Health-Based Screening Levels: Updated 2024 Technical Information

The HBSL methodology has been updated to reflect changes in USEPA procedures for calculating HHBPs since the previous (2018) version of the HBSL methodology.

Changes in USEPA Methods for Calculating Benchmarks

Since 2012, USEPA has developed HHBPs (USEPA, 2021b) for pesticides that do not have MCLs. Through 2018, USEPA included only pesticides registered for food uses, but in 2021, USEPA added pesticides registered for non-food uses if toxicity data were available (USEPA, 2021a).

The Chronic Noncancer and Carcinogenic HHBPs (USEPA, 2021b) are calculated by USEPA using typical methods for developing drinking water health advisories under the Safe Drinking Water Act (SDWA; USEPA, 2021a). Since 2017, USEPA has based the Chronic Noncancer HHBPs on the chronic population adjusted dose (cPAD) instead of the chronic reference dose (cRfD), when these are different. For most pesticides, the cPAD is equivalent to the cRfD. However, for some pesticides, an extra safety factor is applied to the cRfD to derive a cPAD, as mandated by the Food Quality Protection Act to take into account uncertainty (i) related to pre/post-natal exposure or toxicity or (ii) in the overall completeness of the toxicity database (USEPA, 2017). For Chronic Noncancer HHBPs, USEPA selects the cPAD (or if none is available, the cRfD) derived for either the general adult population or females of reproductive age (USEPA, 2021a).

Beginning in 2021, USEPA (2021a) updated the default exposure assumptions using data from the 2005−2010 National Health and Nutrition Examination Survey (NHANES). Instead of using separate default values for drinking water intake (DWI) and body weight (BW) as was done previously (USEPA, 2017), USEPA (2021a) began using normalized ratios of drinking water intake to body weight (DWI/ BW) to represent the 90th percentile values of the two-day average, consumer-only estimates of combined direct and indirect community water ingestion. In calculating HHBPs, USEPA used default values for adults (typically 0.0338 L/kg/day for the general adult population; or when appropriate for selected pesticides, 0.0354 L/kg/day for females of reproductive age), which were based on the 2005-2010 NHANES data as reported in USEPA’s update for Chapter 3 of the Exposure Factors Handbook (USEPA, 2019).

Calculation of HBSLs

HBSLs are calculated only for contaminants that do not have USEPA MCLs or Chronic Noncancer or Carcinogenic HHBPs. Equations for the Noncancer HBSLs and Cancer HBSLs are identical to those for Noncancer HHBPs and Carcinogenic HHBPs (USEPA, 2021a), respectively.

Chronic Noncancer HBSLs use the cPAD, when available, or alternately the cRfD for the most sensitive subpopulation, following the methodology for HHBPs. Chronic Noncancer HBSLs are calculated using the same default exposure assumption for the general adult population that USEPA used to derive Chronic Noncancer HHBPs, namely the default DWI/BW ratio value for the general adult population of 0.0338 L/kg/day. Equation 1 for Noncancer HBSLs 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 the total exposure from all sources to a contaminant comes from drinking water. Consistent with USEPA procedures for calculating HHBPs, the RSC is used only in deriving Chronic Noncancer HBSLs, and not cancer HBSLs.

Noncancer HBSL (µg/L)=	(cPAD or cRfD [mg/kg-day]) × (1,000 µg/mg) × RSC	(1)
	(adult DWI/BW ratio [L/kg/day])

where RSC = 0.2, and the adult DWI/BW ratio = 0.0338 L/kg/day.

The Cancer HBSLs (like USEPA’s Carcinogenic HHBPs) represents a concentration in water that corresponds to a specified cancer risk (not a no-effect concentration), so the risk level is included as a variable in the equation for the Cancer HBSL (eq. 2). Both Cancer HBSLs and Carcinogenic HHBPs consist of a concentration range corresponding to a cancer risk ranging from 10^-6 to 10^-4. Both Cancer HBSLs and Carcinogenic HHBPs use an oral cancer slope factor (CSF) to convey dose-response information for the contaminant, and both are calculated using the same default exposure assumptions for the general adult population. The CSF is developed from USEPA health effects data, and is also called the oral cancer potency factor (Q1^*). The Cancer HBSL range is calculated as shown in eq. 2:

Cancer HBSL (µg/L)=	(risk level) × (1,000 µg/mg)	(2)
	(adult DWI-BW ratio [L/kg/day]) × (CSF [mg/kg/day]-1)

where CSF is the contaminant’s cancer slope factor, in units of (mg/kg/day)^-1, and a default value of 0.0338 L/kg/day (for the general adult population) is used for the adult DWI/BW ratio. 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). This cancer risk range corresponds to an estimated lifetime cancer risk ranging from 1 in 1 million to 1 in ten thousand from consumption of drinking water containing this chemical.

The default exposure assumptions used in equations 1 and 2 represent an adult's lifetime exposure to a contaminant in drinking water. The USGS consults with USEPA when HBSL calculations are not straightforward.

