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.
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,
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
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).
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
The equation used to calculate HBSLs for noncarcinogens is: