They also release heavy metals like mercury and lead, plus airborne pollutants, including sulphur dioxide and nitrogen oxides, which cause acid rain. Some toxic chemicals, such as dioxins and furans, build up in the environment and persist for a long time. Removing them can be difficult, if not impossible. These persistent chemicals come from many sources, including combustion and chemical processes used in industry. They can cause cancer, liver damage, reproductive problems, and birth defects in animals and humans.
Some persistent pollutants build up in the fatty tissue of plants, animals, and humans.
This process is called bioaccumulation. When plants or animals are eaten, the contaminants in their fat enter the body of the consumer. This process is called bioconcentration, because the consumer gets higher concentrations of contaminants through its food. Bioaccumulation of contaminants is especially high in the Arctic, where many species have a lot of fatty tissue. Airborne PAHs fall to the earth in Arctic snow after blowing on air currents and jet streams from industrial areas thousands of kilometres away.