Toxicity End Points Based on Body Burdens for Ecological Receptors

Toxicity End Points Based on Body Burdens for

Ecological Receptors

Toxicity end points based on body burdens for ecological receptors received a

score of 3 for high magnitude of uncertainty. Data are available for nonpolar

organics, but fewer data are available for metals.

The relationship between toxicity end points and body burdens is complicated

when the contaminant is metabolized by the organism. Some organisms can

metabolize contaminants to more or less toxic forms, but prediction of how a

particular chemical will be metabolized by a particular organism is difficult.

Also, since most metabolites are not detected by conventional analytical

methods, exposure of animals may be underestimated.

Dose-Response Models for Human Receptors

Dose-response models for human receptors received a score of 3 for high

magnitude of uncertainty. This uncertainty encompasses both the shape of the

dose-response curve as well as the uncertainties encountered when extrapolating

from animal to human studies. A national scientific commission recently

reviewed problems associated with such extrapolations (Presidential/

Congressional Commission on Risk Assessment and Risk Management 1997).

To extrapolate from animal toxicity data to an "acceptable" concentration or dose

in humans, USEPA applies uncertainty factors that range from 10 to 10,000. The

Presidential/Congressional Commission on Risk Assessment and Risk

Management made practical recommendations for future animal studies to

minimize these uncertainties.

Toxicity of Complex Mixtures

The toxicity of complex mixtures is not well understood for humans or

ecological receptors. Approaches are being developed to address this uncertainty

(Swartz et al. 1995; van Wezel and Opperhuizen 1995; and Giesy, Ludwig, and

Tillitt 1994) and should be pursued within the dredged material management

program. Prediction of human health impacts from chemical mixtures is equally

if not more problematic, because there is such a small database for chemical

mixture toxicity data for such mixtures as tobacco smoke and diesel exhaust.

Alternatively, researchers often must rely on models (e.g., toxicity equivalency

factors for dioxinlike compounds). To enlarge the toxicity database for chemical

mixtures, coordinated research efforts are needed among epidemiologists and

toxicologists.

Chapter 6 Preliminary Ranking and Recommendations