Groundwater & Pesticides -- Section 3.2 The Threats to Human Health

The impacts of pesticides on human health -- like any human health effects -- can be broadly categorized as acute and chronic.

The former relates to those effects that occur relatively quickly upon exposure to elevated levels of pesticides; these can encompass adverse effects ranging from headaches, dizziness, irritability and nausea to significant nervous system impairment and death. There is little argument that pesticide poisoning remains a commonly under-diagnosed and under-reported illness, and definitive statistics are not readily available.

According to the American Association of Poison Control Centers, an estimated 79,000 children were involved in common household pesticide-related poisonings or exposures in the U.S. in 1995.¹ Looking into similar statistics for 1989, Stanley Schuman notes that of the total number of cases in that year (about 60,000 for 1989), more than 18,500 cases resulted in a visit to a health care facility. ² EPA also estimates that there are approximately 250 to 500 physician-diagnosed cases of pesticide poisoning for every 100,000 agricultural workers, and many believe that the number of cases is actually twice that figure. ³

Other health impacts from pesticide exposure, particularly those that relate to long-term exposures to relatively low doses of the chemicals, are even more difficult to track and fully understand.

In looking at the impacts of chronic exposures to chemicals, scientists and policy-makers rely on two primary sources of information: animal studies and human "epidemiological" studies. And more often than not, it takes a significant number of studies of both types to build a clear case against a given chemical or product -- even such a widely used and damaging product as tobacco.

A disturbing and growing body of evidence, however, clearly implicates pesticides in a variety of human health problems, from increased rates of leukemias and other cancers to reproductive problems and immune system suppression.

For purposes of regulatory decisions, many chemicals, including those used in pesticides, have been placed into one of several categories dealing with "carcinogenicity" or the capacity to cause cancer in humans. As reported by former EPA Assistant Administrator Lynn Goldman , EPA and other national and international bodies have classified nearly 165 pesticidal chemicals as "known," "probable" or "possible" carcinogens. ⁴

Of those, five are listed as known human carcinogens ⁴ -- a categorization that requires a very significant body of evidence from both animal and human studies. Sixty-eight pesticidal chemicals were classed as probable human carcinogens, ⁴ based on animal and limited human data. While the use of some of these chemicals has been restricted or canceled, others remain in common use.

For those chemicals that fall in the larger grouping of possible human carcinogens, there may be a paucity of data or study results may conflict or confound. There may also be significant scientific and/or policy disagreement as to the validity or relevancy of certain health studies.

A variety of animal studies also suggest a possible link between pesticides and adverse health effects other than cancer. For example, notes Pimentel, "…a large body of evidence suggesting pesticides can produce immune dysfunction has been accumulated over recent years from animal studies." ⁵

Beyond animal studies, evidence suggesting a link between exposure to agricultural chemicals and a variety of adverse health effects comes primarily from studies of occupationally-exposed populations -- people who farm or reside in farming areas. This reliance on occupational exposure studies to better understand health effects is a common approach in epidemiological work, and the data collected thus far has troubling implications for the farming community as well as the general public.

In 1995, Dr. Aaron Blair of the National Institutes of Health summarized the literature on agricultural exposures and cancer. ⁶ According to Blair, "[f]armers, despite a generally favorable mortality, appear to experience elevated rates for several cancers, including leukemia, non-Hodgkin's lymphoma, multiple myeloma, soft-tissue sarcoma, and cancers of the skin, lip, stomach, brain, and prostate." ⁶

As Blair explains, the studies do not pinpoint a definitive cause but show an association with either naturally occurring or induced immuodeficiencies. This, in turn, suggests a linkage to agriculturally related exposures to pesticides, solvents, dusts, engine exhausts or other contaminants. Much of the literature behind this report implicates agricultural chemicals in the incidence of a variety of cancers.

A more recent epidemiological study -- published in March of 1999 in Environmental Health Perspectives -- looked at cancer mortality in Minnesota. That study found elevated mortality ratios for seven cancers in four regions of the State. The statistical significance of the findings was greatest in those areas with the most intensive use of pesticides. ⁷

Parental occupational exposures to pesticides and home use of pesticides have also been implicated in elevated rates of childhood brain cancers and childhood leukemia. ⁸ Several studies report increased incidence of miscarriages and stillbirths among women agricultural workers, and at least one study found similar problems in women whose husbands were employed in pesticide application. A Minnesota study found an association of pesticide exposure and birth defects in agricultural workers and in the general population. ⁹

Kids Need Protection

Children are at a greater risk for some pesticides for a number of reasons. Children's internal organs are still developing and maturing and their enzymatic, metabolic, and immune systems may provide less natural protection than those of an adult. There are 'critical periods' in human development when exposure to a toxin can permanently alter the way an individual's biological system operates. Children may be exposed more to certain pesticides because often they eat different foods than adults.

For instance, children typically consume larger quantities of milk, applesauce, and orange juice per pound of body weight than do adults. Children's behaviors, such as playing on the floor or on the lawn where pesticides are commonly applied, or putting objects in their mouths, increase their chances of exposure to pesticides.

