Toxic Effects of Pesticides Amplified When Combined


(Beyond Pesticides, January 27, 2006)

A new report finds significant harmful effects of pesticide mixtures on frogs, even though levels of the individual pesticides were thought not to cause harm and were 10 to 100 times below EPA standards. This finding, published Tuesday by University of California Berkley professor Tyrone Hayes in the online version of the journal Environmental Health Perspectives, suggests that current efforts to asses health risks of chemicals in isolation may significantly undermine their danger.

Frogs treated with the mixture of pesticides, all commonly found in agricultural runoff, were, on average, 10 to 12 percent smaller than the untreated control group. Nearly 70% of the treated frogs became infected by a common pathogen that the untreated group fought off. They also developed holes, or plaques, in their thymus. High levels of corticosterone, a hormone similar to one found in humans, were also found. Corticosterone is associated with stress and known to decrease growth and slow development. In a related paper also published by Dr. Hayes on Tuesday, these chemicals, and atrazine in particular, switched testosterone to estrogen, causing the testes of exposed male frogs to produce eggs instead of sperm. Effects were seen in frogs at concentrations of 0.1 parts per billion, a level far below any health threshold.

Dr. Shanna Swan a professor at the University of Rochester, has also found that pesticide concentrations as low as 0.1 ppb may cause problems in humans as well. In particular, she found a link between this concentration and low fertility in men. As a reference, the urine of a farm worker contains 2,400 parts per billion of some of these compounds.

Safety tests performed by the US EPA and FDA study only one compound in isolation. By ignoring the real-world interactions between different chemicals, the safety reports may be significantly underestimating the danger these chemicals cause. Though it may be more difficult to replicate real-world environments in studies, it is important to do so in order to fully understand the implications chemicals may have on human health and the environment.

Amphibians are declining at alarming rates across the globe, and many scientists believe that industrial chemicals and pesticides may be partially to blame. Numerous scientific studies have definitively linked pesticide use with significant developmental, neurological and reproductive effects on amphibians. Recent studies by Dr. Tyrone Hayes at the University of California have strengthened the case for banning atrazine, the most common contaminant of ground, surface, and drinking water. Dr. Hayes demonstrated that atrazine is an endocrine disruptor that chemically castrates and feminizes male amphibians.

Additionally, a study by Penn State University researcher Joseph Kiesecker found that wild tadpoles exposed to low-level agricultural chemicals along with the deformity causing parasite trematode were five times more likely to develop leg deformities than frogs only exposed to the trematode. The presence of the pesticides are thought to weaken the frog’s immune system thereby making them more susceptible to infection by the parasites.

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