Health concerns about pesticides have tended to focus on their potential to act as acute poisons or their ability to cause cancer. But pesticides may pose other important risks as well. Recently, evidence has accumulated that many commonly used pesticides can suppress the normal response of the human immune system to invading viruses, bacteria, parasites, and tumors. The immune system is the body’s primary line of defense against disease agents, so weakening its response could increase the toll of disease. Laboratory studies show that a variety of organochlorine, organophosphate, carbamate, and metal-based pesticides (such as those based on arsenic, copper, or mercury) can suppress the immune system of mammals. Because substances toxic to other mammals are usually toxic to humans (since human immune systems are structured similarly to mammals), these laboratory studies indicate the kinds of immunosuppression humans may also experience.
Such tests provide an abundance of evidence. For example, exposure to the organochlorines aldrin and dieldrin reduces mouse resistance to viral infection, while DDT decreases antibody production in mammal and bird species. The organophosphate parathion delays antibody production and suppresses T-cell response in cell cultures, while chronic low-dose exposures of the commonly used organophosphate malathion can depress several different immune responses. Many solvents, inert ingredients, and contaminants that are part of pesticide formulations can also suppress immune responses in laboratory tests.
Epidemiological evidence, though limited, also indicates that pesticides can be toxic to the human immune system. Among Indian factory workers chronically exposed to several pesticides, blood lymphocyte levels one element of immune system health decreased by as much as two thirds from baseline levels and returned to normal only after pesticide exposure ceased.
Epidemiologists in the former Soviet Union have long observed that T-cell counts and functions are suppressed after pesticide exposure. For example, residents of agricultural districts in southern Russia where pesticide use was substantial had lower T-cell counts than control groups in the general population, and the former group also had higher rates of infectious diseases.
Likewise, in Moldova also part of the former Soviet Union teenagers in villages where pesticide application levels were greatest exhibited rates of infections of the respiratory and digestive tracts several times higher than teenagers from areas of lower pesticide use. From the 1960s through the 1980s, per hectare pesticide application rates in farming regions in central and southern Moldova were almost 20 times the world average.
Immune suppression from pesticide exposure may also play a role in the development of some cancers. As a group, farmers face higher risks than the general population for contracting Hodgkin’s disease, melanoma, multiple myeloma, and leukemia all of which are cancers of the immune system.
Unfortunately, despite current evidence, the study of the immune suppressive potential for most pesticides is still in its infancy, and little work has been done to clarify the relationship between dose and effect. Thus, consensus has not been reached on how much pesticide exposure is required to compromise the immune system enough to affect health, or what kinds of immune-suppressing effects chronic low-dose exposures might cause in the public at large. Nonetheless, it is clear that the potential risk to public health, especially in agricultural communities where exposure is widespread, is significant.
As with other pesticide health risks, the dangers of immune suppression from pesticide exposure may be greatest in parts of the developing world and in countries of the former Soviet Union, where much larger fractions of the populace still live in the countryside and work on farms. In these developing regions, pesticide use is growing rapidly, yet pesticide regulations and handling practices are often grossly inadequate.
To make matters worse, living conditions for many people in the developing world put them at especially high risk for immune suppression. Their immune responses are already weakened by widespread malnutrition; at the same time, contaminated water supplies, lack of sanitation, and poor housing conditions expose them to more disease agents. The result is particularly high fatality rates due to common diseases measles and whooping cough, for example diseases from which most patients in wealthy countries recover.
Adding pesticide-induced immune suppression on top of these other risks may substantially increase the burden of common diseases. The consequences could remain undetected because people would not necessarily die of acute pesticide poisoning; rather, deaths would be attributed to such diseases as pneumonia, or gastroenteritis, or to complications of measles. The fact that pesticide exposure weakened their immune responses and increased their vulnerability to illness or death would remain unrecognized.
Adapted from: Robert Repetto and Sanjay Baliga, Pesticides and the Immune System: The Public Health Risks (World Resources Institute, Washington, D.C., 1996).