Pesticides in pregnant women: some cumulative exposures exceed safe levels.

Following passage of the 1996 Food Quality Protection Act, the U.S. Environmental Protection Agency (EPA) created new guidelines for assessing risks associated with pesticide exposure. In contrast to earlier risk assessment methodologies, the new guidelines provide a framework for estimating the cumulative risk from multiple pesticides sharing a common mechanism of toxicity. This month, Rosemary Castorina of the Center for Children's Environmental Health Research at the University of California, Berkeley, and colleagues report on one of the first case studies using the new guidelines [EHP 111:1640-1648]. Their results indicate that approximately 15% of the pregnant women they studied may have experienced cumulative organophosphate (OP) pesticide exposures exceeding a health-protective value.

OP pesticides are commonly used against insects in home and agricultural environments, and exposure is widespread. Abundant data indicate that low-level exposure to OP pesticides, prenatally and postnatally, affects the growth and neurodevelopment of young animals. These chemicals' mechanism of toxicity is inhibition of cholinesterase, an enzyme that helps control nerve transmission.

In its revised guidelines, the EPA has determined the quantity of each of 33 OP pesticides that reduces brain cholinesterase activity in test animals by 10%--the so-called oral benchmark [dose.sub.10] (BM[D.sub.10]). The BM[D.sub.10] can be used to calculate a relative potency factor to weigh the toxicities of different related pesticides in terms of a single "index pesticide."

Castorina and colleagues drew their study population from participants in the Center for the Health Assessment of Mothers and Children of Salinas study, a longitudinal birth cohort study designed in part to investigate pesticide exposures and their effects in pregnant women and children. Urine samples were collected twice during pregnancy and once after delivery, and were analyzed for 6 OP metabolites. Complete data were available for 446 women. The team also obtained reported pesticide use data for the corresponding time period from the California Department of Pesticide Regulation.

The researchers used two methods to calculate pesticide dose based on urinary metabolites. In the first, they assumed that all relevant metabolites in a sample derived from exposure to a single pesticide. This method yielded an upper limit for exposure to each of 8 pesticides representing many that are used heavily in the Salinas Valley, an area of intensive year-round agricultural production. In the second method, the metabolites were assumed to result from exposure to multiple pesticides. A likely mixture was calculated based on reported chemical use in the Salinas Valley, and a relative potency factor for each constituent pesticide was calculated using chlorpyrifos as the index chemical.

For this study, the team calculated a health-protective pesticide reference dose by dividing each pesticide's oral BM[D.sub.10] by 100. Doses higher than this were deemed to be of concern.

The results using the first method suggested that between 0% and 36% of the study population may have exceeded safe levels of exposure, depending on the pesticide analyzed. The results using the second method indicated that 14.8% of the women had excessive exposure, but due to uncertainty about the actual mixture, the range spanned from 1% to 34%.

The researchers note that each method introduces its own uncertainty. However, they believe that they have proposed a reasonable approximation of exposures, and future studies will incorporate chemical-specific biomonitoring data to counter some of this uncertainty. These preliminary results indicate a need for further research, especially as the fetal dose from maternal exposure is unknown.--Julia R. Barrett