In addition to inhaling or absorbing pyrethroids that linger in households, people ingest traces of pyrethroids in their food, since the chemicals are used on some vegetable, fruit and grain crops.
A 2006 EPA review found that the risk of exposure through diet was at or below the agency’s level of concern for most people. But the study also found that infants and toddlers are highly exposed in some foods, especially bananas, pineapple and dried-oat baby food.
“Now that we know people are exposed to pyrethroids widely, we need to determine what the exact health effects are,” said Barr.
So far, there is little scientific data evaluating the potential threat to human health.
Studies with lab animals have linked pyrethroid exposure to damage of the thyroid, liver and nervous system, as well as impairment of behavioral development, changes in the immune system and disruption of reproductive hormones, according to the 2006 EPA review. These animal studies are relevant to human health because pyrethroids act on functions of the nervous system common to all animals, according to the EPA.
Some pyrethroids imitate the hormone estrogen and can increase levels of estrogen in breast cancer cells, and some are suspected carcinogens. Other data suggest that people using the chemicals are at risk of aggravated allergies or asthma, although the EPA concluded last year that there is no clear link.
Pesticide manufacturers say that pyrethroids are safe and that they are vital to agriculture and to combating mosquitoes that carry West Nile Virus and other diseases.
“Pyrethroids are an extremely important class of insecticidal compounds with major public health and agricultural uses,” Rex Runyon, a vice president at CropLife America, a trade group that represents pesticide companies, said in an email. Runyon added that pyrethroids “do not pose unreasonable effects to human health or the environment” when used according to the directions on the label.
Although little data exist about human health concerns, evidence is growing that pyrethroids might be harming aquatic ecosystems. Studies of streams and rivers in California, Texas and Illinois suggest that the pesticides might be wiping out small organisms that live in the waterways and form the base of the food chain.
A 2009 study found the pesticides in urban stream sediments in central Texas, where they are widely used to control fire ant and grub worm infestations. The concentrations are lethal to a small, shrimp-like crustacean called Hyalella azteca - a species commonly used in laboratories to investigate the effects of pesticides on invertebrates necessary for healthy rivers.
“All of our sampling sites were very close to neighborhoods with manicured lawns,” said Jason Belden, an Oklahoma State University zoologist and author of the study published in the journal Environmental Pollution. “Some people are not following the best management practices. They’re not being careful enough with pesticides. We all need to make an effort to only use pesticides when we need them.”
Pyrethroids are showing up not only in sediments, but also in the currents of California rivers, at levels toxic to insects and aquatic invertebrates that fish and other animals feed on.
Biologist Donald Weston of the University of California, Berkeley, looked for the insecticides in urban runoff, sewage treatment plant effluent, and agricultural drains in California’s Sacramento-San Joaquin River Delta. In the laboratory, Weston tested the toxicity of these samples on the shrimp-like Hyalella azteca.