Introduction Chronic occupational exposure to organophosphorus pesticides (OPs) is consistently associated with deficits on neurobehavioral tests when compared to unexposed groups. However, a dose-response relationship has not been established, leading some to doubt an association between occupational OP exposure and deficits on behavioural performance tests. We studied pesticide application teams in Egypt who are primarily exposed to one OP, chlorpyrifos (CPF), differing in exposure experience based on their job category.

Methods Trailmaking A and B were administered to 54 engineers (who typically watch the applications from beside the field), 59 technicians (who typically guide the applicators in the field), 31 applicators (using knapsack sprayers), and 150 controls (who did not work in the fields) at 2 different times during the OP application season and also immediately and 1.5 months after applications had ended.

Results A consistent dose-response relationship was seen in performance speed: Controls had the best performance through most of the application season on Trailmaking A (p<0.04) and B (p<0.001). Applicators had slower performance than engineers (p=0.015) and technicians (p=0.032). On the more complex Trailmaking B test, applicators and technicians had comparable performance that was significantly slower (p=0.003 and p=0.012 respectively) than performance of the engineers. Test performance at 1.5 months after applications ended and in the following year revealed that differences between the groups were persistent, and some differences were significant. 3,5,6-Trichloro-2-pyridinol (TCPy) levels in urine confirmed the pattern of higher to lower exposures across the job categories of the pesticide application teams, and these were all greater than exposures in controls. Increasing TCPy concentrations were significantly correlated with slower Trailmaking B performance at 1.5 months after the exposures had ceased, but not during or immediately after exposures.

Conclusion This study identifies a dose-response based on job category and establishes that the OP chlorpyrifos is neurotoxic.