Do standard risk assessment procedures adequately account for cumulative risks?: an exploration of the possibilities using California's Air Toxics Hot Spots guidelines.

Existing risk assessment data and procedures can be used to address the estimation of cumulative risk, but there are several uncertainties. These are explored in the context of the State of California's Air Toxic Hot Spots program. Hazard identification for single agents is an established procedure but is much more complex for incompletely characterized or variable mixtures. Hazards from exposure to multiple agents are often only identified by chance. Similar concerns affect dose-response assessment. Although additivity is assumed by default for similar effects at low doses, exceptions are known for specific mixtures and for higher dose rates. Exposure assessment is especially complex for multiple sources, multiple agents from different sources, and target populations or individuals who face cumulative, but not necessarily simultaneous, impacts. With these contributory uncertainties, providing an integrated analysis that can inform risk management and presenting this to a diverse and often already stressed community are challenging.