Although it is well established in the medical literature that acute and subacute exposure to pesticides and other chemicals poses major health issues, less is currently known about low-level chronic occupational or environmental exposures to residues of pesticides and chemicals. However, evidence exists for potential chronic health effects of exposure to several pesticide classes at chronic low levels such as the association of chronic neurological effects with exposure to several pesticide classes. Examples include the association of increased vibration sense, motor-sensory neuropathy, and cognitive and affective deficits after exposure to organophosphates; the association of olfactory, cognitive, and behavioral deficits after exposure to methylbromide; and the association of symptoms of Parkinson's disease after paraquat exposure. Another example is association of oligospermia and azoospermia after exposure to dibromochloropropane (DBCP), which is now banned in the United States. There is also evidence of associations of chronic low-level exposure to pesticide residues and cancer (5,6,53-56).
A major area of interest in relation to pesticides and cancer has concentrated on pesticides acting as endocrine disrupters, mostly organochlorinated insecticides, and on hormone-related cancers. Research has largely focused on the association of breast cancer and exposure to DDT and its metabolites, although a causal inference has not been established. A recent study carried out in India, a country in which exposure to organochlorinated pesticides is expected to be higher and more recent than in populations from developed countries, found significantly higher levels of organochlorinated pesticides (DDT and its metabolites and others) in the blood of women with breast cancer as compared to reference women. In a Danish study, a modifying effect of p53 mutations on the breast cancer risk associated with exposures to organochlorines was observed, suggesting a potential for gene-environment interactions as an important factor in pesticide-related carcinogenicity (56-58).
Other pesticides have also been linked to cancer. For example, an Italian study observed a significantly increased risk for ovarian cancer in women exposed to triazines, a class of herbicides including the frequently used atrazine, simazine, and others (56,59).
Workplace factors and work practices influence the magnitude and amount of exposure. In addition, workers are often exposed to mixtures of pesticides and chemicals in the occupational setting. Other relevant factors contributing to the significance of the occupational exposure to pesticides and chemicals in the agricultural setting are the nature of the pesticide, shorter versus longer duration pesticide, type of work activity (e.g., pesticide operators versus reentry workers), and length of exposure. For example, a study in California determined that certain organophosphate application variables were significantly related to systemic illness. These included application to fleshy fruit, vegetables, and melons; air application drift; and specific OPs such as mevinphos, demeton, oxydemeton-methyl, methamidophos, and azinphos-methyl. California's unique pesticide mandatory reporting requirements make it the only state in which data are available on both pesticide use and suspected pesticide-related illnesses (59,60).
Studies evaluating the health effects of pesticides have mainly addressed the oral route of exposure after consumption. However, exposure to pesticide and chemical residues primarily involves the dermal route and, to a lesser extent, the inhalation route and typically occurs intermittently. However, despite the relatively high dermal exposure in occupational settings, existing regulations such as the FIFRA in the United States have primarily evolved from concerns about the oral route of exposure. Therefore, to accurately estimate occupational exposures to residues in agricultural work, more dermal toxicodynamic studies focusing on intermittent exposures are needed. Furthermore, the bioavailability of bound skin residues of pesticides and chemicals and the effects of the parent compound or relevant metabolite(s) in the context of various agricultural practices and work activities are other areas that need to be researched (9,59,61-63).
Gender-specific research is also needed. There are a number of major gender-related variables in agriculture that may lead to occupational exposure in females to pesticides. For example, compared to men, women working in agriculture may be found in lower-paid and lower-status jobs, with less access to promotion, information, and safety measures. In a survey of over 500 farmers in Thailand, in which all male and female farmers applied pesticides, 53% of the women were not able to read, compared to 29% of men, decreasing their ability to heed the safety warnings written on the labels of pesticides. Another occupational group that has often been overlooked is children. Child labor persists globally. The International Labor Organization estimates that approximately 250 million children between the ages of 5 and 14 work part-time or full-time around the world. Although they engage in various jobs, by far the largest number work in agriculture where they may be exposed to various hazards, including toxic chemicals (see Chapter 12) (56,64-67).
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