Each day, animals take hundreds of chemicals into their bodies by eating, breathing, drinking, and absorbing through the skin. Some of these chemicals are beneficial to the animals, some are not. Once the chemicals have entered the body, they are either processed for energy, stored in the animals' organs and tissues, or eliminated as waste.

Bioaccumulation is a process by which stored chemicals gradually build up in the tissues of living things. This process is a natural part of the growth of animals. All animals, including humans, bioaccumulate vitamins and minerals that they need to stay healthy. However, the process can become harmful if the bioaccumulated chemicals are toxic, or if the levels of the chemicals become too high.

Several factors affect bioaccumulation. One factor is whether the chemical is soluble in water. Have you ever tried to mix oil and water? They don't mix easily. When two substances tend to stay separate after mixing, we call them insoluble. Oil is insoluble in water, and water is insoluble in oil.

Chemicals that are insoluble in water will leave the body more slowly because bodily fluids like blood and urine have a high water content. However, chemicals that are insoluble in water can be soluble in fat. That means that they are easily stored in the animal's fatty tissue. As animals eat more contaminated food, the chemicals collect in their fat and do not leave. Low levels of chemicals which would normally be harmless to animals can be harmful or deadly at the bioaccumulated levels.

A second factor affecting bioaccumulation is the animal's metabolism. Metabolism is the biological breakdown of chemicals that goes on in all living things. Different species have differing abilities to break down chemicals. For example, horses can easily metabolize grass, but humans cannot. Individual animals within a species can also have differing abilities to metabolize chemicals. For example, some people are unable to metabolize dairy products and are allergic to milk, while other people can easily metabolize milk. Differences in metabolism mean that a chemical which has little effect on one species or individual can be harmful for another.

Biomagnification is a process related to bioaccumulation. Biomagnification describes the process in which the concentration of a substance increases as it moves up the food chain.

Read more about biomagnification.