Human and environmental receptors are co-exposed to multiple synthetic chemicals from multiple sources. The question has been raised whether chemical-by-chemical assessments of risk underestimate the total toxicity to individuals. This talk presents a simple screening tool, the Maximum Cumulative Ratio (MCR), which can be used to prioritize cumulative exposures to chemicals, including pesticides, for human and ecological receptors. MCR values, along with a measurement of cumulative hazard such as the Hazard Index (HI) have shown a consistent pattern. Specifically, for any population of receptors exposed to multiple chemicals from one or more sources, the individuals with the largest HI values tend to have risks that are dominated by a single chemical (MCR values close to 1). This suggests that receptors with the largest cumulative stresses are more likely due to a single chemical (either from high toxicity or high exposures), rather than multiple chemicals operating in combination. This further implies that chemical-by-chemical approaches are protective for the majority of cumulative and aggregate exposures. These studies also demonstrated the ability of the MCR to identify certain exposures whose cumulative risks could be missed using chemical-by-chemical approaches under certain conditions. The MCR and the HI provide a basis for risk management of cumulative exposures. Cumulative exposures can be divided into four groups based upon MCR and HI values, Group I, where one or more individual compounds have hazard quotient values greater than 1; Group II, where there is low concern for cumulative effects; Group IIIA, where cumulative exposures are a concern but one chemical is the clear driver; and Group IIIB, where information on mode of action is needed for refining the assessment. Using this approach, assessors can identify exposures where chemical-by-chemical approaches would have missed cumulative toxicity, and whether there are specific groups of chemicals that warrant additional investigation.

Cumulative risks: When do they add up? Using the Maximum Cumulative Ratio (MCR) to understand risks from concurrent exposures to multiple chemicals

246th National Meeting (2013)

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