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 and default exposure assumptions 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.
HBSLs also are different from other commonly used guidelines such as USEPA's Regional Screening Levels (RSLs) for Chemical Contaminants at Superfund Sites. RSLs are risk-based screening levels calculated using the latest toxicity values, default exposure assumptions, and data about physical and chemical properties. RSLs for tapwater are computed differently from HBSLs in that they may utilize information for dermal and inhalation routes of exposure in addition to oral exposure. RSL computations include information about exposure frequency and duration and use toxicity values from USEPA as well as from external agencies.