Whether the presence of a particular contaminant in the environment is of potential concern depends on the toxicity and the levels found. It also depends on how the contaminant behaves in the environment and how long its life-time is.

Most chemicals break down quickly in the environment. Some can, however, persist over days, years or even decades. This way, they can accumulate to over time to levels that may be harmful to the environment and organisms.

Persistent contaminants often have the potential to accumulate in living organisms, such as plants, wildlife and humans. They can also cycle through the food web and concentrate through the food chain.

Different contaminants have different toxic potencies and can impact upon different biological functions or parts of the environment. There is relatively little known about the combined impacts of many contaminants and it is difficult to assess what risks they pose.

To evaluate the risks associated with environmental contamination, it is important to understand how these chemicals behave and move in the environment, at what levels they are present, and how (and at what levels) they affect biological function.