The paths of chlorpyrifos: quantifying aggregate exposures. (Science Selections).

Chlorpyrifos, once one of the most widely used pesticides in the United States, was banned from home and garden use in June 2000 after federally mandated risk assessments concluded that children are more sensitive to the pesticide than previously estimated. Under the Food Quality Protection Act, U.S. Environmental Protection Agency pesticide exposure risk assessments must now use aggregate exposure estimates that account for all exposure routes, including inhalation, ingestion, and dermal absorption. But comprehensive measurements of how one person is exposed via all three routes are rare; few studies have combined direct pesticide concentration measurements from different sources with exposure estimates. In this issue, Yaohong Pang of the University of Georgia at Athens and colleagues present results from just such a study on residential chlorpyrifos exposures--the National Human Exposure Assessment Survey (NHEXAS) in Maryland [EHP 110:235-240]. During NHEXAS-Maryland, researchers were able to quantify aggregate chlorpyrifos exposure by multiple routes among a sample population as well as uncover the surprising dominance of one particular route of exposure.

In 1995-1996, Pang and the other NHEXAS-Maryland researchers measured chlorpyrifos concentrations in indoor air, carpet dust, exterior soil, and diet samples from 80 people over age 10 who lived in Baltimore and the surrounding counties. None of the participants used pesticides for six months before or during the study. The team combined the measured chlorpyrifos concentrations in each medium with self-reported time spent indoors at home, time and frequency in contact with carpet, frequency of contact with soil, and amounts of pesticide in the diet samples to derive the exposure to chlorpyrifos for each medium as well as the average daily aggregate exposure.

They found that aggregate daily exposures for chlorpyrifos ranged from 13.5 ng/day to 12,821.0 ng/day, with a mean daily aggregate exposure of 1,390.0 ng/day. Inhalation of indoor air accounted for 76.1% of the aggregate exposure to the population, while solid food intake contributed 22.8% of the population exposure. The importance of the inhalation pathway was somewhat surprising to the authors because chlorpyrifos is not very volatile. They also point out that little is known about how much chlorpyrifos is actually absorbed through inhalational exposure.

The distribution of chlorpyrifos concentrations in each medium varied by over three orders of magnitude. This variation and the concentrations for indoor air, carpet dust, and soil measured in this study were in the same range as those measured in earlier comparable studies. The aggregate exposure estimates determined in this study are lower than some previous exposure estimates, according to the authors. But some of the previous estimates reflected exposures following pesticide applications and those for young children, whose crawling and mouthing behaviors could lead to higher exposures.

The study indicates that a single short-term measurement of exposure may not yield an accurate estimate of an individual's long-term exposure, but that knowing the relationship between short-term exposure measurements and long-term exposure could be used to improve the efficiency of future epidemiologic study designs. The study also showed that multiple environmental media are important contributors to aggregate exposure, so epidemiologists should account for both dietary and nondietary exposure in their assessments. Finally, better quantification of aggregate exposure to pesticides will help environmental health scientists to evaluate the utility of biological markers of exposure for future epidemiologic studies.