Toxicity end points based on body burdens

for chronic exposures from these data introduces varying levels of uncertainty

into the risk assessment.

If an NOAEL is available for a test species NOAELt, then the equivalent

NOAEL for a species of wildlife NOAELw can be calculated by using an

adjustment factor for differences in body size:

NOAELw = NOAELt (bwt/bww)1/3

(9)

where bwt and bww are body weights of the test species and the wildlife species,

respectively.

However, important differences between taxonomic groups may be over-

looked by this approach. For example, one of the test species for the effects of

2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) is the chicken (Opresko,

Sample, and Suter 1994). However, the chicken is often listed as one of the most

sensitive species to the effects of 2,3,7,8-TCDD-like compounds (USEPA 1994).

Therefore, use of the chicken NOAEL to estimate effects in wild birds may

overestimate the potential for risk. It is important for the assessor to consider the

taxonomic relationship between the test species and the one of concern.

To predict adverse effects, one must know at what point during the life of an

organism that exposure occurs. For example, for many species, exposure to

certain chemicals during gestation may produce severe adverse effects on

reproductive success.

Toxicity end points based on body burdens

Typical toxicity tests use concentrations of contaminant in an external

medium, such as water or sediment, as a surrogate for actual dose to the

organism. However, complex behavioral interactions between certain organisms

and environmental media (e.g., benthic invertebrates and sediment) can confound

approaches that attempt to predict exposure and adverse effects for sediment-

dwelling organisms. Studies that relate measured body burdens of contaminants

to toxic effects appear to have better predictive ability for some compounds

(McCarty and Mackay 1991). Databases such as ERED (http:// www. army.

mil/el/ered) that have been developed by USACE and others relate measured

body burdens of chemicals to adverse effects. Comparison of measured body

burdens in the field to levels known to produce adverse effects in similar

organisms should decrease the uncertainty associated with estimates of risk to

these receptors.

Another theoretical approach, the narcosis model (van Wezel and

Opperhuizen 1995), shows promise for accurately predicting effects of chemicals

that act as nonpolar narcotics. This approach predicts that, for fish with a lipid

content of about 5 percent, narcosis should be observed at concentrations of 2 to

8 mmol of narcotic chemicals per kg wet weight of animal. The range in

expected toxicity is ascribed, at least in part, to interspecific variation in the lipid

content of the animals. A more limited range in toxic body burdens is observed

when the toxicity end point is based on a concentration in lipid of 40 to

160 mmol of chemical per kg of lipid (van Wezel and Opperhuizen 1995).

Chapter 5 Uncertainty in Tier IV Risk Assessments