The level of physical and psychological demand confronted by farmworkers and their level of control over their daily tasks is reported in . Farmworkers reported having moderate levels of psychological job demand and physical demand, as indicated by the mean values located at the approximate midpoint of the possible range of scores. Self-reported exposure to hazardous conditions, on average, and levels of control over work conditions were both lower than the midpoint of the possible range of scores. Psychological demands was moderately correlated with physical exertion (r = 0.61), while hazardous conditions had a small to moderate association with psychological demand (r = 0.29) and physical exertion (r = 0.41). Control was modestly associated with hazardous conditions (r = 0.17) but unrelated to psychological demand or physical exertion.
Descriptive statistics, estimated correlations among, and reliabilities (on the diagonal) for job demand and control variables.
The percentage of farmworkers with detectable levels of DAP pesticide metabolites generally increased across the agricultural season (). This pattern is clearly evidenced with the DMTP metabolite. In the beginning of the season 53.6% of farmworkers had DMTP in their urine, but the percentage of workers with detectable levels of this metabolite increased to 83.3%, 93.5%, and 85.2% at the subsequent observations. Similarly, at the beginning of the season 16.8% of farmworkers had the DETP urinary metabolite, and the percentage of workers with detectable levels of this metabolite increased to 28.2%, 48.9% and 38.8% across the season. More detailed descriptions of seasonal trends in farmworker pesticide exposure are available elsewhere (Arcury et al., 2009a
Percentage of Latino farmworkers with detectable levels of DAP organophosphorus pesticide metabolites across the agricultural season
Bivariate analyses yielded little consistent evidence that job demands and control may influence farmworkers exposure to pesticides as indicated by detection of DAP urinary pesticide metabolites (). Significant associations that did emerge were generally in the opposite direction of predictions from the demands-control model articulated in the first hypothesis. A one unit increase in psychological demands was associated with a 6% reduction in the odds of detecting the DETP urinary pesticide metabolite and a 7% reduction in the odds of detecting the DEP metabolite. Similarly, for every unit increase in physical exertion the odds of detecting DETP in provided urine samples decreased by 6%. There was little evidence linking hazardous conditions to any of the DAP metabolites, apart from a trend-level (p. < .10) association suggesting that greater exposure to hazardous conditions was associated with lower odds of detecting DETP. Control was not associated with detection of any of the individual DAP urinary pesticide metabolites.
Odds ratios estimating unadjusted associations of job demand and control with each DAP urinary pesticide metabolite †
Results from multivariate analyses yielded no support for our first hypotheses from the demands-control model that distinct types of job demands and control would have independent effects on our specified measures of pesticide exposure (). Only one association was found and it contradicted the hypothesis: for every one unit increase in psychological demands, the odds of detecting the DEP metabolite in provided urine samples decreased by 10%. Additionally, trend-level (p < .10) suggests that increased psychological demand may be associated with an increased odds of detecting the DMTP urinary metabolite.
Multivariate odds ratio estimating associations of job demand and control with each DAP urinary pesticide metabolite †
However, the multivariate analyses yielded several interaction effects. The interaction of physical exertion with control was significant for the DEDTP and the DMTP urinary metabolites. The interaction of hazardous conditions with control also emerged for DMTP. Figures and illustrate the interaction effects for DEDTP and DMTP, respectively, and they show associations consistent with our second hypothesis that associations of job demands with detection of DAP urinary pesticide metabolites would be greater for workers with little job control. Among individuals with high levels of control, there is no association between physical exertion and detection of DEDTP in provided urine samples (). However, among individuals with low control, greater physical exertion is associated with a greater probability of detecting DEDTP. The potential buffering effect of control on physical exertion is clearer for the DMTP metabolite: for individuals with low control there is a positive association between physical exertion and detection of DMTP, but this association is negative for individuals with a high level of control ().
The effect of physical exertion on the probability of detecting the DEDTP urinary pesticide metabolite by level of job control.
The effect of physical exertion on the probability of detecting the DMTP urinary pesticide metabolite by level of job control.
Models exploring our research question about potential interaction effects of high psychological demands and high physical demands (i.e., physical exertion and hazardous conditions) on pesticide exposure yielded consistent effects for five of the six DAP metabolites (). The interaction of psychological demands and physical exertion on detection of DMP () illustrates the pattern that is observed across all of the double demands interaction effects. shows that greater physical exertion is associated with greater probability of detecting DMP among individuals with low psychological demand, but that physical exertion is associated with lower probability of detecting DMP among those with high psychological demand. A similar pattern emerges for the other four significant interaction effects of psychological demands and hazardous conditions: each unit increase in hazardous conditions is associated with greater probability of detecting DAP metabolites when psychological demand is low, but when psychological demand is high, increases in hazardous conditions are associated with lower probability of detecting pesticide.
The interactive effect of psychological job demands and physical exertion on the probability of detecting the DMP pesticide metabolite.