Although much remains to be learned about the acute and chronic health impacts of herbicide use, public health reports and epidemiological studies indicate that certain herbicides can be responsible for direct, unintentional poisoning and may be associated with increased incidence of cancer and other disorders. Farmers, farm families, and agricultural workers are exposed to herbicides at higher concentrations than the general public and consequently may be subjected to greater health risks. Health issues relating to exposure to herbicides and other pesticides are particularly important in developing countries, where safe use is difficult because of unavailable or prohibitively expensive protective equipment, inadequate and poorly enforced safety standards, poor labeling, illiteracy, and insufficient knowledge of hazards by handlers and applicators (Pimentel et al., 1992; Repetto & Baliga, 1996, pp. 9-16).
Acute symptoms of pesticide poisoning include headache, skin and eye irritation, fatigue, dizziness, nausea, cramping, fever, diarrhea, and difficulty in breathing (Stone et al., 1988). Most incidents of pesticide poisoning go unreported (Jeyaratnum, 1990), but it is conservatively estimated that one million serious accidental pesticide poisonings occur throughout the world each year (World Health Organization, 1990, p. 86). Pesticide poisonings of farmers and agricultural workers occur in industrialized countries, such as the USA (Stone et al., 1988), but are more frequent in developing countries (Repetto & Baliga, 1996, pp. 9-16).
Public health data from Costa Rica suggest that herbicides may contribute to a significant portion of acute pesticide poisonings in developing countries. Hilje et al. (1992, p. 79) reported that bipyridilium, chloroacetamide, dinitroa-niline, phenoxy, picolinic acid, substituted urea, and triazine herbicides accounted for 19% of the 787 pesticide poisonings registered in 1984 by the Costa Rican National Poison Control Center. Similarly, Dinham (1993, p. 105) noted that various herbicides were responsible for 22% of the acute pesticide poisonings in the region of Limón, Costa Rica, in the first six months of 1990. Hilje et al. (1992, p. 79) stated that the actual number of pesticide poisonings in Costa Rica is higher than that reported to government agencies, but that available data accurately reflect the percentage of poisonings attributable to different types of pesticides.
Chronic health effects of chemical exposure can include cancer and disorders of the immune, endocrine, neurological, and reproductive systems. Unambiguous cause-and-effect relationships are often difficult to establish for these types of health problems because a long lag period typically exists between exposure to causative agents and presentation of clinical symptoms, and because exposure to other chemicals or behaviors such as smoking may be contributing factors. Epidemiological studies can be conducted, however, to determine patterns of risk associated with exposure to herbicides and other pesticides.
Thirty-nine herbicide active ingredients are classified by the US Environmental Protection Agency (1999) as probable, likely, or possible carcinogens, and a number of epidemiological studies have examined possible links between herbicides and cancer in human populations. Significant correlations between herbicide use and several types of cancer were noted by Stokes & Brace (1988) in a study of cancer deaths in 1497 nonmetropolitan counties in the USA. The percentage ofland area treated with herbicides in each county was significantly correlated with the incidence of genital, lymphatic, hemato-poietic, and digestive system cancers. Herbicide use had no relationship with urinary system cancers, however, and was negatively correlated with respiratory system cancers. On Saskatchewan farms of less than 400 ha, death of male farmers due to non-Hodgkin's lymphoma (NHL) rose significantly with increasing numbers of hectares sprayed with herbicides (Blair, 1990; Wigle et al., 1990). No significant relationship was found on farms of more than 400 ha, where farmers may have been less likely to apply herbicides personally or may have used aircraft for applications.
Hoar et al. (1986) reported that the incidence of NHL among men in Kansas increased significantly with the number of days per year that they used herbicides; men who used herbicides more than 20 days per year had a six-fold higher chance of contracting NHL than did nonfarmers or farmers not using herbicides. Increased risk of NHL was specifically associated with use of phenoxy herbicides, especially 2,4-D, which is widely used in field crop production in Kansas. Exposure to phenoxy herbicides has been linked to increased risks of NHL, Hodgkin's lymphoma, and soft-tissue sarcoma in a number of other studies (Hardell & Sandstrom, 1979; Hardell et al., 1981; Blair, 1990), although reviews of the subject have concluded that no consistent cause-and-effect pattern exists (Smith & Bates, 1989; Ibrahim et al., 1991).
In addition to concerns about possible links to various cancers, concerns also exist about potential effects of herbicide exposure on other aspects of human health. Repetto & Baliga (1996, pp. 17-49) noted that three widely used herbicides - atrazine, 2,4-D, and paraquat - are immunotoxic to laboratory animals whose immune systems are similar to that of humans, and they suggested that exposure to these and other pesticides may increase human susceptibility to infectious diseases and certain types of cancer because of immune system suppression. They noted, however, that the epidemiological studies necessary to test that hypothesis have not been conducted. The herbicides alachlor, atrazine, 2,4-D, metribuzin, and trifluralin have been identified as potentially disruptive to the human endocrine system (Colborn, vom Saal & Soto, 1993), but how actual exposure through agricultural use affects endocrine function is unknown. Public health data from Minnesota suggest that exposure to 2,4-D and MCPA significantly increased the rate of birth defects in offspring ofpesticide applicators and members ofthe general population in areas with high application rates (Garry et al., 1996). However, exposure to 2,4-D and MCPA was confounded with exposure to a number of fungicides, making it impossible to draw firm conclusions about the reproductive system effects ofspecific compounds.
Because manipulative experiments with human subjects and possible toxins are unethical, uncertainty about the chronic health effects of herbicides will continue. How should this uncertainty be dealt with.? Many proponents of herbicide use do not find available data sufficiently compelling to assume that herbicides pose important human health risks. Opponents believe there is adequate evidence that they do, particularly in developing countries. We suggest that it is prudent to err on the side of safety by minimizing herbicide exposure and toxicity. Greater safety could be obtained by producing and distributing superior application and protective equipment, and by developing new herbicides whose chemistries limit their persistence, mobility, and toxic-ity to nontarget organisms, including people. The development of effective nonchemical weed management strategies would address the problem at its source and is the focus ofthis book.
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