From January 27, 2006
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.
.