While much public attention is given to identifying and measuring the toxicity of chemicals, the amount of exposure is equally important. Even very toxic substances can do no harm if we are not exposed to them in sufficient amounts. The first step in exposure assessment is to determine possible pathways by which toxic chemicals may reach humans.

The three most important pathways are inhalation (air transport), eating (food transport), and drinking (water transport). Pathways can involve many steps, and many substances have a number of different, often complex pathways.

In addition to mobility, the persistence of a substance is also important. Persistence refers to whether the substance remains intact long enough to be transported long distances.

In the case of exposure by food, a crucial property is the tendency toward bioconcentration. Bioconcentration (sometimes called bioaccumulation or biomagnification) is the tendency of a substance to accumulate in living tissue; it involves two steps. First, an organism takes in substance, but does not excrete or metabolize it.

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Then, a predator eats that organism and in the process ingests the substance already accumulated by its prey. This second step is common in fishes, shellfishes, carnivores such as eagles and cats, and other organisms that prey on smaller organisms so that the chemical is concentrated by passage through the food pyramid. Mollusks, such as scallops, are exceptionally prone to bioconcentration because they feed by filtering millions of tiny plankton from the water.