The study, published in the current issue of the journal Toxicology and Industrial Health, suggests that combinations of commonly used agricultural chemicals, in concentrations that mirror levels found in groundwater, can significantly influence the immune and endocrine systems as well as neurological health.
Conducted over five years, the study of mice suggests that current methods used by the Environmental Protection Agency (EPA) and others for studying the toxic effects of low-levels of pesticides may be flawed.
"The single most important finding of the study is that common mixtures, not the standard one-chemical-at-a-time experiments, can show biological effects at current concentrations in groundwater," said Warren P. Porter, the lead author of the study and a UW-Madison professor of zoology and environmental toxicology. Although used worldwide, "tests for these compounds in combination are very rare, although they frequently co-occur."
The experiments performed by Porter's group suggest that children and the developing fetus are most at risk from the pesticide-fertilizer mixtures. Their influence on developing neurological, endocrine and immune systems, said Porter, portend change in ability to learn and in patterns of aggression.
The privately funded Wisconsin study focused on three commonly used farm chemicals: aldicarb, an insecticide; atrazine, a herbicide; and nitrate, a chemical fertilizer. All three are in wide use worldwide and are the most ubiquitous contaminants of groundwater in the United States.
In the series of experiments, when mice were given drinking water laced with combinations of pesticides and nitrate, they exhibited altered immune, endocrine and nervous system functions. Those changes, according to Porter, occurred at concentrations currently found in groundwater.
Effects were most noticeable when a single pesticide was combined with nitrate fertilizer. This was true for herbicide as well as insecticide, said Porter, and chips away at the notion that herbicides have no significant influence on animals.
"Herbicides can have neurological impacts and hormonal impacts and immune impacts," he said. "They are not the harmless chemicals they are sometimes portrayed to be. They can be every bit as biologically active as insecticides or fungicides."
The apparent influence of pesticide and fertilizer mixtures on the endocrine system -- the system of glands such as the thyroid that secrete hormones into the bloodstream -- may have a cascade effect, spilling over to the immune system and affecting fetal brain development.
"Thyroid disruption in humans has multiple consequences," Porter said. Some of these include effects on brain development, level of irritability, sensitivity to stimuli, ability or motivation to learn, and altered immune function.
A curious finding of the study is that animals may be more vulnerable to the influence of such chemicals depending on the time of year: "Our current working hypothesis is that animals are seasonally vulnerable because of subtle modulation of natural seasonal variation in hormone levels," according to Porter. "For example, thyroid hormone level varies seasonally and we have now shown that two different chemical mixtures will modulate thyroid hormone levels."
The new study, Porter contends, adds to a growing body of evidence that current testing methods required for the registration and use of chemical pesticides are fundamentally flawed. In addition to a lack of testing of combinations of chemical compounds, Porter said there is a narrow focus on looking principally for carcinogenic effects or obvious cell mutations.
Neurological, immune and endocrine tests for pesticides have been mandated by federal law for almost three years, but there has been no enforcement of these laws, Porter said.
"Toxicological testing so far has been extremely limited in scope and focused on mechanisms that require extensive mutations or cell damage to show any effects. They do not adequately assess the potential for biological effects under real world exposure scenarios."
Co-authors of the paper include James W. Jaeger of the UW-Madison Department of Zoology, and Ian H. Carlson of the Endocrinology Laboratory, University of Wisconsin Hospital.
Last Updated on 8/3/99
By Karen Lutz