Science Advisory Board to Review Co-exposure Approach to TSCA Risk Evaluations

This October, the Science Advisory Board (SAB) received an EPA briefing on a proposed “screening level analysis” that could account for cumulative chemical exposures in Toxic Substances Control Act (TSCA) risk evaluations.

Released in September 2023, EPA’s draft approach is designed to support the identification of potentially exposed or susceptible subpopulations (PESS) and co-exposures with other chemicals.  It relies heavily on AirToxScreen, a modelling tool already used by EPA to estimate chemical inhalation exposure and risk at the national level.

The draft approach “can be used to better inform chemical co-exposure, highlight geographic areas or population groups that may experience disproportionate impacts, and identify areas that may need more targeted or higher tier exposure and risk characterization.”  It is not intended to be “used as a sole basis for health or regulatory action,” however.

Limitations to the draft approach include its narrow focus on industrial releases to air and reliance on annual data, which makes it impossible to determine whether releases occurred concurrently.  In addition, the model does not calculate “a total additive exposure or total additive risk across the chemicals included in the analysis.”

Historically, TSCA risk evaluations have evaluated a single chemical and considered routes of exposure separately.  However, in a slideshow from the October 15–16 SAB meeting, EPA noted that there is “wide acknowledgement in [the] scientific and regulatory community [that] multiple facility and chemical exposures” meaningfully intersect.

The agency also emphasized the environmental justice concerns tied to cumulative exposures, which disproportionately affect marginalized communities.

The SAB is expected to provide formal comments on the approach in the coming months.