Global use of pesticides creates substantial health impacts in all parts of the world, although the exact toll is difficult to pinpoint, given both the various chemicals and types of exposure. In short, not all pesticides are equally risky, and not all people are equally at risk. Effects can be divided broadly into two categories: acute effects, which appear immediately or very soon after exposure; and chronic effects, which may manifest themselves many years later and whose origins are often difficult to trace.
Exposure to pesticides can lead to an array of acute effects, depending on the pesticide’s toxicity and the dose absorbed by the body. For pesticides with high acute toxicity, exposure can produce symptoms within minutes or hours, most of which diminish in time. These acute effects known as poisonings or intoxications run from mild headaches and flu-like symptoms, to skin rashes, to blurred vision, and other neurological disorders (37). For example, a fieldworker who gets a few drops of methyl parathion on his skin might experience severe sweating, headache, and nausea, and even an inability to walk. Other acute responses include chemical burns, paralysis, blindness, and even death (38).
Air temperature and the exposed person’s general health condition influence the severity of these symptoms. Toxic reactions may be worse for those suffering from poor nutrition or dehydration, and warmer temperatures also may increase the toxic effects. These factors mean that field laborers working in the heat may be more susceptible to poisoning (39).
The majority of pesticide poisonings and deaths occur in the developing world, although far greater quantities of pesticides are used in the developed world. Reasons for this include the following: developing countries have a higher proportion of the populace involved in agriculture, they have poorer pesticide handling practices, they commonly use unsafe equipment (such as leaky backpack sprayers), and they generally employ more toxic pesticides than those used in developed countries. A 1991 survey of farmworkers involved in export agriculture in Ecuador revealed that more than 60 percent of farmworkers participating in the study suffered from one or more symptoms of acute pesticide poisoning such as headaches, allergies, dizziness, dermatitis, or blurred vision (40). More typical estimates for the percentage of workers experiencing acute pesticide toxicity range from 7 to 13 percent (41)(42).
A high percentage of pesticide poisonings worldwide is attributable to two particularly toxic classes of pesticides: organophosphates and carbamates, many of which are banned or restricted in the developed countries. The toxicity of these compounds stems from their ability to inhibit the action of cholinesterase, an enzyme essential to nervous system functioning (43)(44)(45).
Worker exposure can often be prevented if proper application methods are used and protective clothing is worn (46). However, farm managers often fail to make such equipment available or do not insist that it be used. A complicating factor is that field conditions may be too hot for protective clothing to be worn comfortably. Even in developed countries where equipment is more routinely provided, studies show that farmers and farm laborers do not diligently use protective clothing, especially in hot weather (47).
Although occupational exposures account for the majority of serious pesticide poisonings, workers are not the only ones at risk from acute pesticide effects. People who live adjacent to farms or plantations can also experience significant pesticide exposures from wind drift from aerial spraying, from volatilization and redeposition of applied pesticides, from use of empty pesticide containers as drinking water storage, or from contamination from spills or the dumping of pesticide wastes. One study in Nicaragua found that residents living next to cotton fields regularly sprayed with insecticides had depressed cholinesterase levels an effect typical of exposure to organophosphate or carbamate pesticides (48). Children living near treated fields are especially vulnerable, because they may spend considerable time playing among contaminated soil or plants, or they may help out in the field work. In Colombia, 18 percent of recorded pesticide poisonings from 1978 to 1989 occurred in children younger than age 14 (49).
The number of people exposed to damaging doses of pesticides through their association with agriculture is not clear, because only the most serious poisoning incidents are usually recorded. One estimate admittedly uncertain shows that between 50 million and 100 million people in the developing world may receive intensive pesticide exposure, and another 500 million receive lower exposures; these exposures may result in some 3.5 million to 5 million acute pesticide poisonings per year, with a much larger number of people suffering from subacute effects (50).
Even in developed countries where regulations are stricter and protective gear more available, occupational exposures can be significant. In the United States, the U.S. Environmental Protection Agency (U.S. EPA) estimates that nearly 4 million people in the agricultural workforce are at risk of significant occupational exposure because they handle or apply pesticides, and that these exposures result in about 10,000 to 20,000 pesticide poisonings medically treated per year (51)(52). U.S. EPA estimates that at least that number of cases probably goes untreated, because studies show that exposure is greatly underreported (53). For example, a study of 98 Nebraska farmers and pesticide applicators who routinely handled organophosphate pesticides found that 30 percent of the group had reduced levels of blood cholinesterase, an indication of significant exposure, and 22 percent actually exhibited symptoms including headache, nausea, or diarrhea. None of the affected workers sought medical treatment (54).
