What are HBSLs?
Health-Based Screening Levels (HBSLs) are non-enforceable benchmark concentrations of unregulated contaminants in water that may be of potential concern for human health, if exceeded. HBSLs were
developed by the USGS in collaboration with the U.S. Environmental Protection Agency (USEPA) and others using: (1) USEPA Office of Water methodologies for establishing drinking-water guidelines, and (2) the most recent, USEPA peer-reviewed, publicly available human-health toxicity information (Toccalino and others, 2003; 2006; Toccalino, 2007).
HBSLs are calculated for contaminants that are not regulated by the USEPA in drinking water under the Safe Drinking Water Act and therefore do not have Maximum Contaminant Levels. The USGS and its cooperators continue to refine the HBSL methodology. HBSLs do not consider all potential human exposure pathways (only drinking water ingestion), nor are they used to assess ecological health.
For noncarcinogens, the HBSL represents the contaminant concentration in
drinking water that is not expected to cause any adverse effects over a lifetime
of exposure. For carcinogens, the HBSL range represents the contaminant concentration in drinking water that corresponds to an excess estimated lifetime cancer risk of 1 chance in 1 million to 1 chance in 10 thousand. HBSL calculations adopt USEPA assumptions for establishing drinking-water guidelines, namely, lifetime ingestion of 2 liters of water per day by a 70-kilogram adult. For noncarcinogens, it also is assumed that 20 percent of the total contaminant exposure comes from drinking water sources and that 80 percent comes from other sources (for example, food and air). If data are available to quantify the percentage of contaminant exposure that comes from water, then a data-derived percentage is used instead of the default of 20 percent. See "How
are HBSLs calculated?" for more information.