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Bioaccumulation means an increase in the concentration of a chemical in a biological organism over time, compared to the chemical's concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted. Understanding the dynamic process of bioaccumulation is very important in protecting human beings and other organisms from the adverse effects of chemical exposure, and it has become a critical consideration in the regulation of chemicals.

Bio-concentration is the specific bioaccumulation process by which the concentration of a chemical in an organism becomes higher than its concentration in the air or water around the organism. Although the process is the same for both natural and manmade chemicals, the term bio-concentration usually refers to chemicals foreign to the organism. For fish and other aquatic animals, bio-concentration after uptake through the gills (or sometimes the skin) is usually the most important bioaccumulation process.

Biomagnification describes a process that results in the accumulation of a chemical in an organism at higher levels than are found in its food. It occurs when a chemical becomes more and more concentrated as it moves up through a food chain -- the dietary linkages between single-celled plants and increasingly larger animal species. Biomagnification is illustrated by a study of DDT, which showed that where soil levels were 10 parts per million (ppm), DDT reached a concentration of 141 ppm in earthworms and 444 ppm in robins. Through biomagnification, the concentration of a chemical in the animal at the top of the food chain may be high enough to cause death or adverse effects on behavior, reproduction, or disease resistance and thus endanger that species, even when levels in the water, air, or soil are low. Fortunately, bioaccumulation does not always result in biomagnification.

Bioaccumulation is a normal and essential process for the growth and nurturing of organisms. All animals, including humans, daily bioaccumulate many vital nutrients, such as vitamins A,D and K, trace minerals, and essential fats and amino acids. What concerns toxicologists is the bioaccumulation of substances to levels in the body that can cause harm. Because bioaccumulation is the net result of the interaction of uptake, storage and elimination of a chemical, these parts of the process will be examined further.

Bioaccumulation begins when a chemical passes from the environment into an organism's cells. Scientists have learned that chemicals tend to move, or diffuse, passively from a place of high concentration to one of low concentration. The force or pressure for diffusion is called the chemical potential, and it works to move a chemical from outside to inside an organism. Factors influencing the chemical potential positively lead to higher rate of bioaccumultion. For example, a chemical capable of being dissolved in water has least potential of bioaccumulation, while the same for any fat loving chemical is higher. However, there are a few exceptions like mercury, which bind tightly to specific sites within the body.

Another factor affecting bioaccumulation is whether an organism can break down and/or excrete a chemical. The biological breakdown of chemicals is termed metabolism. This ability varies among individual organisms and species and also depends on characteristics of the chemical itself. Chemicals that dissolve readily in fat but not in water tend to be more slowly eliminated by the body and thus have a greater potential to accumulate.

When a chemical enters the cells of an organism, it is distributed and then excreted, stored or metabolized. Excretion, storage and metabolism decrease the concentration of the chemical inside the organism, and consequently increase the potential of the chemical in the outer environment to move into the organism. A constant environmental exposure to a chemical, therefore maintain a state of dynamic equilibrium for the amount of a chemical accumulated inside the organism, and the amount leaving. Detailed explanation of the situation suggests an environmental chemical will at first move into an organism more rapidly than it is stored, degraded, and excreted, until the concentration of the chemical inside the organism reaches a equilibrium with the concentration of the chemical outside the organism. Eventually, the amount of intake of the chemical will be equal to the amount excreted or degraded. Therefore, a constant level of chemical will be maintained within the body.

As the environmental concentration of the chemical increases, the amount inside the organism increases until it reaches a new equilibrium. Exposure to large amounts of a chemical for a long period of time, however, may overwhelm the equilibrium (for example, overflowing the tub) potentially causing harmful effects. On the other hand, when the concentration in the environment decreases, the amount inside the organism starts decline. Consequently when the organism move to a clean environment, the exposure ceases, and the chemical eventually is eliminated from the body.

Bioaccumulation is a major concern of toxicologists as toxicity of elements depends on the amount and nature of accumulated elements in the body of living organisms. The extent of bioaccumulation depends on the concentration of a chemical in the environment, the amount of chemical coming into an organism from the diet, water, or air, and the time it takes for the organism to acquire the chemical and then excrete, store, and/or degrade it. However, bioaccumulation is a normal process that can result in injury to an organism only when the equilibrium between exposure and bioaccumulation is overwhelmed, relative to the harmfulness of the chemical.

But, the accumulated materials, when start traveling through food chain and get biomagnified, may become a matter of concern in respect of its toxicity. Sometimes we consider the mammalian milk and even mothers milk as an important rout of transmission of the bioaccumulated materials through food chain. Bioaccumulation varies between individual organisms as well as between species. Large, fat, long-lived individuals or species with low rates of metabolism or excretion of a chemical will bioaccumulate more than small, thin, short-lived organisms. Thus, an old lake trout may bioaccumulate much more than a young bluegill in the same lake. Other factors like exposure time, fat content of the body etc. may also influence the bioaccumulation.