Multiple models reveal pesticide exposure routes
Combining modeling with biomonitoring data reveals pesticide exposure routes in pregnant Latina women in the Salinas Valley (Calif.).
Thomas McKone and his co-workers set out to answer a simple question: can the levels of pesticides in pregnant women in the Salinas Valley (Calif.) be correlated with the amounts of these chemicals used in surrounding farmlands? A simple statistical model suggested no direct correlation, but McKone and his colleagues at the Lawrence Berkeley National Laboratory and the University of California, Berkeley, didn't stop there.
The team combined multiple fate and exposure models with biomonitoring data. The results, published today on ES&T's Research ASAP website (DOI: 10.1021/es0618447), show that the study population has a significantly higher intake of organophosphorus (OP) pesticides than the average woman in the same age group. The difference is attributed to exposure through air, water, and soil, but not from food. By using these models with hard data, the researchers could track how pesticides travel from the fields through different channels into the human body.
Farmers use large amounts of pesticides in the Salinas Valley, an important agricultural region of California. In 2001, 240,000 kilograms of OP pesticides were applied in and around the valley. Humans metabolize commonly used OP pesticides, such as chlorpyrifos, diazinon, and malathion, into simpler compounds that eventually are excreted in urine. Researchers can monitor levels of exposure by analyzing the levels of these metabolites in urine. Understanding the routes of exposure, however, is trickier.
McKone and his team studied OP pesticide exposure in about 600 pregnant Latina women in the Salinas Valley. They were all part of a larger study on women and children's environmental health, called the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) project. The concentration of the pesticide metabolites in the CHAMACOS population was compared with the average levels in women from across the U.S. (from data collected by the National Health and Nutrition Examination Survey, or NHANES, in 1999–2000). This revealed that the Salinas Valley women are being exposed to "significantly higher" levels of the pesticides, says McKone.
The next step was to determine how these women are being exposed. The team predicted that many different pathways were involved. And it was crucial to determine how much each route or source was contributing to the measured levels of pesticide metabolites in the women. First, they adapted the CalTOX fate and exposure model to estimate OP pesticide concentrations in outdoor air and soil near participants' homes. They then combined the results with an indoor mass-balance model to estimate indoor air, dust, and surface concentrations. The team modeled nondietary exposures through inhalation, nondietary ingestion, and skin contact, and they estimated dietary exposures using the U.S. Food and Drug Administration's Total Diet Study, which provides information on the levels of pesticide intake through "table-ready" foods.
When McKone and colleagues combined the results from the models with biomonitoring data, they found that the Salinas Valley population was receiving similar levels of the pesticides from their diet as women elsewhere in the U.S. The real culprit behind the higher levels of OP pesticide in this population was exposure through air, water, and soil.
Interpreting biomonitoring data on chemicals like OP pesticides is difficult because the chemicals don't persist in the environment for very long and the data have an "inherent uncertainty and variability," says Dana Barr, the chief of the pesticides laboratory at the Centers for Disease Control and Prevention's National Center for Environmental Health, "If you take one spot, you may get no exposure where you've been exposed previously, and another time you can get peak exposure," she says. McKone's approach provides a unique way of getting at the complex mechanism of exposure to pesticides, Barr comments.
The study is also very informative in terms of understanding the fate of pesticides, says Don MacKay of the Canadian Environmental Modeling Center at Trent University. Most environmental models merely predict "that if you use this amount of chemicals in this area you probably get this concentration in air, and this in water, and that in fish, and that's about as far as they go," he says. McKone's work goes beyond that and "quantifies the whole journey [of pesticides] from sources into the environments . . . into outdoors, indoors, into human body," he says.
Few studies "include how much [of a contaminant] is actually reaching us as organisms and how this results in concentrations in our tissue and fluids," adds MacKay. This study provides "a shining example of the approach that should be taken for more substances, including pesticides and industrial chemicals. If that can be done, it will help the whole risk assessment process enormously."
