Evidence integration in TSCA risk evaluation: The value of tiered risk assessment – A case study using HHCB

As part of the evidence integration step of its proposed systematic review protocol supporting chemical risk evaluations under the Toxic Substances Control Act (TSCA), the United States Environmental Protection Agency (EPA) describes a data hierarchy for various sources within the risk assessment. EPA identifies "less preferred" exposure data sources (e.g., modeled data) and "more preferred" sources (e.g., monitoring data). This approach to risk assessment data contrasts with substantial EPA guidance regarding tiered approaches for risk-based decision-making. We examined environmental exposure data for the fragrance material 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta[g]-2-benzopyran (HHCB), a TSCA high-priority substance, using EPA's deterministic model the Exposure and Fate Assessment Screening Tool (E-FAST), a publicly available probabilistic environmental exposure model (iSTREEM), and data from the United States Geological Survey's National Water Information System. Exposure estimates for HHCB decreased progressively from deterministic modeling to probabilistic modeling to monitoring data. However, this case study illustrates that higher-tier analyses may reduce uncertainty but may not improve the risk conclusions. Over the course of an iterative risk characterization, the need for higher tier data may be demonstrated. However, in other cases, it may be more efficient and effective to draw risk conclusions at a lower tier of assessment and forego further analysis of existing data.