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The use of insecticides has been intensified in Brazilian agricultural production and control of endemic diseases. Plasma – or Butiryl -Cholinesterase (BCHE) is used for biological monitoring of organophosphates (OP) and carbamate exposure. Despite legal requirements, there are controversies about this monitoring.Cross – sectional study carried out among pesticide applicators in tobacco farming. Sociodemographic, occupational data, PPE use, pesticide- related-symptoms (PRS) and standardised medical examination were collected BCHE was measure at low and high exposure period. Pesticide exposure was characterised by using chemical types and days from last exposure. The assessment by physicians and toxicologists classified poisoning by pesticides as possible or probable. The analysis was made by chi-square test, t test for comparison of two means and Poisson regression.Out of 492 smokers studied, 43% had recent exposure to OP or Carbamates, with average BCHE increase in intensive period (p<0.001). Five people (2.4%) presented a reduction of ≥20% in BCHE. There was more BCHE reduction in the most exposed group. There were no association with any pesticide poisoning criteria, BCHE levels or use of PPE.Although consistent with some studies, the findings of BCHE contrast with other authors. Possible explanations include tolerance to long term exposure, differences in analytical methodology, genetic polymorphism, other diseases and limitations with the gold standard. Future studies should investigate these results to define better recommendations for occupational monitoring, including periodicity and cutoff points. New biomarkers should be developed to exposure monitoring of cholinesterase inhibitors and other pesticides.