About HBSLs and the HBSL Database

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 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 guidelines that denote concentrations in water that correspond to a range in estimated lifetime cancer risk of one-in-one million (10^-6) to one-in-ten thousand (^10-4) from consumption of drinking water containing this chemical.

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).

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 many 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.

The USEPA issues legally enforceable drinking-water standards (MCLs) as well as non-enforceable drinking-water guidelines. The USEPA’s drinking-water guidelines (Health Advisories [HAs], Risk Specific Level Concentrations, and Human Health Benchmarks for Pesticides [HHBPs]) are issued in an advisory capacity. USGS HBSLs are also 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 HAs, Risk Specific Level Concentrations, 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 has developed HHBPs for pesticides that do not have federal drinking water standards or HAs (USEPA, 2021a). Most pesticides with HHBPs are registered for uses on food crops, but starting in 2021, USEPA began developing HHBPs for pesticides that were not registered for use on foods, if appropriate toxicity data were available (USEPA, 2021b).

Because USGS HBSLs are calculated using the same methodology and equations that USEPA uses to calculate chronic HHBPs, HBSLs are not calculated for pesticides that have HHBPs (or for any contaminants with MCLs). If any HBSLs differ from existing EPA HA and Risk Specific Level Concentration values, this is because HBSLs have been calculated more recently using updated default exposure assumptions for the general adult population (as per USEPA, 2021b) or more recent toxicity information from USEPA, if available.

The HBSL database contains 835 contaminants, listed below by primary chemical class. Except for 27 PFAS chemicals added in 2024, the contaminants in the database are currently or were historically analyzed by the USGS National Water Quality Program. The database includes drinking-water standards or benchmarks for 429 contaminants (93 USEPA MCLs or proposed MCLs, 144 USEPA HHBPs, and 192 USGS HBSLs) and detailed supporting toxicity information used to calculate HBSLs. HBSLs are not available for the remaining 406 contaminants because of a lack of USEPA toxicity information. The 835 contaminants in the HBSL database by chemical class (with the number having benchmarks in brackets) consist of:

• 227 Pesticides [212 with benchmarks]
• 160 Pesticide degradates [21]
• 27 Per- and polyfluoroalkyl substances (PFAS) [27]
• 154 Emerging contaminants: [11]
  • 18 Hormones [0]
  • 111 Pharmaceuticals [2]
  • 25 Waste-water effluent contaminants and other emerging contaminants [9]
• 138 Volatile organic compounds (VOCs) [81]
• 61 Semi-volatile organic compounds (SVOCs) [36]
• 8 Polychlorinated biphenyl (PCB) compounds [3]
• 27 Trace elements [26]
• 14 Major ions [2]
• 10 Nutrients [4]
• 9 Radionuclides [6]

USEPA is in the process of developing regulations for PFAS chemicals in drinking water under the Safe Drinking Water Act and continues to develop human health toxicity assessments for additional PFAS chemicals (USEPA, 2023b). On April 10, 2024, USEPA issued the Final PFAS National Primary Drinking Water Regulation, which applies to 5 PFAS chemicals and some mixtures (USEPA, 2024). Over time, USEPA may regulate and(or) develop human health toxicity assessments for additional PFAS chemicals. The PFAS chemicals listed in this January 2024 HBSL update represent those PFAS chemicals currently included in the USEPA (2024) Final Rule, plus some additional chemicals for which USEPA has toxicity assessments available. We have updated the HBSL database to reflect these April 2024 changes. The HBSL database will continue to be updated to capture future USEPA regulations and supporting toxicity information. For recent updates, readers can consult USEPA’s online summary of key actions to address PFAS chemicals.

USEPA, 2021a. 2021 Human Health Benchmarks for Pesticides. https://www.epa.gov/sdwa/2021-human-health-benchmarks-pesticides.

USEPA, 2021b. Human Health Benchmarks for Pesticides: Updated 2021 Technical Document. https://www.epa.gov/system/files/documents/2021-07/hh-benchmarks-technical-document-2021.pdf.

USEPA, 2023a. Human Health Benchmarks. https://www.epa.gov/sdwa/human-health-benchmarks.

USEPA, 2023b. EPA’s PFAS Strategic Roadmap: Second Annual Progress Report, December 2023. https://www.epa.gov/system/files/documents/2023-12/epas-pfas-strategic-roadmap-dec-2023508v2.pdf.

USEPA, 2024. Per- and Polyfluoroalkyl Substances (PFAS). Final PFAS National Primary Drinking Water Regulation. https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas.