What are HBSLs?
Health-Based Screening Levels (HBSLs) are benchmark concentrations of contaminants in water that may be of potential concern for human health, if exceeded. HBSLs are non-enforceable benchmarks that were developed by the USGS in collaboration with USEPA and others using USEPA methodologies for establishing drinking-water guidelines and the most current, USEPA peer-reviewed, publicly available human-health toxicity information (Toccalino and others, 2003, 2006). The USGS and its cooperators are continuing 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.