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Abstract: Organophosphate (OP)-containing pesticides are widely used worldwide for domestic and industrial purposes. Studies on acute and chronic exposure to OPs have revealed numerous health effects attributed mainly to acetylcholinesterase (AChE) inhibition. The enzyme human serum paraoxonase (PON1) is involved in the detoxification of OP compounds. PON1 polymorphisms have been shown to affect susceptibility to OP exposure. We studied the effect of OP exposure on pest control workers and assessed the distribution of two common PON1 polymorphisms in our local population. The exposed group consisted of 103 workers from various pest control companies under the Singapore Pest Management Association while the 91 unexposed workers were from a lead stabilizer factory. For all workers, the mean age was 36.9 (20–70) years and the ethnic distribution was 38.1% Chinese, 44.3% Malay and 17.5% Indian. The mean±S.D. exposure duration among the pesticide workers was 10.4±8.4 years. The mean±S.D. RBC cholinesterase level was 18436.2±2078U/L and 18079.6±1576U/L for the exposed and unexposed groups, respectively (p =0.216). The mean±S.D. serum pseudocholinesterase was 11028.4±2867.4U/L and 9433.6±2022.6U/L in the exposed and unexposed groups, respectively (p <0.0001). Mean paraoxonase activity was similar among Chinese and Malays (266.5 and 266.3U/L, respectively) whereas that of the Indians was significantly lower (165.6U/L). Our study showed that cholinesterase levels among the exposed were not lower than those in the unexposed group. PON1 polymorphisms differed among ethnic groups, implying that ethnicity could be an important surrogate for identifying susceptible groups in case of OP exposure. Although OP poisoning is rare among occupationally exposed workers in Singapore, this information is useful for other developing countries that have large populations of Chinese, Malays and Indians where OP exposure could be very high especially in agricultural settings. [Copyright &y& Elsevier]