Basic Course: Supplemental Topics - Health (g)

Quantitative Risk Assessment Data Needs: Toxicology

Toxicology provides information on the nature of the adverse effects that can be caused by the pollutant under consideration and the doses that cause the effect—that is, the toxic effects and the dose-response properties.

Effects can range from mild dermatitis to birth defects or cancer. In toxicology, noncancer effects are generally considered separately from cancer effects.

The specific dose-response terms that are used in the derivation of the human health criteria are as follows. 

For Cancer Effects

• Risk Specific Dose (RSD). The concentration in water that has a specific associated risk, such as a one-in-a-million extra risk for an adverse effect. This dose-response parameter is used for chemicals that have no safe dose and where the risk increases linearly as dose increases from no exposure to doses where the tumors have been experimentally observed in one or more toxicological studies. (See also, definition of CSF.)
• Point of Departure (POD). The lower confidence bound on the lowest experimental dose that showed an effect. The critical study used for all quantitative risk assessments has a Point of Departure. However, at present, the POD acronym is primarily used in deriving the AWQC for a type of cancer that does not show a linear response to dose.

For Noncancer Effects

• Reference Dose (RfD). RfD terminology is used for noncarcinogens. It is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to humans (including sensitive subgroups) that is likely to be without an appreciable risk of adverse effects over a lifetime.

Resource. Reference dose information for individual chemicals (i.e., risk values and complete toxicological assessments) can also be found on EPA’s Integrated Risk Information System (IRIS) database.

Resource. Reference doses for pesticides are included in Reregistration Eligibility Documents (REDs) that are available from the Office of Pesticide Programs (OPP) website.

Another consideration for toxicology can be uncertainty factors:

• Uncertainty Factors (UFs). UFs are applied to the POD in determining the RfD and the human health AWQC for a nonlinear carcinogen. UFs range from 1 to 3,000 depending on the nature of the data used in the assessment. The final UF value depends on whether the data are from human or animal studies, the duration of exposure in the study that supplies the POD, the type of POD used, and the completeness of the database that supports the POD.