People are exposed throughout their lifetimes to mixtures of chemicals and other agents; this may result from numerous exposure circumstances, including contaminated drinking water, polluted air, intake of pharmaceuticals, use of cosmetics and consumer products, occupational exposures, proximity to industrial facilities, and pesticide exposures. Conventional agriculture relies heavily on pesticides, often applied as mixtures of products. Each chemical in the mixture targets different soil pests, and co-application saves farmers time and fuel. With a few exceptions for known interactions between pesticides that alter their pesticidal activity, there are no label restrictions on combining pesticides. Exposure to multiple pesticides is thus widespread, from the most heavily exposed farm workers, to neighbors adjacent to or downstream from pesticide application sites such as agricultural fields or structural fumigations.

Increasingly, research shows that cumulative exposures can have larger than anticipated impacts on public health. While the potential for interactive effects is recognized in both scientific and regulatory communities, pesticide testing requirements by the California Department of Pesticide Regulation and the U.S. Environmental Protection Agency — the state and federal agencies responsible for pesticide regulation — are currently focused primarily on the effects of a single chemical. While there are some efforts underway to assess pesticides with common mechanisms of action, pesticides are still assessed and regulated assuming exposure to only a single chemical.

Assessing the risks of multiple exposures is challenging for already resource-stressed regulatory agencies, but is essential for fully understanding the potential for adverse health effects. This report evaluates the possible cumulative health effects of three pesticides commonly used in California, and makes policy suggestions for implementing a framework for cumulative risk assessment that accounts for potential interactive effects, not just those occurring by the same mechanism of action.

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