How do HBSLs differ from drinking-water standards and guidelines?
A U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) is a legally enforceable standard that sets the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs are set as close as feasible to the maximum level of a contaminant at which no known or anticipated adverse effects on human health would occur, taking into account the best available technology, treatment techniques, cost considerations, expert judgment, and public comments. Health-Based Screening Levels (HBSLs) are not legally enforceable drinking-water standards or clean-up levels. Rather, HBSLs are non-enforceable guidelines based on health effects alone and do not consider cost or technical limitations.
HBSLs were calculated using standard USEPA Office of Water methodologies for establishing drinking-water guideline values for the protection of human health. HBSLs are, therefore, equivalent to existing USEPA Lifetime Health Advisory (Lifetime HA) and Cancer Risk Concentration values (when they exist), except for compounds for which more recent toxicity information has become available. For some compounds, the toxicity information used to derive Lifetime HA and RSD values has been updated in recent years, and the most recent USEPA toxicity information is used to calculate HBSLs.
HBSLs are also different from other commonly used guidelines such as EPA Region 9 Preliminary Remediation Goal (PRG) values and EPA Region 3 Risk Based Concentration (RBC) values for tap water. PRGs and RBCs are risk-based concentrations, derived from standardized equations combining exposure information assumptions with USEPA toxicity data. Although HBSLs are also derived using standard equations and USEPA toxicity data, PRGs and RBCs use additional exposure information (such as exposure frequency and exposure duration) and they also account for inhalation of volatile organic compounds present in a water source.