Dichlorvos (DDVP) Risk Characterization Document - California ...

V. RISK APPRAISAL
The health risk assessment of DDVP was conducted for the exposures of workers through work
activities and the general population. Risk assessment is the process used to evaluate the potential
for human exposure and the likelihood that the adverse effects of a substance will occur in humans
under the specific exposure conditions. Every risk assessment has inherent limitations on the
application of existing data to estimate the potential risk to human health. Therefore, certain
assumptions and extrapolations are incorporated into the hazard identification, dose-response
assessment, and exposure assessment processes. This, in turn, results in uncertainty in the risk
characterization which integrates all the information from the previous three processes. Qualitatively,
risk assessments for all chemicals have similar uncertainties. However, the degree or magnitude of
the uncertainty can vary depending on the availability and quality of the data, and the types of
exposure scenarios being assessed. Specific areas of uncertainty associated with this risk
assessment for DDVP are delineated in this section.
Toxicology
Pharmacokinetic studies with rats and humans showed that DDVP was rapidly metabolized, and the
exhaled air was the primary route of excretion in both species (Blair et al., 1975; and studies cited in
the III.A. PHARMACOKINETICS). Acute and subchronic toxicology studies in humans showed the
inhibition of plasma or/and erythrocyte activities. In several studies, the inhibition of erythrocyte ChE
activity was transient (Rasmussen et al., 1963 and Menz et al., 1974). None of the subjects in those
studies reported cholinergic signs. Since there is insufficient information on the effects of DDVP on
humans and uncertainty in the interpretation of the human data based on plasma and erythrocyte
cholinesterase inhibition, results from animals studies were used in the risk assessment of DDVP.
There was sufficient information in the toxicology database of animal studies to show that DDVP by
the oral (acute and chronic exposures) and inhalation routes (acute exposure) was a potent
cholinesterase inhibitor as both ChE inhibition and cholinergic signs were observed. The inhalation
study used for the acute NOEL was not acceptable to DPR according to FIFRA guidelines as a
developmental toxicity study (Thorpe et al., 1971b). The deficiencies (inadequate number of fetuses
and inadequate examination of fetuses) of the study did not affect the usefulness of the observations
on maternal toxicity.
Chronic toxicity of DDVP by the inhalation route is less certain as only one study was available (Blair
et al., 1974). Rats were exposed to DDVP by whole body exposure which included inhalation, and
potential oral (from grooming) or dermal exposure. The study was unacceptable to DPR according to
FIFRA guidelines because of only partial brain examination, higher survival in the high dose group
compared to controls, and lack of optimal high dose exposure. However, these deficiencies should
not affect the interpretation of the data used to establish the NOEL at the lowest dose studied.
Exposure Assessment
Exposures to DDVP by humans in the work place were determined for warehouse workers, structural
PCOs, and livestock applicators. The workers in livestock and other farm buildings were not
specifically addressed. However for risk assessment purposes, the exposure levels from the
warehouse workers may be used as a surrogate to evaluate the exposure.
The use of DDVP on raw agricultural commodities, livestock products, and processed commodities
may result in DDVP residues in the diet. Of all the food-related uses, potential exposure from
residues in livestock products represents the most significant concern. The direct applications of
DDVP on livestock (cattle and poultry) and secondary residues in the milk may result in residues
because there are no "pre-slaughter" or "pre-milking" intervals (similar to a preharvest interval) on the
75