Objective To identify differences between erythrocyte cholinesterase levels, haemoglobin and persistent symptoms in labourers in three stages of the agricultural process.

Methods Three cross sectional studies were conducted in an agricultural company in northwestern Mexico. The first study was done at the beginning of the season. The second during the planting and pruning, each one included 106 workers. The third was held in the collection stage and included 172 workers. During the first baseline measurement of biological indicators were performed, a questionnaire was used to assess socio-demographic characteristics, diseases and 19 symptoms which have been associated with cholinesterase inhibiting pesticides. The symptoms were classified into: non-specific, probable and specific. We considered persistent, those who remained in the last 15 days. For the measurements two and three, the same procedures were repeated, further evaluation of working conditions and exposure to pesticides were added. SPSS for analysis was used.

Results In the first, second and third measurement, there were significant decreases in cholinesterase and haemoglobin (p < 0.001), but there were within normal ranges. The prevalence of persistent symptoms was 37.7%, 51.8% and 62.2%, respectively, p < 0.05. There was an increase in the number of symptoms per worker. We didn´t find statistical association between symptoms and cholinesterase levels, but we found it among symptoms, frequency of pesticides application, re-entry time, hours of work per week, use of personal protective equipment, work clothing change and shower after work day (p < 0.05). The risk of having symptoms was 55% higher in the group of workers more exposed to pesticides.

Conclusions The increased frequency of persistent symptoms and their relation with exposure to pesticides and poor working conditions was evident. However, as has been observed in other studies, it was not associated with cholinesterase levels.