Risk-Based Screening Level Calculator

This calculator derives risk-based screening levels (RBSLs) — the contaminant concentration in soil or groundwater below which exposure is considered acceptable — by rearranging the standard EPA risk equation to solve for concentration instead of risk. For each medium it runs two parallel calculations: a cancer-based level using the oral slope factor and a target cancer risk (default 1-in-a-million), and a non-cancer level using the oral reference dose and a target hazard quotient (default 1.0), then reports the lower, more conservative of the two as the protective screening level, along with which endpoint drives it. The underlying exposure assumptions switch based on land use, matching the default exposure factors published in EPA's Regional Screening Levels (RSL) User's Guide: residential parameters use EPA defaults of 350 exposure days/year over 26 years with a 200 mg/day soil ingestion rate (child-adjusted), while commercial/industrial uses 250 days/year over 25 years at 100 mg/day, reflecting that workers are exposed less frequently and for less of their lifetime than residents. Averaging time for the cancer calculation always uses a 70-year lifetime regardless of land use, per EPA convention, while non-cancer averaging time is limited to the actual exposure duration.

Groundwater screening levels use the same logic through a direct ingestion pathway at 2 L/day (residential) or 1 L/day (commercial). This tool only models direct ingestion — it doesn't include dermal contact, vapor inhalation, or the vapor intrusion pathway, so a site with volatile contaminants needs those pathways evaluated separately (see the vapor intrusion calculator). Toxicity values (slope factor, reference dose) must come from EPA's IRIS database or a state-approved source; using outdated or wrong values will silently produce a wrong screening level with no warning from the math itself.