This study identified several work activities that were associated with BuChE inhibition, and some PPE use practices that appeared to prevent BuChE inhibition. Results were generally consistent with the findings of other studies.[21
] Handlers who mix/load pesticides are generally considered to have relatively high exposures,[25
] and this activity may be particularly hazardous due to potential exposure to OP/CBs in their concentrated forms (i.e., before being diluted for application). We also found that handlers who cleaned spray equipment had significantly greater BuChE inhibition. Similarly, Arbuckle et al.
found that washing spray equipment was associated with elevated urinary levels of the herbicide 2,4-D.[34
] Although 2,4-D is not a ChE-inhibiting pesticide, the exposure pathway is likely to be similar. There was also some suggestion in our study that handlers may have been exposed while cleaning out pesticide containers or cleaning up after pesticide spills, although these factors were not significantly associated with BuChE inhibition after adjustment.
Use of PPE has been shown to minimize pesticide exposures effectively.[16
] In our study, handlers who wore full-face respirators and chemical-resistant footwear had significantly lower levels of BuChE inhibition. Storing PPE in a locker was also protective against BuChE inhibition. Handlers who change into chemical-resistant boots for applications and store PPE in a locker at work may have less “take home” exposure. Although such PPE use and storage practices may afford greater protection, it is also possible that these variables could be surrogates for safer handling practices in general.
Other studies have shown that glove use is associated with lower levels of exposure while mixing or applying pesticides.[34
] We did not see any strong associations between glove use and BuChE inhibition in this study, but it should be noted that only one participant did not wear chemical-resistant gloves. As such, we could only evaluate differences in BuChE inhibition between handlers who wore chemical-resistant gloves alone (67%) and handlers who wore chemical-resistant gloves in combination with disposable gloves (18%) or cloth gloves (13%).
We found that handlers who wore chemical-resistant aprons had somewhat greater BuChE inhibition than other handlers; this association was not in the anticipated direction. Chemical-resistant aprons are generally worn by handlers while mixing/loading pesticides, which is an activity with an inherently higher risk of exposure. Although we attempted to control for handling activities in this analysis, there may have been residual confounding due to generally higher exposures among handlers who wore aprons relative to other handlers.
In terms of decontamination practices, Curwin et al.
found that hand washing significantly reduced the concentration of acephate residues on the hands of tobacco harvesters.[36
] We did not see an association between hand washing practices and BuChE inhibition, which may be due to exposure misclassification. We collected self-reported information about “usual” hand washing practices, but were unable to ascertain the frequency and consistency of such practices after each application. Such misclassification might also explain the lack of associations between decontamination before breaks (i.e., potential contamination from using a cellular phone, eating, or urinating) and BuChE inhibition. More detailed observations of decontamination practices as potential sources of exposure are warranted.
In addition to potential sources of exposure, we evaluated BuChE inhibition in relation to exposure to specific OP/CBs. In the unadjusted analysis, chlorpyrifos users had less BuChE inhibition relative to handlers who were exposed to other OP/CBs, including compounds that were more acutely toxic (e.g., azinphos-methyl). However, the association between BuChE inhibition and chlorpyrifos use was not significant after adjustment for covariates, suggesting that confounding may have been present. In particular, days since baseline test may have been an important confounder in this analysis because chlorpyrifos is typically applied early in the spray season, whereas other OP/CBs (including azinphos-methyl) are usually applied later in the season when there is greater potential for cumulative BuChE inhibition over time. The association between use of multiple OP/CBs and BuChE inhibition is more plausible; recent studies suggest that mixed exposures can potentiate the toxic effects of specific OPs.[37
We did not observe any association between recency of exposure and degree of BuChE inhibition. However, it should be noted that approximately two-thirds of our study population had handled pesticides within the week preceding their follow-up ChE test, and only 8% of the sample was exposed >30 days previously. This pattern suggests that there may not have been enough heterogeneity in our sample to determine the association between recency of exposure and BuChE inhibition.
Relative to handlers who reported “excellent” health status, handlers who reported “poor” or “fair” health status had significantly greater BuChE inhibition. It is possible that handlers with poorer health were susceptible to BuChE inhibition, or their BuChE activity may recovered more slowly following OP/CB exposure. However, it is also possible that handlers with greater BuChE inhibition may have experienced symptoms of pesticide-related illness, and reported poorer health as a consequence of OP/CB exposure. Because self-reported health status was determined cross-sectionally at the time of follow-up ChE testing, we were unable to characterize the temporal relation between health status and BuChE inhibition. Future studies with prospective data collection may provide additional information about this association.
