US Geological Survey

What are HBSLs?

Health-Based Screening Levels (HBSLs) are non-enforceable benchmark concentrations of unregulated contaminants in water that may be of potential concern for human health, if exceeded. HBSLs were developed by the USGS in collaboration with the U.S. Environmental Protection Agency (USEPA) and others using: (1) USEPA Office of Water methodologies for establishing drinking-water guidelines, and (2) the most recent, USEPA peer-reviewed, publicly available human-health toxicity information (Toccalino and others, 2003; 2006; Toccalino, 2007).

HBSLs are calculated for contaminants that are not regulated by the USEPA in drinking water under the Safe Drinking Water Act and therefore do not have Maximum Contaminant Levels. The USGS and its cooperators continue 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.

How are HBSLs Used?

Concentrations of contaminants in water are compared to human-health benchmarks in screening-level assessments to provide an initial perspective on the potential relevance of detected contaminants to human health and to help prioritize further investigations. Two human-health benchmarks are used in USGS screening-level assessments: U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Levels (MCLs) and USGS's Health-Based Screening Levels (HBSLs). Concentrations of regulated contaminants (those with MCLs) are compared to their MCLs and concentrations of unregulated contaminants (those without MCLs) are compared to their HBSLs, when available.   See "Guidance on Use of Benchmarks in Screening-Level Assessments" and SIR 2007-5106 for more information.

Comparisons of measured contaminant concentrations in water to MCLs and HBSLs are useful for local, State, and Federal water-resource managers and others charged with protecting and managing drinking-water resources. For example, these comparisons can indicate when measured concentrations may be of potential human-health concern and can provide an early indication of when contaminant concentrations in ambient water resources may warrant further study or monitoring.

What are the benefits of using HBSLs?

• The use of Health-Based Screening Levels (HBSLs) more than doubles the number of human-health benchmarks available for unregulated compounds measured by the National Water-Quality Assessment (NAWQA) Program. Of the 435 unregulated compounds in the HBSL database, 95 had human-health benchmarks before HBSL calculations and 221 have benchmarks after HBSL calculations. Benchmarks are not available for about half of the unregulated compounds in the database because of a lack of toxicity values.

• The most recent U.S. Environmental Protection Agency (USEPA) toxicity information is used to calculate HBSLs, therefore, HBSLs provide a mechanism for the timely incorporation of updated toxicity information in the interpretation of water-quality data.
• HBSLs can be used as planning tools to help prioritize contaminants that may merit further study or monitoring and to provide an early indication of contaminant concentrations of potential human-health concern in water resources.
• Because HBSLs supplement existing USEPA drinking-water standards and guidelines, they provide a basis for a more comprehensive evaluation of contaminant-occurrence data in a human-health context than by using USEPA benchmarks alone.

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

A U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level (MCL) is a legally enforceable standard that sets the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. 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. Health-Based Screening Levels (HBSLs) are not legally enforceable drinking-water standards or clean-up levels. Rather, HBSLs are non-enforceable guidelines based on health effects and default exposure assumptions and do not consider cost or technical limitations.

HBSLs were calculated using standard USEPA Office of Water methodologies for establishing drinking-water guideline values for the protection of human health. HBSLs are, therefore, equivalent to existing USEPA Lifetime Health Advisory (Lifetime HA) and Cancer Risk Concentration values (when they exist), except for compounds for which more recent toxicity information has become available.

HBSLs also are different from other commonly used guidelines such as USEPA's Regional Screening Levels (RSLs) for Chemical Contaminants at Superfund Sites. RSLs are risk-based screening levels calculated using the latest toxicity values, default exposure assumptions, and data about physical and chemical properties. RSLs for tapwater are computed differently from HBSLs in that they may utilize information for dermal and inhalation routes of exposure in addition to oral exposure. RSL computations include information about exposure frequency and duration and use toxicity values from USEPA as well as from external agencies.

How are HBSLs calculated?

Three U.S. Environmental Protection Agency (USEPA) Office of Water (OW) equations are used to calculate HBSLs for unregulated contaminants as determined by the USEPA cancer classification for each chemical. HBSLs are calculated using (1) standard USEPA OW equations for establishing drinking-water guideline values (Lifetime Health Advisory and Cancer Risk Concentration values) for the protection of human health and (2) the most current, USEPA peer-reviewed, publicly available human-health toxicity information. Complete information about the calculation of HBSLs is presented in SIR 2007-5106.

1.  Carcinogens
The OW equation for calculating Cancer Risk Concentration values (also called Risk-Specific Dose values) is used to calculate an HBSL range for contaminants with the following cancer classifications:  Group A (known), Group B (probable), known/likely, carcinogenic to humans, and likely to be carcinogenic to humans.

