The route of exposure is an important consideration in the determination of toxicity for PAHs, especially for aquatic species. In the aquatic environment, species exhibit a variety of feeding modes, including sediment ingestion (selective and nonselective), detritus feeding, predation, suspension feeding, and filter feeding. Each of these modes may have a dramatic impact on the degree that the organism is exposed to contaminants and final bioaccumulation values. There are several studies that demonstrate the importance of the mode of feeding by invertebrates in relation to the degree ofbioaccumulation. An additional consideration is the physical-chemical properties for PAHs. Toxicants, such as PAHs, usually exist in all phases (water, sediment, air, tissue) in various proportions, depending on their fugacity (f), which is a property that determines the degree to which a specific compound will occur in each phase. Several simulation models exist that use chemical, environmental, and organism-specific properties (e.g., f Kow, and rate of elimination) to predict the occurrence of individual contaminants in these various compartments. These models and experimental work have found that many contaminants with low hydrophobicity will be taken up predominately by aqueous exposure. This generally occurs when water is passed over the gills (ventilation). Other compounds that are more hydrophobic will be mostly accumulated from dietary sources. This is generally true because the concentration of the compound is orders of magnitude higher in food and sediment that is ingested compared to the amount found in the water phase. Knowing the route of exposure is often important when assessing the likelihood of environmental contaminants to accumulate and cause adverse effects.
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