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Pesticide risk assessments are fraught with uncertainties that arise from the process of estimating exposure to and toxicity of chemicals. Regulatory agencies resolve those uncertainties in a health-protective (conservative) manner, typically acknowledging only inter- and intraspecies uncertainties quantitatively. Other uncertainties may be acknowledged qualitatively, but those safety factors (SF) are not enumerated. Quantitative risk appraisal may be used to enumerate the multiplicative SF generated by conservative assumptions regarding uncertainties. The magnitude of SF derived from decision points dealing with historically unquantified uncertainty may far exceed explicit SF used to gauge acceptable margins of exposure (MoE). Examination of the basis for some previously unenumerated SF may justify potential changes in regulatory practices and policies. Using past risk assessments of 3 pesticides (mevinphos, parathion, and methyl iodide) for which the California Department of Pesticide Regulation found unacceptable risk as examples, the previously unquantified SF ranged from 47 to 1 × 10 6 for scenarios involving handlers, reentry workers, and bystanders.

Fipronil is a pyrazole acaricide and insecticide that may be used for insect, tick, lice, and mite control on pets. Residents' short-term and long-term postapplication exposures to fipronil, including secondary environmental exposures, were estimated using data from chemical-specific studies. Estimations of acute (24-h) absorbed doses for residents were based on U.S. Environmental Protection Agency (U.S. EPA) 2012 standard operating procedures (SOPs) for postapplication exposure. Chronic exposures were not estimated for residential use, as continuous, long-term application activities were unlikely to occur. Estimated acute postapplication absorbed doses were as high as 0.56 μg/kg-d for toddlers (1-2 yr) in households with treated pets based on current U.S. EPA SOPs. Acute toddler exposures estimated here were fivefold larger in comparison to adults. Secondary exposure from the household environment in which a treated pet lives that is not from contacting the pet, but from contacting the house interior to which pet residues were transferred, was estimated based on monitoring socks worn by pet owners. These secondary exposures were more than an order of magnitude lower than those estimated from contacting the pet and thus may be considered negligible.