The equation used to calculate HBSLs for carcinogens is:

Where µg/L = micrograms per liter, kg = kilogram, wt = weight, risk level = 10^-6 to 10^-4 cancer risk range,

SF = oral cancer slope factor, and mg/kg/day = milligrams of chemical per kilogram of body weight per day.

2.  Possible Carcinogens
The Lifetime Health Advisory (Lifetime HA) approach for Group C carcinogens is used for contaminants with the following cancer classifications:  Group C (possible), suggestive evidence of carcinogenicity but not sufficient to assess human carcinogenic potential, and suggestive evidence of carcinogenic potential.

The equation used to calculate HBSLs for possible carcinogens is:

Where RfD = oral reference dose, RSC = Relative Source Contribution (defaults to 20 percent in the absence of other data), and RMF = Risk Management Factor (defaults to 10 in the absence of other data).

3.  Noncarcinogens
The Lifetime HA approach is used for contaminants with the following cancer classifications:  Group D (unclassifiable), Group E (evidence of non-carcinogenicity), cannot be determined, data are inadequate, inadequate information to assess carcinogenic potential, and not likely to be carcinogenic to humans.  The Lifetime HA approach is also used for contaminants that do not have a cancer classification, but have a reference dose (RfD) value available.  Lastly, the Lifetime HA approach is used for contaminants with a multiple narrative descriptor similar to:  likely to be carcinogenic to humans under high-dose conditions but not likely to be carcinogenic to humans under low-dose conditions because concentrations detected in the environment are typically low.

The equation used to calculate HBSLs for noncarcinogens is:

What hierarchy of toxicity information is used to calculate HBSLs?

Two types of toxicity information are used in the calculation of Health-Based Screening Levels (HBSLs): cancer classifications (as discussed in "How are HBSLs calculated?") and cancer and noncancer toxicity values.  The toxicity value for carcinogens is the oral Slope Factor (SF or Q1*) and the toxicity value for noncarcinogens and possible carcinogens is the chronic oral Reference Dose (RfD).

An oral SF is an upper bound, approximating a 95% confidence limit, on the increased cancer risk from a lifetime exposure to a contaminant. This estimate is generally reserved for use in the low-dose region of the dose-response relationship. If the model selected for extrapolation from dose-response data is the linearized multistage model, the SF value is also known as the Q1* (carcinogenic potency factor) value. Units for SF are (mg/kg/day)^-1.

An oral RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. Units for RfD are mg/kg/day.

The hierarchy of toxicity information used in HBSL development represents the current "state of the science" in the United States. "Acceptable" toxicity information for developing HBSLs meets four criteria: (1) United States information (limited to USEPA data); (2) internally (e.g., USEPA) or externally peer reviewed; (3) publicly available (so anyone has access to the information); and (4) most recently available.

The hierarchy of sources of cancer classifications and toxicity data to use in the development of HBSLs is the most recent of the five USEPA sources listed below. Use of this hierarchy has the advantage of providing a mechanism for the timely incorporation of updated toxicity information in the interpretation of water-quality data.

¹Data source abbreviations are provided in the detailed HBSL reports to indicate the sources of cancer classifications and toxicity values used to calculate HBSLs. See data output for detailed HBSL reports at "Search for HBSL values".

²Toxicity information from the following types of OPP TRR documents is used in HBSL calculations: Reregistration Eligibility Decision (RED), Interim Reregistration Eligibility Decision (IRED), Reports on FQPA Tolerance Reassessment Progress and [Interim] Risk Management Decisions (TRED), Risk Assessments in support of RED, IRED, or TRED documents, and human health assessment scoping documents. Note that RED documents are the only source of OPP toxicity information used by USEPA OW in the calculation of Lifetime Health Advisory and Cancer Risk Concentration values.

The USEPA data sources listed in the table above have changed somewhat since the publication of SIR 2007-5106. Toxicity information from the Reference Dose Tracking Report is no longer used in the calculation of HBSL values because those data no longer meet the criteria of being publicly available.

Exceptions to how HBSLs are calculated

When a Health-Based Screening Level (HBSL) differs from an existing U.S. Environmental Protection Agency (USEPA) Lifetime Health Advisory (Lifetime HA) value and the reason for the difference is not apparent (i.e., the same Office of Water (OW) toxicity value and equation are used for the HBSL and Lifetime HA), the USGS will consult with the USEPA OW to identify the reason for the discrepancy. The USEPA OW may have technical or policy reasons for modifying the equations for calculating Lifetime HA values for some compounds. The rationale for any exceptions to the HBSL methodology will be captured in the HBSL database.