Exceptions to how HBSLs are calculated
When a Health-Based Screening Level (HBSL) differs from an existing U.S. Environmental Protection Agency (USEPA) Lifetime Health Advisory (Lifetime HA) value and the reason for the difference is not apparent (i.e., the same Office of Water (OW) toxicity value and equation are used for the HBSL and Lifetime HA), the USGS will consult with the USEPA OW to identify the reason for the discrepancy. The USEPA OW may have technical or policy reasons for modifying the equations for calculating Lifetime HA values for some compounds. The rationale for any exceptions to the HBSL methodology will be captured in the HBSL database.