US Geological Survey

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.