control weeds, including weeds that are resistant to some PPO herbicides and other modes of action. EPA’s memorandum in support of its final decision to approve can be found here.

The approvals have garnered attention because of the potential identification of these active ingredients as PFAS — depending on the definition used.

In 2023, EPA’s Office of Pollution Prevention and Toxics (OPPT), in a final rule, defined PFAS as a chemical containing at least one of these three structures:

1. R-(CF2)-CF(R’)R’’, where both the CF2 and CF moieties are saturated carbons

1. R-CF2OCF2-R’, where R and R’ can either be F, O, or saturated carbons

1. CF3C(CF3)R’R’’, where R’ and R” can either be F or saturated carbons.

OPPT explained that molecules containing a single, fully or partially fluorinated carbon are less likely to persist in the environment or pose greater bioaccumulation or toxicity risks than molecules with two or more fluorinated carbons.

Under the EPA’s definition, EPA-approved single fluorinated compounds are not PFAS and do not pose any risks of concern when used as labeled. In contrast, other organizations such as the Organisation for Economic Co-operation and Development (OECD) define PFAS more broadly and inclusive as any chemical that has at least one fully fluorinated methyl group (-CF₃) or a fully fluorinated methylene group (-CF₂-).

These new ingredients qualify as PFAS under the latter definition.