- Related Topics:
- dioxin
- DDT
- polychlorinated biphenyl
- toxin
- chlordane
Bioaccumulation
What is bioaccumulation?
What is an example of bioaccumulation affecting humans?
What is the difference between bioaccumulation and biomagnification?
What are persistent, bioaccumulative, and toxic (PBT) contaminants?
Bioaccumulation, the increase in the concentration of heavy metals or other toxic chemicals in a living organism’s fatty tissues and muscles over time due to the organism’s inability to break down and excrete these substances quickly enough. The gradual accumulation of these substances in the body can disrupt normal physiological functions, ultimately leading to disease, impaired health, and, in severe cases, death. Because such substances often persist in tissues and accumulate over time, their adverse effects may not become apparent immediately after the initial exposure.
Substances that bioaccumulate
Not all toxic contaminants bioaccumulate. Bioaccumulative substances are characterized by highly stable molecular structures, which make them resistant to degradation by sunlight, microorganisms, and common chemical reactions. As a result, they persist in soil and water for long periods. Most are also fat-soluble rather than water-soluble, causing them to accumulate in the fatty tissues and organs of living organisms. In addition, many of these chemicals undergo biomagnification. They enter the food chain through soil, water, or sediment and are then taken up by smaller organisms near the base of the food chain. Their concentrations then increase at successively higher trophic levels (levels of the food chain) as organisms consume contaminated prey.
- Bioaccumulation factor is the ratio of the concentration of a contaminant in an organism to its concentration in the surrounding environment, accounting for all potential exposure routes.
- Bioavailability is the extent to which a contaminant is absorbed by organisms following exposure through inhalation, dermal contact, or ingestion.
- Half-life is the time required for one-half of the original mass of a chemical to be degraded, transformed, or otherwise removed from a specific environmental medium under defined conditions.
- Octanol/water partition coefficient (Kow) is the ratio of a contaminant’s concentration in n-octanol—a nonpolar, nonaqueous solvent commonly used as a surrogate for lipids—to its concentration in water at equilibrium. It quantifies a molecule’s lipophilicity (fat solubility) relative to its hydrophilicity (water solubility) and is a key physicochemical parameter used to predict its potential for environmental bioaccumulation.
Major bioaccumulative substances on the U.S. Environmental Protection Agency’s priority list include polychlorinated biphenyls (PCBs), dioxins, organometallic compounds such as methylmercury and alkyl lead, and organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT), aldrin, hexachlorobenzene, and chlordane. These chemicals are classified as persistent, bioaccumulative, and toxic (PBT) contaminants because of their environmental persistence and propensity to accumulate in living organisms and because they cause adverse health effects. In 2023 an international consortium of researchers proposed that plastics should also be classified as PBT pollutants, citing their long-term environmental persistence, capacity to accumulate in biological tissues, and potential to induce damage at the genetic, molecular, cellular, and organ levels.
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Routes of exposure and fate within living systems
The uptake of these contaminants can occur in the atmosphere, in fresh water or salt water, or on land. Plants may take in contaminated water or pollutants in the soil through their roots. Animals may inhale the chemicals, absorb them through their skin, or consume them in contaminated food and water. The sources of bioaccumulative substances that end up in the environment range from agricultural runoff and industrial pollution to oil tanker spills, vehicle exhaust, and landfill leakage.
Within living systems, bioaccumulation is heavily influenced by the physiological, anatomical, and biochemical properties of living organisms. These specific biological features influence the rapid uptake, transport, and long-term storage of the substances. In aquatic organisms such as fish, contaminants are absorbed mainly through the gills, whose large surface area and countercurrent flow system enable rapid diffusion of waterborne chemicals, and through the digestive system, which continuously transfers ingested chemicals into the bloodstream. Once inside the body, lipophilic contaminants bind strongly to blood plasma proteins and red blood cells, reducing their biotransformation, prolonging their residence time, and promoting storage in lipid-rich tissues, which they reach via blood circulation. In contrast, heavy metals are sequestered by proteins such as metallothioneins or accumulate in bony tissues. Bioaccumulation is further influenced by body size, as larger fish generally have higher lipid content and exhibit greater bioconcentration than smaller individuals.
