When an animal ingests a chemical, the long-term fate of that chemical depends on two properties: how fat-soluble it is, and what its half-life is. A chemical that is water-soluble rather than fat-soluble will be excreted in urine and so does not have a chance to build up in the body. However, if a chemical is fat-soluble (lipophilic) it will remain in the body, especially if the animal does not have the required enzymes to break it down. Over time, if more of the chemical is added to the environment, the concentration of the chemical will build up in the fatty tissues. This process is known as bioaccumulation.

Chemicals that bioaccumulate are a particular problem in ecosystems with complex food webs. A predator might eat several prey animals, each contaminated with small concentrations of the contaminant. The chemical is absorbed through the gut and stored in the fatty tissues of the predator. If the chemical has a long half-life and does not naturally biodegrade, it can rapidly become concentrated in top predator animals in the ecosystem. This process is known as biomagnification and is the reason why tuna—a marine predator—can contain dangerous levels of methylmercury [48].

Two contaminants of concern that lend themselves to bioaccumulation and biomagnification are polychlorinated biphenyls (PCBs) and per- and polyfluoroalkyl substances (PFAS). PCBs were originally used in electronic components, particularly transformers, before being banned in 1977 due to their harmful effects on the liver, lungs, thyroid, and other organs. However, due to their long half-life, PCBs continue to cycle in the environment. Because they are fat-soluble, they bioaccumulate in bottom-feeders which ingest the PCB-laden sediment. PCBs then undergo biomagnification as they move up the food-chain: PCB levels in aquatic organisms can be up to a million times greater than those found in the aquatic environment [49].

PFAS are used in coatings to make products resistant to grease, oil, heat, stains, and water. They are found in a variety of products, including flame retardants, furniture, food packaging, adhesives, and heat-resistant nonstick cookware. Because PFAS encompass such a wide suite of chemicals, it is hard to pin down the exact health impacts of the group as a whole. However, CDC studies indicate that PFAS can cause damage to the liver and the endocrine and immune systems and are also carcinogenic. Like PCBs, PFAS are very long-lived and fat-soluble, making them prime candidates for biomagnification in the environment [50].

[48] J. Wiener, et al., "Ecotoxicology of mercury," in Handbook of ecotoxicology, 2nd ed., D. Hoffman, et al., eds., Boca Raton, FL: Lewis Publishers, 2003, pp. 409-463.

[49] Agency for Toxic Substances and Disease Registry, "Polychlorinated Biphenyls (PCBs) Toxicity," May 14, 2014. [Online]. Available: https://www.atsdr.cdc.gov/csem/polychlorinated-biphenyls/where_found.html. [Accessed June 6, 2022].

[50] Centers for Disease Control and Prevention (CDC), "Per- and Polyfluorinated Substances (PFAS) Factsheet," May 2, 2022. [Online]. Available: https://www.cdc.gov/biomonitoring/PFAS_FactSheet.html#:~:text=Many%20PFAS%2C%20including%20perfluorooctane%20sulfonic,bioaccumulate)%20in%20fish%20and%20wildlife. [Accessed June 6, 2022].