Much of the HBSL methodology and the guidance on the use of human-health benchmarks have not changed since the previous revision in 2007 (Toccalino, 2007). Only changes made since 2007 are included in this 2014 update.

Changes in USEPA Methods for Calculating Benchmarks

In 2012, USEPA introduced a new policy of calculating noncancer Lifetime Health Advisories for all drinking-water contaminants, regardless of their carcinogenicity status. Historically, noncancer benchmarks were not calculated for carcinogens. The new policy also discontinues the use of a 10-fold risk management factor to account for possible carcinogenicity for Group C carcinogens (USEPA, 2012).

These policy changes were implemented in the development of USEPA HHBPs, which include Chronic Noncancer HHBPs and Carcinogenic HHBPs representing a 10^-6 (one-in-one million) to 10^-4 (one-in-ten thousand) cancer risk range (USEPA, 2013).

The 2014 HBSL update also incorporates these USEPA policy changes. As a result, there may be as many as three HBSL values for carcinogens and possible carcinogens for which both a chronic oral reference dose (cRfD) and an oral cancer slope factor (SF) are available. The cRfD is used to calculate a Noncancer HBSL, and the SF (or oral carcinogenic potency factor, Q1*) is used to calculate a Cancer HBSL range, which consists of two HBSL values representing a 10^-6 to 10^-4 cancer risk range. Click here for definitions of Noncancer and Cancer HBSLs.

Calculation of HBSLs

HBSLs are calculated only for compounds that do not have USEPA Maximum Contaminant Levels (MCLs) or HHBPs.

Noncancer HBSLs are calculated using the same default exposure assumptions that are used to derive USEPA Lifetime Health Advisories and Chronic Noncancer HHBPs. Equation 1 incorporates a drinking-water Relative Source Contribution (RSC) factor that defaults to 20% in the absence of contaminant-specific data. The RSC assumes that 20% of all exposure to a contaminant comes from drinking water.

Noncancer HBSL (µg/L)=	(cRfD [mg/kg-day]) × (70 kg body weight) × (1,000 µg/mg) × RSC	(1)
	2L water consumed/day

The Cancer HBSL range (concentrations representing a 10^-6 to 10-⁴ cancer risk range) is calculated using the same default exposure assumptions that are used to derive USEPA Cancer Risk Concentration values and Carcinogenic HHBPs (eq. 2):

Cancer HBSL (µg/L)=	(70 kg body weight) × (risk level)	(2)
	(2L water consumed/day) × (SF or Q1* [mg/kg-day]-1) × (mg/1,000 µg)

Risk levels of 10^-6 and 10-⁴ are used to calculate the low end and high end, respectively, of the Cancer HBSL range (eq. 2) The default assumptions used in equations 1 and 2 represent an adult's lifetime exposure to a contaminant in drinking water. The USEPA is consulted when HBSL calculations are not straightforward. There are a few contaminants for which HBSLs were calculated using exceptions to the HBSL methodology. Learn more.

Sources of Toxicity Information

The human-health toxicity information (cancer classifications, cRfD values, and SF or Q1* values) used to calculate HBSLs meets all of the following criteria: (1) published by USEPA, (2) peer reviewed, (3) publicly available, and (4) the most recently available.

For a given contaminant, toxicity information from the most recent of these four USEPA data sources is used:

• Integrated Risk Information System (IRIS) — Office of Research and Development
• Drinking Water Standards and Health Advisories document — Office of Water (OW)
• Human-health risk assessments and other documents from USEPA's pesticide registration and review process — Office of Pesticide Programs (OPP)
• Chemicals Evaluated for Carcinogenic Potential document — OPP

This guidance describes how HBSLs are used by the USGS NAWQA Program. In NAWQA water-quality assessments, measured concentrations of contaminants in surface water or groundwater sources of drinking water are compared to human-health benchmarks to provide perspective on the potential relevance of detected contaminant concentrations to human health. Concentrations of contaminants that are regulated by USEPA in drinking water under the Safe Drinking Water Act are compared to USEPA MCLs. Concentrations of unregulated contaminants are compared to either USEPA HHBPs or to USGS HBSLs, when available. Learn more.

Comparing Contaminant Concentrations to Benchmarks

USEPA MCLs, USEPA chronic HHBPs, and USGS HBSLs generally are calculated assuming an adult's lifetime of exposure to drinking water (see eqs. 1 and 2). As a result, contaminant concentrations (or summary statistics for concentrations) indicative of long-term exposure are most appropriate to compare to these benchmarks in most USGS applications (Toccalino, 2007). In groundwater, concentrations measured in individual samples (wells) are compared to these benchmarks. In surface waters, time-weighted annual mean concentrations are used.

Learn more about comparing contaminant concentrations to benchmarks and interpreting the potential human-health significance of detected contaminants.

Multiple Benchmarks for the Same Contaminant

Some contaminants have as many as three HBSLs or chronic HHBPs:

• Chronic noncancer
• Low end of cancer risk range (10^-6)
• High end of cancer risk range (10^-4)

For these contaminants, comparing measured concentrations (or summary statistics for concentrations) to benchmarks in NAWQA water-quality assessments is a three-step process:

1. Compare concentrations or summary statistics to the lowest HHBP or HBSL because this will be the most conservative (protective) benchmark. For nearly all carcinogens or possible carcinogens, the lowest of the three values will be the HHBP or HBSL that corresponds to a 10^-6 cancer risk.
2. If concentrations are less than the lowest HHBP or HBSL, then no further benchmark comparisons are needed.
3. If concentrations are greater than the lowest HHBP or HBSL, and the lowest value is the low end of the cancer risk range (10^-6), then compare concentrations to the high end of the cancer risk range (10^-4) to determine where concentrations fall within the 10^-6 to 10^-4 cancer risk level range. USEPA advises consideration of carcinogenic benchmarks that represent a 10^-6 to 10^-4 cancer risk level range, when appropriate (USEPA, 2012).

See Toccalino (2007) for additional information about:

• Exceptions to the HBSL methodology
• Guidance on the use of benchmarks
• Comparisons of contaminant concentrations to benchmarks
• Interpretation of the potential human-health significance of detections
• Limitations of screening-level assessments

• Representatives from USEPA Office of Water and OPP helped to shape the 2014 revisions to the HBSL methodology during webinars on October 12, 2011 and June 24, 2013, an informal meeting in Washington DC on June 24, 2014, and in supplementary discussions.
• Terri Arnold, Leslie DeSimone, and Lisa Nowell of the USGS provided insightful reviews.
• Christine Marsh, Kathryn Schoephoester, and Martin Wernimont from the USGS Center for Integrated Data Analytics developed this website.

Toccalino, P.L., 2007, Development and application of health-based screening levels for use in water-quality assessments: U.S. Geological Survey Scientific Investigations Report 2007-5106, 12 p.

Toccalino, P.L., Nowell, L.H., Wilber, W.G., Zogorski, J.S., Donohue, J.M., Eiden, C.A., Krietzman, S.J., and Post, G.B., 2003, Development of health-based screening levels for use in state- or local-scale water-quality assessments: U.S. Geological Survey Water-Resources Investigations Report 03-4054, 22 p.

USEPA, 2012, 2012 Edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, Office of Water EPA 822-S-12-001, April 2012, 20 p.

USEPA, 2013, Human health benchmarks for pesticides: Updated 2013 technical document: U.S. Environmental Protection Agency, Office of Water and Office of Pesticide Programs EPA 820-R-13-010, August 2013, 5 p.