The lethal response

In general, when aquatic organisms are exposed to waterborne PAHs, acute (short-term, e.g., <96 h) lethal values (e.g., the LC50) occur in the 100-5000 ngml~ range for many fish and invertebrates. The LC50 is a measure of a compound's toxicity that is based on the water or sediment/soil concentration at which half of the individuals die. Ideally, the LC50, or any such measure of toxicity, should be determined at steady state (i.e., no change over time); however, this is infrequently done necessitating the designation of time (e.g., 96 h LC50). As mentioned above, all PAHs exhibit the same lethal toxicity when expressed as a tissue concentration. Most organic compounds that occur at this tissue concentration cause lethality. Because of this commonality in tissue concentrations causing lethality, the observed variability in PAH toxicity based on water, sediment/soil, or prey concentrations as the exposure metric is solely due to differences in bioaccumulation potential and the organism's ability to metabolize these compounds.

The response concentration for tissue described above is designated as the lethal residue (tissue concentration) that causes mortality in 50% of the organisms (LR50) and is expressed as mgg1 or pmolg-1. The LR50 is considered the 'acquired' dose or the amount found in tissue that is correlated to the response. This is distinguished from the administered dose which is generally the amount of a toxicant given at one or several time periods. This is termed lethal dose (LD50) and is expressed as d~ or just mgg1 (mg or mmol administered/g body weight (bw)). This is an important distinction because the LR50 is the final concentration associated with the response and the LD50 is the amount given, which can be very different compared to the actual tissue concentration causing the response. Once a compound is administered to an organism it may be metabolized, excreted, or depurated through passive diffusion. Expressing toxicity as the LR50 is often more informative for interspecies comparisons and toxicity evaluation of organisms in the field because of the similarity in toxic potential and concentration among compounds. The LR50 is generally more common for invertebrates and much less so for vertebrates.

Most of the lethal toxicity data for vertebrates are based on administered dose (LD50). In general, LD50 values are determined for mammals and wildlife such as birds, reptiles, and amphibians by a variety of methods such as dietary uptake, injection, and gavage (introduction by tube to the stomach). Only a few studies have established LD50 values for laboratory animals based on oral exposure to PAHs. Values for the LD50 range from 50 mg/gbw for BaP to 2000 mg/gbw for fluoranthene. Even fewer LR50 values are available because it is difficult to achieve a lethal tissue concentration in vertebrates from dietary uptake and only a few researchers measure tissue concentrations in experimental animals. For vertebrates in the field, mortality from PAH exposure is not likely to occur because of the high levels needed to achieve the lethal tissue concentration; however, mortality from secondary effects such as oiled fur in marine mammals, suffocation, tumor formation, or lung damage from inhalation is possible. Acute mortality is, however, very possible for invertebrates and those species with inefficient biotransformation of PAHs.

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