Effects of bioaccumulation: Examples from history
Bioaccumulation and biomagnification were the processes behind the widespread ecological damage caused by persistent pesticides such as DDT—a crisis that inspired American biologist and author Rachel Carson to write her seminal 1962 book, Silent Spring. In the book, Carson detailed the effects of DDT on organisms and ecosystems. DDT bioaccumulated in organisms and was passed from organisms lower on the food chain, such as fish, to those higher on the food chain, such as eagles. One effect of this biomagnification was the thinning of eggshells of many species of birds, especially those that ate fish, causing parent birds to inadvertently crush their own eggs during incubation and leading to a population decline in many species. In the 1970s DDT was banned in many countries.
Bioaccumulation is a broad term describing the uptake, retention, and sequestration of organic and inorganic contaminants by an organism from its environment through all possible exposure pathways. It represents the cumulative net accumulation of contaminants resulting from respiratory absorption, direct physical contact, and dietary intake, reflecting how contaminant concentrations increase within the organism over time. Bioconcentration, by contrast, is a more specific process that refers exclusively to the uptake of contaminants through non-dietary pathways, primarily via respiration (from water in aquatic organisms and from air in terrestrial organisms).
Biomagnification, also called bioamplification, refers exclusively to the progressive increase in foodborne contaminants through dietary uptake and assimilation in organisms at successively higher trophic levels. Unlike bioaccumulation and bioconcentration, which describe contaminant accumulation within an individual organism relative to its environment, biomagnification is an ecosystem-level process in which contaminant concentrations increase progressively at successive trophic levels as they are transferred through the food web.
A well-known example of bioaccumulation in humans occurred near Minamata Bay in Japan. A chemical plant discharged wastewater containing methylmercury into the bay for more than 30 years, from 1932 to 1968. This methylmercury bioaccumulated in Minamata Bay’s fish and shellfish. Seafood was a large part of local inhabitants’ diets. In 1956 citizens began showing signs of mercury poisoning. Those with the condition, which was eventually named Minamata disease, had symptoms including tremors, headaches, seizures, vision and hearing problems, ataxia, dysarthria, memory loss, and cognitive decline. Babies born after mercury exposure in utero often had severe neurological damage and developmental disabilities. The tragedy led to more than 120 countries signing the Minamata Convention on Mercury, an international treaty that sought to prevent future incidents of mercury poisoning by regulating the handling and use of the metal.
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A more recent example of bioaccumulation from the ecosystem was described in a scientific study published in 2023. This study found that per- and polyfluoroalkyl substances (PFASs)—also known as “forever chemicals”—have been bioaccumulating in fish at different trophic levels along the Lake Huron–Lake Erie corridor, with subsequent biomagnification at higher trophic levels. Many PFASs are endocrine disruptors that can harm the reproductive system and interfere with fetal development. Some PFASs increase the risk of certain types of cancer, while others harm the immune system.
Addressing bioaccumulation
Bioaccumulation cannot be fully reversed and must therefore be prevented by minimizing exposure to toxic substances. Reducing exposure to persistent, bioaccumulative contaminants requires a combination of regulatory measures that prevent their release and remediation strategies that remove or neutralize existing pollution. Remediation approaches include phytoremediation using hyperaccumulator plants (which absorb and store contaminants in tissues such as leaves, which can then be discarded), microbial transformation of toxic metals into less bioavailable forms, and physical and chemical methods such as leaching, thermal treatment, and electro-remediation.
Monitoring is essential for evaluating contamination and the effectiveness of remediation efforts. Scientists analyze tissue samples from resident organisms for priority pollutants in an environment and compare contaminant concentrations between polluted and reference sites using either individual (grab) samples or composite samples prepared by combining tissues from multiple organisms.