Common Biological Responses

Biological effects of exposure to PAHs are diverse. These compounds are known to affect the immune system, impair growth and reproduction, cause tumor formation, and of course lead to death if concentrations are high. Some of the known responses are discussed below. Toxicity values are generally highly variable among species and compounds. This is primarily a result of the inherent toxicity of compounds, species susceptibility, the length of time for exposure, and the uptake and elimination kinetics (toxicokinetics) found among species. A brief general description of the observed toxicity values are presented here, with additional detail for the most commonly reported responses. Most studies on PAHs examine responses to water concentrations, dietary input, or sediment/soil exposure. It appears that the majority of toxicity information on PAHs has been generated for aquatic invertebrates and fish; therefore, most of the specific information and examples are from these studies. Where appropriate, information from studies on terrestrial invertebrates, amphibians, reptiles, birds, and mammals are included, in addition to a general section, for some of these groups, that is presented below.

As mentioned above, hydrophobicity for PAHs ranges about 10 000 fold, which is also generally true for the LC50 toxicity values when water concentrations for a given species are considered. In general, uptake clearance (ki; ml waterg~ h~ ) for water exposure of PAHs increases with increasing Kow while the rate of elimination (k2; h_1) decreases with increasing Kow. The bioconcentration factor (BCF) at steady state (a condition where uptake and elimination are balanced) can be determined by the equation k1/k2. In combination, these opposing trends will lead to large differences in the BCF and toxicity values over Kow. Assessment of bioaccumulation and toxicity based on dietary uptake (sediment or prey) is more complicated and may not show the same range over Kow as was shown for water exposure. The route of uptake generally has no effect on the rate of elimination (k2), which is mostly a function of Kow and the rate of biotransformation.

The rate of elimination (k2) is a parameter that can provide important information about bioaccumulation and toxicity. The half-life for a toxicant in tissue and the time it takes the tissue concentration to reach steady state are both determined solely by k2. The equation for half-life for any toxicant in tissue is determined with the equation 0.693/k2, which also equals the time it takes to reach 50% of steady state. The time to 'complete' steady state (actually «95%) is defined as 2.99/k2, which is characterized by a constant and maximum tissue concentration and constant rate of toxicity (e.g., unchanging LC50).

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