By Lisa M. Campbell, Timothy D. Backstrom, and James V. Aidala
On August 2, 2019, the U.S. Environmental Protection Agency (EPA) Office of Pesticide Programs (OPP) announced that it has decided to reduce the Food Quality Protection Act (FQPA) safety factor for infants and children for pyrethroids from its current value of 3X to a new value of 1X. This decision is based on a July 1, 2019, OPP report entitled “USEPA Office of Pesticide Programs’ Re-evaluation of the FQPA Safety Factor for Pyrethroids: Updated Literature and CAPHRA Program Data Review.” Risk assessments incorporating the new lower FQPA safety factor for pyrethroids will be utilized in developing proposed registration review decisions for these compounds, and EPA has stated it will be taking public comment on the OPP report reducing the FQPA safety factor for pyrethroids after EPA publishes a notice of availability for the proposed registration review decisions.
Pyrethroids are a group of insecticides that includes natural pyrethrins (found in chrysanthemums) and more than 30 synthetic compounds with similar structure and activity. EPA has determined that it is appropriate to establish one FQPA safety factor for all pyrethroid active ingredients because these compounds all have the same mode of action and similar patterns of toxicity. Pyrethroid insecticides are widely used in and around residential structures, on pets, in treated clothing, for mosquito control, and in various agricultural applications. EPA indicates that although pyrethroids have relatively low mammalian toxicity, EPA believes that the principal concern for human risk assessment is a potential to cause acute neurotoxic effects.
The FQPA safety factor is intended to account for “potential pre- and post-natal toxicity and completeness of data with respect to exposure and toxicity to infants and children.” The FQPA safety factor is set by statute at a default value of 10X, but EPA may select a lower value for this safety factor if EPA determines based on “reliable data” that such a lower value will be safe for infants and children. This determination necessarily depends on EPA’s assessment of the quality of the data that address the susceptibility to adverse effects of the pesticide of infants and children. Based on current EPA guidance, OPP evaluates the need for the default FQPA safety factor of 10X in two components: a safety factor of about 3X assigned to pharmacodynamic (PD) differences and a safety factor of about 3X assigned to pharmacokinetic (PK) differences. PD differences refer to the sequence of events at the molecular or cellular level leading to a toxic response to a substance, while PK differences refer to absorption, distribution, metabolism, and excretion of the substance.
EPA previously evaluated the adequacy of the database concerning risks to infants and children posed by pyrethroid active ingredients in 2011. At that time, EPA decided that there were sufficient data concerning the mechanism for potential neurotoxic effects of pyrethroids to allow EPA to reduce the factor for PD differences to 1X, but EPA retained the 3X factor for PK differences because EPA believed that the available pharmacokinetic data for pyrethroids was not sufficient for EPA to conclude that infants and children would not confront a greater risk of neurotoxic outcomes. After EPA made the 2011 determinations, the Council for the Advancement of Pyrethroid Human Risk Assessment (CAPHRA) conducted a variety of additional research to address whether children are more sensitive to the neurotoxic effects of pyrethroid exposure, and this research assessed both PD and PK differences. CAPHRA submitted a peer-reviewed physiologically based pharmacokinetic (PbPk) model for pyrethroids to EPA in 2018. After reviewing the new CAPHRA data and the current public literature for pyrethroids, EPA has now concluded that the factor for PD differences should be maintained at 1X, but the factor for PK differences should be reduced from 3X to 1X. Collectively, these determinations mean that EPA has concluded that there are reliable data to support a determination that infants and children are not more susceptible to the neurotoxic effects of pyrethroids than adults, so there is no need to retain either the default FQPA safety factor of 10X or the previous FQPA safety factor used for pyrethroids of 3X.
The adoption by EPA of a new FQPA safety factor of 1X for all pyrethroid active ingredients will likely facilitate retention of existing use patterns and use directions for a large number of pyrethroid insecticides that are commonly used in and around human residences and workplaces.
From a larger perspective, the process by which EPA evaluated and selected a proposed FQPA safety factor for pyrethroids may be seen as typical for most pesticides or classes of pesticides. The selection of a FQPA safety factor for a particular pesticide usually is based on review of available animal data, including PD and PK data, to determine whether there is any basis for concluding that infants and children may be more susceptible to adverse effects of that pesticide than adults. Where EPA decides that the animal data addressing this question are insufficient, affected registrants and other proponents of registration can consult with EPA concerning studies that will address the uncertainties. Depending on the outcome of such studies, EPA may be able to conclude that there is a scientific basis for a partial or complete reduction of the default FQPA safety factor.
Compared to this typical evaluation process, the recent decision by EPA to retain the default FQPA safety factor for all organophosphate (OP) active ingredients, which was based on EPA’s interpretation of neurodevelopmental effects reported at low exposure levels (below the threshold for acetylcholinesterase inhibition) in epidemiology studies for chlorpyrifos, may be seen as an aberration. EPA’s decision to rely on epidemiology studies that may be susceptible to methodological biases, and the decision to utilize epidemiology studies for chlorpyrifos to set the FQPA safety factors for all OP pesticides, have both been controversial.
EPA’s recent decision to retain the current tolerances and registrations for chlorpyrifos was based in significant part on EPA’s interpretation of a PbPk model for chlorpyrifos previously submitted by DowAgro (now Corteva), which mitigated to some degree EPA’s retention of the default FQPA safety factor for chlorpyrifos. Corteva may submit further data addressing PD and PK differences for chlorpyrifos, and EPA has also stated that it intends to review some new animal studies for chlopyrifos, which purport to show neurodevelopmental effects at low exposure levels. Perhaps these data will allow EPA to establish a point of departure (POD) for chlorpyrifos risk assessment without any need for a further excursion into the unfamiliar risk assessment territory represented by EPA’s use of epidemiology data for chlorpyrifos.