Problems in monitoring overexposure among spray workers in fruit orchards chronically exposed to diluted organophosphate pesticides

Objectives: Farmworkers applying diluted pesticides are chronically exposed to low levels of toxic substances such as organophosphates (OPs). Monitoring of serum and red cell cholinesterase (ChE) levels are often used to determine whether these workers are overexposed to OPs. There are major problems with the interpretation of ChE levels: differing definitions of 'overexposed'; measurement variation and biological variation over time; and confounding factors such as alcohol misuse and certain liver conditions. The purpose of this study was to conduct a survey under field conditions to determine the extent of possible overexposure and the problems with biological monitoring using ChE levels in a spray worker population chronically exposed to OPs.

Methods: A survey of 134 spray workers from 44 deciduous fruit farms in the Western Cape, South Africa, was conducted three times during the production season. Both ChE levels were determined at the end of 6 months lay-off (no spraying); during peak spraying; and at the end of the spray season. A group of 51 packshed workers with no exposure to OPs was used for comparison.

Results: Despite exhaustive statistical analyses, the proportions of lowered ChE levels could not be shown to differ between the exposed and unexposed groups. The variation in ChE levels observed could be explained statistically by the high levels of alcohol misuse among the participants (76% in the exposed group, 59% in the unexposed group). Unhealthy lifestyle habits were highly prevalent amongst these workers (e.g. 83% smoked), which have long-term implications for their health and, importantly, hamper the diagnosis of overexposure. Alcohol intake was statistically associated with lowered ChE levels.

Conclusions: According to different, definitions of 'lowered', different conclusions could be reached, which hamper reliable application in the field. The interpretation of ChE levels as an indicator of overexposure to OPs is complex, and the risk of misinterpretation in such a worker population is considerable.