The acute effects of pesticide exposure are relatively well understood. By contrast, much more uncertainty surrounds long-term or chronic effects, especially those believed to arise from low-level exposures to pesticide residues in food or water. There is little dispute regarding the nature of some chronic effects, such as those that follow high-dose exposures. Several studies have shown that many people who experience acute pesticide poisoning from organophosphates later suffer neurological damage. Symptoms of this problem include weakness, tingling, or even paralysis in the legs due to dieback of some nerve endings, and reduced memory and attentiveness (55)(56). Because organophosphates may account for as many as 70 percent of occupational pesticide poisonings, the number of people suffering such neurological damage could be substantial (57).
Chronic dermatitis, which includes rashes and enhanced sun sensitivity, is one of the most common effects of pesticide exposure seen in farmworkers (58). In California, where agricultural pesticide use is substantial, a 1990 study found that dermatitis accounted for one third of reported pesticide-related illnesses (59).
Pesticide exposure may cause reproductive damage as well. Male sterility has been definitely linked to heavy exposure to dibromochloropropane (DBCP), once commonly used to control nematodes. Several epidemiological studies suggest that exposure to certain pesticides particularly the herbicide 2,4-D, which is widely used on crops, pastureland, rights-of-way, and lawns heightens the risk of birth defects. However, other studies of 2,4-D have not produced conclusive findings (60)(61).
Recently, pesticide exposure has been implicated in cases of immune system suppression as well (62). A compromised immune system makes it more difficult to fight off infectious diseases, parasites, or tumors, and could increase the toll these threats exact on one’s health. This combination could be particularly significant in developing countries, where the population’s exposures to both pesticides and infectious agents may be high and their immune systems may already be compromised by other factors. (See Pesticides and the Immune System: The Public Health Risks.)
Of all the possible health impacts from pesticide exposure, cancer has been the most frequent focus of attention and controversy. Many pesticides show cancer-causing potential in animals; many other pesticides give no indication of causing cancer. Central questions in the debate about pesticides’ role in the development of cancer relate to the level of exposure and dose required to affect cells, the possible synergistic effects of chemicals in the body, the manner in which chemicals accumulate in body tissue, the length of time they remain in the system, and many other issues. Based largely on animal studies, the U.S. EPA reports that of 321 chemicals examined, 146 are probable or possible human carcinogens (63).
Epidemiological studies, too, suggest a link between some pesticides and cancer. For example, epidemiological studies have shown an association between exposure to organochlorines and various cancers, including lymphoma and leukemia, as well as lung, pancreatic, and breast cancer (64). Such findings are significant because organochlorines such as DDT, aldrin, and chlordane are widely dispersed in the environment and can easily accumulate in human tissues.
Additional epidemiological evidence links exposure to several common herbicides with cancer. Several studies have found an association between 2,4-D and non-Hodgkin’s lymphoma (65)(66). In one study, farmers exposed to the herbicide more than 20 days per year had a sixfold higher risk of non-Hodgkin’s lymphoma, and those who mixed or applied the herbicide themselves had an eightfold greater risk (67).
Many pesticides that show carcinogenic potential remain in active use in agriculture. Could exposure to these pesticides, under normal conditions of use, actually lead to cancer? For those who work directly with these compounds on a regular basis, the answer may well be yes. Calculations show that for people who are occupationally exposed, the typical lifetime doses of a number of these pesticides would be enough to raise one’s risk of contracting cancer substantially (68).
For the general public, the answer is much less clear. Although pesticide residues are nearly ubiquitous in food and water even in developed countries, sampling studies show these residues are generally minute. In 1996, an expert panel of the U.S. National Academy of Sciences concluded that although evidence was limited, consuming such small amounts of pesticides in the diet was unlikely to pose an appreciable cancer risk (69).
Even with minimal exposures, concern has arisen that children may be at greater risk than adults because they often consume more per body weight of certain foods such as fruits that are likely to contain pesticide residues. They are also undergoing rapid tissue growth, allowing for greater concentration of these substances in their systems (70). In addition, pesticides are used widely around the home, for instance, on lawns and gardens and inside for pest control. Thus, pesticide exposure for some portion of the nonfarm population could be significant, even if pesticide residues in food and water are minimal.
In developing countries, exposures to the farm and nonfarm population may be greater, and risks may be higher. A lack of training in, or controls on, pesticide use often means that crops are treated excessively or sprayed too close to harvest and then sent straight to market with little washing (71).