Dermal exposure to pesticides in Nicaragua : a qualitative and quantitative approach

Abstract

Background: Pesticide use continues to be a serious public health problem in developing countries, despite decades of safe pesticide use strategies. In Nicaragua, organophosphate insecticides, in particular chlorpyrifos and methamidophos are responsible for about half of the acute pesticide poisonings. Contamination of the skin occurs frequently in the occupational setting. There is extensive research to improve methods to assess dermal exposure. The applicability and feasibility of such methods in developing countries is uncertain.

Aim: This thesis aims at increasing the understanding of risk factors underlying exposure, evaluating dermal exposure among Nicaraguan subsistence farmers, and proposing more suitable methods for developing country conditions.

Methods: A group of 29 subsistence farmers were interviewed in four focus groups and their responses were analyzed using grounded theory. Field data for semi-quantitative and quantitative exposure measurements of 31 farmers were collected during 33 pesticide applications, using observation, supplementary video recording, a fluorescent tracer, and skin wiping.

A visual scoring system developed in the US was modified into a Nicaraguan Visual Scoring System suitable for developing country conditions. Pesticides were traced during application. Skin fluorescence was videotaped in a foldaway darkened room which was later measured through Body Segment Scores (BSS), Contaminated Body Area (CBA) and Total Visual Score (TVS).

TVS was used as a criterion indicator for the identification of main exposure determinants by observation. Univariate and multivariate analyses were performed. Hundred and ten potential exposure determinants were reduced to 27 variables grouped as worksite, spray equipment, work practices, clothing, and hygiene practices. Reliability of the visual score was tested with intraclass correlation coefficients, in a sub-sample of five farmers evaluated by five raters.

Observations of hand exposure events (direct and indirect contacts) were summarized into a Concentrate Contamination Index (CCI) and a Solution Contamination Index (SCI). Chemical residues were quantified for the hands and selected body parts according to fluorescent intensities. Spearman rank correlation coefficients were computed to compare the observational indices (CCI+SCI), fluorescent visual scores and quantitative residues.

Results: Reasons for unsafe practices were connected with poverty, inadequacy of personal protective equipment, climatic factors, and limited knowledge influenced by beliefs and traditions. Farmers felt affection towards their traditional crops and this relationship seemed to have strong meanings for pest removal and pesticide use, contributing to dangerous work practices. The observed fluorescent images on the skin of farmers reflected work practices and contamination mechanisms and pathways.

Novel determinants included spraying on a muddy terrain, dew on plants, sealing of tank lids with a cloth, and wiping sweat from the face. The Visual Scoring System was highly consistent (Cronbach alpha = 0.96) and reasonably reliable (0.75; 95% CI: 0.62-0.83), with scoring of extent being more reliable than scoring of intensity. The highest CBA was 66% and the farmer with the highest TVS scored 60% of the maximum possible. Hands were most frequently contaminated and the back had the highest BSS. Hand contact was most frequently indirect, by touching contaminated surfaces.

All farmers had quantifiable pesticide residues on their hands. Spearman correlation coefficients between the observational contamination scores, fluorescent visual scores and residues in relation to the hands ranged from 0.65 to 0.74 for chlorpyrifos and 0.62 to 0.87 for methamidophos. Differences in scores could be explained by limitations of the different methods.

Conclusions: Poverty and cultural factors contribute to pesticide use and unsafe use conditions. Education programs should be culturally appropriate to achieve pesticide exposure reduction. Each method studied in this thesis can be used independently. However, they can also complement each other, providing a better understanding of the mechanisms of skin exposure.

With further improvements, a combination of observation and fluorescent visual scoring techniques, both lowcost and practical, would become highly accessible methods for surveillance and for epidemiological studies in developing countries.