U.S. EPA ¹⁰

Another study of those who drink rural well water suggests a possible link between exposure to agricultural chemicals in drinking water and Parkinson's disease. ¹¹

In addition to the troubling but unsettled associations of pesticides with a variety of specific diseases and illnesses, recent years have seen a growing concern with the possibility that man-made chemicals, including some pesticides, may disrupt the functioning of the endocrine system -- both in wildlife and in humans. The endocrine system -- made up of various glands and the hormones they secret -- is critical for regulating many biological processes, including growth, development and fertility. A number of synthetic chemicals, unfortunately, have the capacity to mimic these hormones or otherwise interfere with their critical roles in bodily functions.

According to EPA's Special Report on Environmental Endocrine Disruption, "compelling evidence has accumulated that endocrine systems of certain fish and wildlife have been disturbed by chemicals that contaminate their habitats." ¹² For example, Florida alligators exposed to pesticide runoff in Lake Apopka have shown a range of reproductive problems, including reduced fertility, smaller-than-normal penises, and changes in sex ratios with predominantly females being born. ¹³ The intense debate today revolves around just what the disturbing indicators in animal populations may mean for human beings.

EPA's 1997 Special Report notes that "possible human health endpoints" affected by endocrine-disrupting chemicals include breast cancer, endometriosis, testicular and prostate cancers, abnormal sexual development, reduced male fertility, neurobehavioral effects and immune system suppression.¹² Exposure to endocrine-disrupting chemicals has become common, explains the Center for Bioenvironmental Research at Tulane and Xavier Universities, but "human health risks that may be associated with these low-level constant exposures are still largely unknown and highly controversial." ¹⁴

Hormone Levels in Fish Show Signs of Possible Endocrine Disruption

A reconnaissance study of sex hormones in carp collected at 11 NAWQA [National Water Quality Assessment] stream sites indicates that pesticides may be affecting the ratio of estrogen to testosterone in both males and females. The hormone ratio, which is sometimes used as an indicator of potential abnormalities in the endocrine system, was significantly lower at sites with the highest pesticide concentrations. Although the lower hormone ratios may not be associated with measurable effects on fish populations, they are a signal that further investigation is needed.

U.S. Geological Survey ¹⁵

In response to the growing body of evidence associating pesticides with human health threats, the National Cancer Institute and other federal agencies have launched a major, multi-year study. The Agricultural Health Study (AHS) will interview and follow some 90,000 individuals in an attempt to better understand the health outcomes associated with agricultural chemical exposures. That study is not likely to settle the question of health effects from pesticide exposures, but it is hoped that it will significantly advance our understanding of this complex and important field of science. ¹⁶

Not surprisingly, public policy debates on pesticide use and regulation are marked by lively exchange on the issue of health effects. Some, particularly those in the business of making and selling agricultural chemicals, would defer action until the evidence of harm from specific pesticides is iron-clad, while others espouse caution and remind us that inquiries into the genesis of human disease are seldom marked by speed or certainty.

Sadly, a retrospective on pesticide control in the United States illustrates only too well that hindsight often functions as the primary means of protecting the public, with numerous chemicals "pulled" from the marketplace after their adverse impacts had been demonstrated.

2. Schuman, Stanley, "Risk of Pesticide-Related Health Effects: An Epidemiologic Approach," in The Pesticide Question: Environment, Economics, and Ethics, David Pimentel and Hugh Lehman, eds., 1993.

3. US EPA, Pesticides and National Strategies for Health Care Providers: Workshop Proceedings, April 23-24, 1998, EPA 735-R-98-001, July 1998.

4. Goldman, Lynn R., "Chemicals and Children's Environment: What We Don't Know About Risks," Environmental Health Perspectives, 106 Suppl 13, 1998.

5. Pimentel, David, et al, "Assessment of Environmental and Economic Impacts of Pesticide Use," in The Pesticide Question: Environment, Economics, and Ethics, David Pimentel and Hugh Lehman, eds., 1993.

6. Blair, A. and S.H.Zahm, "Agricultural exposures and cancer," Environmental Health Perspectives, 103 No 8, 1995.

7. Schreinemachers, Dina, John Creason and Vincent Garry, "Cancer mortality in agricultural regions of Minnesota," Environmental Health Perspectives, 107 No 3, 1999.

8. California Public Interest Research Group Charitable Trust and Physicians for Social Responsibility, Greater San Francisco Bay Area and Los Angeles Chapters, Generations at Risk: How Environmental Toxicants May Affect Reproductive Health in California, 1998.

9. Garry, V.F., et al, "Pesticide appliers, biocides, and birth defects in rural Minnesota," Environmental Health Perspectives, 104, No 4, 1996.

11. Koller, W.C., et al, "Environmental risk factors in Parkinson's disease," Neurology 40, 1990.

12. US EPA, Special Report on Environmental Endocrine
Disruption: an Effects Assessment and Analysis, EPA/630/R-96/012
February 1997, available at <http://www.epa.gov/ORD/WebPubs/endocrine/>.

13. Eubanks, Mary W., "Hormones and Health," Environmental Health Perspectives, 105, No 5, 1997.

15. US Geological Survey, The Quality of Our Nation's Waters: Nutrients and Pesticides, Circular 1225, 1999.