Bioaccumulative contaminants persist in the environment and accumulate in living organisms and tend to increase up the food chain, with greatest concentrations in high-end predators. A number of metals, organometallics (metals bonded by an organic ligand), and organic chemicals (Persistent Bioaccumulative Toxic (PBT) Organic Pollutants) are known to bioaccumulate and pose a human health risk. Bioaccumulation is the degree of concentration of a chemical contaminant in an organism relative to the concentration of that contaminant in the organism's environment (such as water). The term also refers to the process by which the chemical enters the organism through dietary intake, epithelial tissue (tissue on the surface of the body), gills (for fish), or other sources. For organisms in aquatic ecosystems, chemical uptake directly from water (across cell membranes) and dietary uptake (eating smaller organisms that have accumulated the contaminant) are important routes of exposure. For terrestrial organisms, dietary uptake tends to be the most important exposure route. Biomagnification, also called the "food chain effect," is the process whereby the tissue concentrations of a chemical contaminant increase as it passes up the food chain through two or more trophic levels. Human exposure to bioaccumulative contaminants is usually through consumption of plants, meat, and fish, but drinking water and exposure to nonfood solids (soil, paint chips).