Study strengths and limitations
The implementation of a ChE monitoring program in Washington State provided a valuable opportunity to evaluate potential sources of exposure to OP/CB insecticides among agricultural pesticide handlers. Because participants were unaware of the results of their follow-up ChE tests at the time of the interview, and acute pesticide-related symptoms are relatively uncommon in this population, reporting or healthy worker survivor effect biases on risk estimates in this study were probably minimal. Results were not materially changed after excluding fifteen participants with self-reported symptoms of pesticide-related illness (data not shown). We also repeated the analyses after excluding eight participants with a previous BuChE depression (i.e., >20% BuChE inhibition on the prior follow-up visit). Again, results were essentially unchanged for most exposures, with the following exceptions: 1) cleaning pesticide containers was statistically significantly associated with greater BuChE inhibition (P=0.033); 2) the association between cleaning spray equipment and greater BuChE inhibition was only borderline significant (P=0.053); and 3) the association between wearing a full-face respirator and less BuChE inhibition was no longer significant (P=0.13).
Previous studies have noted the potential for exposure misclassification in self-reported data.[38
] In the present study, reliance on self-reported exposure information may have resulted in missing data for some variables (12% for OP/CB insecticides used) and misclassification of other exposures. In particular, exposure misclassification may have been a concern for worker behaviors, which are somewhat more subjective. However, since participants were unaware of their ChE results at the time of data collection, we would expect any misclassification to be non-differential, resulting in under-estimated associations. Future studies may be able to validate self-reported exposures against direct workplace observations.
We evaluated BuChE inhibition in relation to use of specific OP/CBs during the preceding 30 days. However, due to time constraints we were unable to collect detailed information regarding the degree of exposure to specific OP/CBs. It is likely that this limited our ability to characterize the risk of BuChE inhibition associated with individual OP/CBs.
Due to an administrative change in the statewide monitoring program, ChE assays were performed by different laboratories, with differing measurement methods, in 2006 and 2007. This was unlikely to have been an important bias, however. Absolute BuChE levels did differ by year, yet the percent of BuChE inhibition from baseline levels was not substantially different in 2006 and 2007 (means were 4.8% and 6.6%, respectively). Furthermore, year of participation was included as a covariate in all adjusted analyses. Moreover, when we evaluated follow-up BuChE activity as the outcome variable, results were generally consistent with the findings based on BuChE inhibition reported above (results not shown).
Finally, statistical power was limited in this study, particularly for evaluating associations with relatively uncommon exposures (e.g., not wearing chemical-resistant boots). Thus, our findings should be replicated in other populations with greater heterogeneity of PPE use and other exposure-related factors. Although risk estimates from logistic regression analyses were based on a small number of cases of BuChE depression (N=18), the associations were generally in the same direction as those observed in the linear regression analyses, and several strong associations were observed.
Implications for policy and practice
Findings from this study suggest that continued efforts are needed to promote and enforce safe pesticide handling practices among agricultural pesticide handlers. We investigated modifiable worker behaviors and workplace conditions, as well as specific high-risk handling activities. These findings may ultimately inform future targeted interventions to reduce pesticide exposures.
Furthermore, evidence of an association between OP/CB exposure and BuChE inhibition in this study suggests that current regulatory exposure assessment models may under-estimate exposure.[39
] Estimates of occupational exposure in pesticide risk assessments could be refined based on associations between BuChE inhibition and specific work activities and practices observed in this study. It should be noted that greater use of PPE was reported among participants in this study relative to pesticide handlers in other regions of the U.S. and in developing countries.[25
] Nonetheless, the general pattern of consistency of our findings with those from previous studies of pesticide exposure determinants offers some reassurance that our results have relatively broad generalizeability.
What this paper adds
- Agricultural pesticide handlers who are exposed to organophosphate and N-methyl-carbamate insecticides may experience inhibition of serum cholinesterase enzyme activity, a short-term marker of exposure and early biologic effects.
- In this study, handlers who mixed/loaded pesticides or cleaned spray equipment had significantly greater serum cholinesterase inhibition than handlers who did not perform these activities.
- Several work practices appeared to protect against serum cholinesterase inhibition, including: wearing a full-face respirator (rather than a half-face respirator), wearing chemical-resistant footwear, and storing personal protective equipment in a locker at work.
- Results of this study suggest that models used to characterize occupational pesticide exposure for regulatory risk assessments may under-estimate the degree of exposure attributable to specific work activities and practices.