There are a few contaminants for which HBSLs were calculated using exceptions to the HBSL methodology that is described above. For example, acute RfDs are used for 15 contaminants for which USEPA did not derive a cRfD because acute endpoints are protective of longer-term exposure (noncancer effects). Six additional contaminants had exceptions, in which (i) USEPA advised using a non-standard RSC or RfD value or adding a modifying factor (4 contaminants), or (ii) the most recent toxicity values were under review or no longer used by USEPA (2 contaminants). These exceptions are noted in the Benchmark Remarks column of the online table of HBSL values.

Sources of Toxicity Information

The human-health toxicity values used to calculate HBSLs (cPAD or cRfD values for Noncancer HBSLs, and CSF values for Cancer HBSLs) meet all of the following criteria: (1) published by USEPA, (2) peer reviewed, and (3) the most recently available. For a given contaminant, toxicity information from the most recent of these five primary USEPA data sources is used:

• Integrated Risk Information System (IRIS) — Office of Research and Development (USEPA, 2024a)
• Drinking Water Standards and Health Advisories Tables — Office of Water (OW) (USEPA, 2018a)
• Human-health risk assessments and other documents from USEPA's pesticide registration and review process, which are accessible from the USEPA Public Docket at www.regulations.gov. The Public Dockets for individual pesticides can be identified by Pesticide Chemical Search — Office of Pesticide Programs (OPP)
• Chemicals Evaluated for Carcinogenic Potential (Annual Cancer Report 2022) — OPP (USEPA, 2022a)
• The 2022 Reference Dose Summary Report (OPP) (USEPA, 2022b)

In the absence of available toxicity information from the primary USEPA data sources above, the USEPA Provisional Peer Reviewed Toxicity Values (PPRTV) developed for the Superfund Program (USEPA, 2018b) were used, when available; PPRTVs were used for 16 contaminants. This is consistent with the use of PPRTVs by the Superfund Program as a "second tier" source of toxicity values. PPRTVs were externally peer reviewed and developed following USEPA guidance on deriving human-health toxicity values for the Superfund Program (USEPA, 2018b); however, the toxicity values have not undergone the multi-program consensus review that is required for inclusion in the USEPA IRIS database (USEPA, 2024a). Contaminants with HBSLs derived from PPTRV toxicity information are noted in the Benchmark Remarks column of the online table of HBSL values.

This guidance describes how HBSLs are used in many USGS 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 (SDWA) are compared to USEPA MCLs. Concentrations of unregulated contaminants are compared to either USEPA HHBPs or to USGS HBSLs, when available.

USEPA MCLs, USEPA Chronic Noncancer and Carcinogenic HHBPs, and USGS HBSLs generally are calculated assuming an adult’s lifetime of exposure to drinking water (see equations 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). For groundwater, concentrations measured in individual samples (wells) are compared to these benchmarks. For surface waters, time-weighted annual mean concentrations are used when there are multiple samples.

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

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

For contaminants with multiple benchmarks, data analysts can compare measured concentrations (or summary statistics for concentrations) to benchmarks in water-quality assessments as follows:

1. First, compare concentrations or summary statistics to the lowest HHBP or HBSL because this is 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 benchmark 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 whether concentrations fall within the 10^-6 to 10^-4 cancer risk level range. USEPA typically calculates concentrations (benchmarks) corresponding to an upper-bound excess cancer risk ranging from 1 in a million (10^-6) to 1 in 10,000 (10^-4), when appropriate (USEPA, 2018a; USEPA, 2021a).

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

USEPA is in the process of developing regulations for PFAS chemicals in drinking water under the SDWA and continues to develop human health toxicity assessments for additional PFAS chemicals (USEPA, 2023, 2024b, 2024c). On April 10, 2024, USEPA issued the PFAS National Primary Drinking Water Regulation Final Rule which applies to five PFAS chemicals and some mixtures (USEPA, 2024b; 2024c). Over time, USEPA may regulate and(or) develop human-health toxicity values for additional PFAS chemicals. The 27 PFAS chemicals listed in this January 2024 HBSL update represent those chemicals specified in the USEPA Final Rule for PFAS National Primary Drinking Water Regulation Rulemaking (USEPA, 2024c); associated USEPA technical documents; and existing USEPA toxicity assessments for additional PFAS chemicals. The HBSL database has been updated for PFAS chemicals to reflect the April 2024 values from USEPA’s (2024c) Final Rule. The USGS plans to periodically update the applicable MCLs and HBSLs for PFAS chemicals in the HBSL database to capture future USEPA regulations and toxicity information. Readers are encouraged to consult USEPA’s online summary of their key actions to address PFAS chemicals, to find the current status of USEPA drinking water regulations for PFAS chemicals.

In USEPA’s (2024c) PFAS National Primary Drinking Water Regulation Rulemaking, Final Rule, and associated technical documents, PFAS chemicals are listed by name and Chemical Abstract Services Registry Number (CASRN). It is recognized that PFAS chemicals exist in multiple forms, including linear and branched isomers, acids, and salts, and each form has a unique CASRN. However, USEPA specifies in its Final Rule that, “This rule covers all salts, isomers and derivatives of the chemicals listed, including derivatives other than the anionic form which might be created or identified” (USEPA, 2024c). Therefore, it is reasonable for users to apply a PFAS benchmark in the HBSL database to the anionic form (i.e., conjugate base) of the PFAS chemical, as well as to derivatives (e.g., salts) and isomers (e.g., branched and linear) of that chemical that might be created or identified in a water sample.

