While the increase in toxicity was relatively modest, the effects from the interaction of chemicals were unexpected, the researchers said.
The study's results, reported in the July 26 online edition of Environmental Science & Technology, send a message to environmental regulators to reassess how they evaluate a product's toxicity, said Donald Weston, UC Berkeley adjunct professor of integrative biology and lead author of the study. "This interaction of compounds is a whole new issue when doing pesticide risk assessments that has been largely ignored by regulators," said Weston.
The findings came from water and sediment samples collected from six creeks in Sacramento, Calif., after the county's mosquito control authorities took the rare step of applying an insecticide by air in August 2005. UC Berkeley researchers point out that aerial applications can cover far more acreage than ground spraying from trucks, and are used if the human health risk from mosquito-borne infections is perceived to be greater than the risk of exposure to the pesticide.
In this case, the decision to spray by air came after two dozen people in Sacramento County had become infected that summer by the West Nile virus. An 85-square mile region north of the American River was treated on Aug. 8, 9 and 10, and a 104-square mile area south of the American River was treated Aug. 20, 21 and 22.
Sediment and water samples were taken from the northern creeks on Aug. 12, about 34 hours after the third night's application of the pesticide. In the southern creeks, samples were collected on Aug. 14, six days before the first application, and Aug. 22, about 10 hours after the second night's application.
Source:University of California - Berkeley