Physiology and biology of receptors

Other empirical approaches predict the probability that mixtures of sediment-

associated contaminants will result in toxicity to benthic invertebrates. For

example, the Sum-PAH model (Swartz et al. 1995) established a regression

relationship between the Kow of individual PAHs and the concentration in

sediment that results in acute toxicity to benthic invertebrates in spiked sediment

bioassays (10-day LC50 = 1 toxic unit). The regression relationship is used to

predict the LC50 of PAHs that have not been tested in bioassays. In this approach,

the concentration of each PAH in a field-collected sediment is expressed as a

fraction of its LC50. A large data set that contains matched information on

concentration of PAHs in field-collected sediment and results of toxicity tests

was used to develop a predictive relationship. The Sum-PAH approach estimates

the probability that a sediment will be acutely toxic on the basis of the sum of the

PAH toxic units of individual compounds. Results of the model are more

uncertain for sediments with low levels of PAHs (sum of toxic units < 0.25),

where 79 percent (n = 379) of the samples in the data set were predicted to be

nontoxic, but only 35 percent (n = 292) were observed to be nontoxic.

Other empirical approaches that predict the probability of toxicity to benthic

invertebrates from the concentration of chemicals in field-collected sediment

show promise in their ability to screen sediments for potential toxicity of

mixtures of contaminants (e.g., Long, Field, and McDonald 1998), but

uncertainties remain.

Physiology and biology of receptors

The exposure of an organism to a persistent environmental contaminant, such

as PCBs, can be assessed directly by measurement of body burdens of these

chemicals. However, the exposure to some chemicals, such as PAHs, cannot

always be assessed directly by measuring their concentration in tissue because

some organisms can rapidly convert PAHs to a variety of metabolites (Melancon

et al. 1992). The metabolites, which are usually more water soluble than the

parent compound, are not typically detected in environmental samples by

standard analytical techniques. Exposure of organisms to chemicals that are

rapidly metabolized may be underestimated if only the concentration of the

parent compound is examined.

The metabolites may be either more or less toxic than the parent compound.

For example, some PAHs undergo biotransformation into metabolites that are

mutagenic and carcinogenic (Thakker et al. 1976). The metabolites of these

chemicals can accumulate to high levels, especially in some aquatic organisms

(James 1989). If the metabolites are more toxic than the parent compound,

inability to detect the presence of metabolites in exposed organisms may result in

an underestimate of risk.

Risk Characterization

The goal of the risk characterization is to provide a quantitative estimate of

potential risk. For an ecological risk assessment, risk estimates for each chemical

Chapter 5 Uncertainty in Tier IV Risk Assessments