For PFAS chemicals, parameter codes from the National Water Information System (NWIS) are not included in the HBSL database because CASRNs are not available for PFAS chemicals in the USGS National Water Quality Laboratory (NWQL) analytical schedules. However, most PFAS chemicals are expected to dissociate in water (at environmentally relevant pHs) to their anionic form (conjugate base), and the resulting anion for any group of acids and salts that share a common structure (after deprotonation) will appear as a single analyte for the purposes of detection and quantitation. Technically, USGS measures concentrations of PFAS chemicals in their anionic forms. However, based on USEPA guidance in the Final Rule (p. USEPA, 2024c), quoted above, if the anionic form of a given PFAS analyte is not included in the HBSL table, it is appropriate for the user to apply the benchmark for the corresponding acid form to concentrations of the same PFAS chemical (including various isomers) measured in the anionic form. Moreover, if concentrations are measured separately for branched and linear isomers of the same PFAS chemical (for example, branched and linear perfluorooctane sulfonic acid, or PFOS), then it is reasonable to sum the concentrations of the branched and linear isomers before comparison to the benchmark for that PFAS chemical (in this example, to the PFOS benchmark).

In its Final PFAS National Primary Drinking Water Regulation, USEPA (2024c) has proposed to use a Hazard Index (HI) approach to protect public health from mixtures of four PFAS chemicals because of their known and additive toxic effects, and their occurrence and likely co-occurrence in drinking water. These four PFAS chemicals are: perfluorohexane sulfonic acid (PFHxS), hexafluoropropylene oxide dimer acid (HFPO–DA) and its ammonium salt (also known as GenX chemicals), perfluorononanoic acid (PFNA), and perfluorobutane sulfonic acid (PFBS). USEPA has set an HI of 1.0 (unitless) as the MCL for these four PFAS chemicals and any mixture containing one or more of them (USEPA, 2024b, 2024c). This HI information is included in the Benchmark Remarks column for the affected chemicals within the HBSL table.

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

See Toccalino (2014) for the 2014 updates to the HBSL methodology, including:

• Adopting HHBPs and removing HBSL values for pesticides with HHBPs
• Calculating noncancer HBSLs for contaminants regardless of carcinogenicity status
• Discontinuing the use of a 10-fold risk management factor for Group C carcinogens

See Norman et al. (2018) for 2018 updates to the HBSL methodology, including:

• Use of the cPAD (when available) instead of the cRfD.

• Three USGS colleague reviewers provided insightful and helpful reviews.

Norman, J.E., Toccalino, P.L., Morman, S.A., 2018, Health-Based Screening Levels for evaluating water-quality data (2nd ed.), Updated 2018 Technical Information. U.S. Geological Survey web page, accessible at https://water.usgs.gov/water-resources/hbsl/methods-guidance-2018.html.

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., Norman, J.E., and Schoephoester, K.M., 2014, Health-Based Screening Levels: Updated 2014 technical information. U.S. Geological Survey web page, accessible at https://water.usgs.gov/water-resources/hbsl/methods-guidance-2014.html.

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, 2017, Human health benchmarks for pesticides: Updated 2017 technical document: U.S. Environmental Protection Agency, Office of Water and Office of Pesticide Programs, EPA 822-R-17-001, January 2017, 5 p.

USEPA, 2018a, 2018 Edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, Office of Water, EPA 822-F-18-001, March 2018, 20 p. https://www.epa.gov/system/files/documents/2022-01/dwtable2018.pdf.

USEPA, 2018b, Provisional Peer Reviewed Toxicity Values for Superfund (PPRTV): U.S. Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation, electronic library, https://hhpprtv.ornl.gov/.

USEPA, 2019. Update for Chapter 3 of the Exposure Factors Handbook, Ingestion of Water and Other Select Liquids, EPA/600/R-18/259F February 2019, https://www.epa.gov/sites/default/files/2019-02/documents/efh_-_chapter_3_update.pdf.

USEPA, 2021a, Human health benchmarks for pesticides: Updated 2021 technical document: U.S. Environmental Protection Agency, Office of Water and Office of Pesticide Programs, August 2021, 5 p., https://www.epa.gov/system/files/documents/2021-07/hh-benchmarks-technical-document-2021.pdf.

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

USEPA, 2022a, Chemicals Evaluated for Carcinogenic Potential (Annual Cancer Report 2022), Office of Pesticide Programs. http://npic.orst.edu/chemicals_evaluated.pdf.

USEPA, 2022b. The 2022 Reference Dose Summary Report, Office of Pesticide Programs.

USEPA, 2023. 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, 2024a. Integrated Risk Information System. https://www.epa.gov/iris.

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

USEPA, 2024c. PFAS National Primary Drinking Water Regulation Rulemaking, Final Rule. https://www.epa.gov/system/files/documents/2024-04/pfas-npdwr_prepubfederalregisternotice_4.8.24.pdf.