The potential of PAHs to cause impairment to the immune system has been demonstrated for many species. These studies generally examine specific (e.g., antibody response) and nonspecific (e.g., T-cells, phagocytic activity, secondary plaque-forming cells, and B-cell-mediated immunity) components of the immune system. Very few studies examine whole organism responses to immunolo-gical impairment. This would be accomplished in a controlled experiment by dosing a species with a range of PAH concentrations, then exposing them to a known pathogen to test for the in vivo response.

There are only a handful of studies examining immu-nocompromise in fish exposed to PAHs and even fewer for invertebrates. Some studies have documented immu-nological responses at very low concentrations of PAHs. For example, one study found alterations to biochemical and molecular processes related to the immune system in fish and noted an LOEC of 17 ngml~ for rainbow trout exposed to creosote in water. When only the PAH fraction was considered, which is likely the toxic component, the effect concentration was 0.61 ng of total PAH per ml. This LOEC is a concentration that is often observed in urban waterways.

Vertebrates in general appear to exhibit immunologi-cal responses at very low PAH exposure concentrations. One article reports an effective dose for a 50% response (ED50) based on immunosuppression in deer mice in the range of0.026-0.14 mg/g organism/day for three different PAH compounds (methylcholanthrene, dibenz[a]anthra-cene, and dimethylbenz[a]anthracene).

Only a handful of studies have examined immunotoxi-city for invertebrates exposed to PAHs. One notable study found severe immunological alterations in oysters exposed to oil from a spill. The authors also suggested that environmental stress (e.g., altered temperature and salinity) may exacerbate the immunological response and should always be considered in such an assessment.

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