Biomonitoring. Source: NIEHS

Introduction Bioaccumulation is a process resulting in the concentration of substances in living tissues. The term is used often in reference to such chemical contaminants that may do harm to organisms as chlorinated pesticides and heavy metals. Nonetheless, organisms do accumulate chemicals and minerals needed for their survival-this may be referred to as bioaccumulation. Many substances entering organisms are eventually eliminated in wastes; whereas such others as heavy metals and fat-soluble organic substances (for example, persistent organic pollutants or POPs) may remain in the body for long periods of time. The U.S. Environmental Protection Agency uses the term persistent, bioaccumulative and toxic pollutants (PBTs) to categorize substances that raise human health and environmental health concerns. Depending on their affinity for fatty tissue and the length of the body's exposure, PBTs may accumulate in high concentrations and may cause physiological problems. PBTs enter the organism through a variety of active and passive means, including respiration, food intake, and epidermal (or skin) contact.

Organismal - Compounds present in an organism's environment may concentrate in the body over time. For example, fish that swim frequently in contaminated water may build up pollutants in their fatty tissues. This type of accumulation, in organisms experiencing continuous environmental exposure, is linear.

Trophic transfer - Here, fat-soluble compounds pass from prey to predator. The more prey that are eaten, the greater the magnification of the compound as it travels up the food chain. Such organisms at the top of the food chain as humans or polar bears, can receive the highest concentrations. This process is sometimes referred to as biomagnification, since the concentration of pollutant in the predator is a large multiple of that measured in the prey.

Soil Bioaccumulation - PBTs that are dumped into surrounding environments from specific waste sites or that leak from specific factories are point-source pollutants. Often, the substances bind to soil particles and persist until they are removed through erosion, water percolation, or uptake by plants, microorganisms or animals in soils or sediments. This process includes both linear accumulation through abiotic processes, and biomagnification-as pollutant-laden bacteria and fungi are consumed by earthworms and other detritivores (or detritus eaters). Often, soil bioaccumulation is the initial source of PBT exposure for terrestrial organisms. Such methods as phytoremediation (using plants to remediate) and mycoremediation (using fungi to remediate) have proven useful in removing or even breaking down pollutants in soil, and could be increasingly important solutions in the future.

ExamplesChlorinated pesticides

DDT, a chlorinated pesticide used heavily on U.S. farms in the middle of the 20th century, was found to bioaccumulate through earthworms to organisms higher on the food chain. Various songbirds, waterbirds and birds of prey experienced drastic population decreases during the 1950s due to such severe reproductive problems as overly thin, breakable eggshells. These problems were associated with widespread spraying of DDT. Writer and biologist Rachel Carson affected U.S. policy and public perception regarding pesticides greatly by describing the devastating effects of DDT in her 1962 best-selling book, Silent Spring.

Heavy metals

A study of bioaccumulation in cyprinid fish of the Kor River in Iran found high levels of mercury in the fishes' muscles, liver, kidneys, brain, and gonadal tissue. One species was sampled in three river stretches: the Upper, Central, and Lower Kor, and the monitored fish proved to have significantly higher mercury contamination in the middle reach of the river where two cities (Shiraz, Marvdasht) produced industrial metals.

In Japan during the 1950s, strange cases of human neurological disorders and death began to appear in Minamata City and other industrial areas. The "Minamata Disease" was traced to fish and shellfish that contained high levels of methylmercury, and that had been eaten by local townspeople. The methylmercury was traced to the Chisso Corporation's Minamata factory, the largest Japanese producer of acetaldehyde (a chemical used in manufacturing plastics) during the 1950s. Methylmercury is formed during the production of acetaldehyde, and the factory had been discharging a large amount of methylmercury via wastewater directly into the Yatsushiro Sea. People who ate a lot of the contaminated fish over a long period of time were especially at risk for Minamata Disease.

Synthetic industrial compounds

Polychlorinated biphenyls, or PCBs, are synthetic chemical substances used in a large number of mechanical and electrical systems because of their high thermal and chemical resistance. They do not break down easily-remaining in the environment for long periods of time and that build up as inputs continue. Exposure to, and the consequent bioaccumulation of, PCBs has been associated with cancer in animals, as well as negative effects to the immune, reproductive, nervous and endocrine systems. When PCBs are disposed of through incineration, dioxins are formed-another type of bioaccumulated compound that has shown extremely disconcerting long-term effects on the human body. The result of PCB and dioxin bioaccumulation has revealed itself in the fat of polar bears and in human breast milk of Inuit women, sometimes at toxic levels.

Related terms: bioconcentration, biomagnification, biotransformation, biomonitoring

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