Health-Based Screening Levels (HBSL)

About HBSLs and the HBSL Database

About the HBSL project

To supplement federal drinking-water standards and guidelines, the U.S. Geological Survey (USGS) began an interagency pilot effort in 1998 to develop non-enforceable Health-Based Screening Levels (HBSLs) that would help communicate the potential relevance of water-quality findings from the USGS National Water Quality Program, National Water-Quality Assessment (NAWQA) Project in a human-health context. Federal drinking-water standards or guidelines are not available for many of the hundreds of contaminants analyzed by USGS in sources of drinking water. Collaborators on the initial HBSL effort included:

  • U.S. Environmental Protection Agency (USEPA)
  • New Jersey Department of Environmental Protection (NJDEP)
  • Oregon Health & Science University (OHSU)

What are HBSLs?

HBSLs are non-enforceable water-quality benchmark concentrations of contaminants in water that were developed using (1) the latest USEPA Office of Water methods for establishing drinking-water guidelines and (2) the most recent USEPA peer-reviewed publicly available toxicity information. Specifically:

  • Noncancer HBSLs are non-enforceable concentrations of contaminants in water below which adverse noncarcinogenic health effects are not expected over a lifetime of exposure.
  • Cancer HBSLs are non-enforceable concentrations of contaminants in water that correspond to a range in excess estimated lifetime cancer risk of one-in-one million (10-6) to one-in-ten thousand (10-4).

HBSLs do not consider all human exposure pathways (only potential drinking-water ingestion), nor are they used to assess ecological health. HBSLs are calculated for contaminants that:

  1. Are not regulated by the USEPA in drinking water under the Safe Drinking Water Act and therefore do not have Maximum Contaminant Levels (MCLs).
  2. Do not have USEPA chronic Human Health Benchmarks for Pesticides (HHBPs).

How are HBSLs used?

HBSLs can be used to supplement USEPA MCLs and HHBPs to put measured concentrations of a broad array of contaminants in drinking-water sources into a human-health context. In USGS water-quality assessments, measured concentrations of regulated contaminants are compared to USEPA MCLs, and concentrations of unregulated contaminants are compared to USEPA HHBPs or to USGS HBSLs, when available.

Comparisons of contaminant concentrations in water to MCLs, HHBPs, and HBSLs are useful for local, state, and federal water-resource managers and others charged with protecting and managing drinking-water resources. These comparisons:

  • Provide an initial perspective on whether contaminants detected in surface water or groundwater sources of drinking water may indicate a potential human-health concern.
  • Can help prioritize which contaminants and water resources may warrant further study or monitoring.

See HBSL Methods & Guidance for details about how HBSLs are calculated and guidance on their use.

How do HBSLs differ from USEPA drinking-water standards and guidelines?

The USEPA issues legally enforceable drinking-water standards (MCLs) as well as non-enforceable drinking-water guidelines. The drinking-water guidelines (Lifetime Health Advisories (LHAs), Cancer Risk Concentrations (CRCs), and Chronic Noncancer and Carcinogenic HHBPs) are issued in an advisory capacity. USGS HBSLs also are non-enforceable guidelines.

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. By contrast, USEPA LHAs, CRCs, and HHBPs, as well as USGS HBSLs, are based on health effects alone, calculated using default exposure assumptions, and do not consider cost or technical limitations.

USEPA HHBPs are calculated for pesticides that (1) do not have MCLs, LHAs, or CRCs and (2) are registered for uses on food crops. Because USGS HBSLs and USEPA chronic HHBPs are calculated using identical methodologies, HBSLs are not calculated for pesticides that have HHBPs (or for any compounds with MCLs). Although HBSLs are calculated using the same equations used by USEPA to calculate LHAs and CRCs, some HBSL values differ from existing LHA and CRC values because HBSLs have been calculated more recently using updated default exposure assumptions and more recent toxicity information from USEPA, if available.

Contaminants included in the HBSL database

The HBSL database contains 808 contaminants currently or historically analyzed by the NAWQA Project, listed below by chemical class. The database includes drinking-water standards or guidelines for 393 contaminants (79 USEPA MCLs, 140 USEPA HHBPs, and 174 USGS HBSLs) and detailed supporting toxicity information used to calculate HBSLs. HBSLs are not available for the remaining 415 contaminants because of a lack of toxicity information.

  • 226 Pesticides
  • 161 Pesticide degradates
  • 154 Emerging contaminants:
    • 18 Hormones
    • 111 Pharmaceuticals
    • 25 Other emerging contaminants
  • 138 Volatile organic compounds (VOCs)
  • 61 Semi-volatile organic compounds (SVOCs)
  • 8 Polychlorinated biphenyl (PCB) compounds
  • 27 Trace elements
  • 14 Major ions
  • 10 Nutrients
  • 9 Radionuclides