Humans reside in an environment where exposure to toxic chemicals occurs through inhalation, ingestion, or skin contact. These chemicals can be air and environmental pollutants, agricultural chemicals, organic solvents, or heavy metals. Air pollutants—mostly gases—cause several respiratory problems, such as hypoxia, lung cancer, or in extreme cases, death. Environmental pollutants like polychlorinated biphenyls are highly stable, not easily degradable, and persist in the environment. They enter the human body through bioaccumulation in the food chain, causing dermatologic or hepatic problems. Agricultural chemicals like organochlorine pesticides are persistent chemicals that bioaccumulate, particularly in aquatic ecosystems. Animal testing for chronic, long-term use of such chemicals shows results such as enhanced oncogenesis. Organic solvents like halogenated hydrocarbons are lipid soluble, and capable of diffusing through the blood-brain barrier, causing depression of the central nervous system. Chronic occupational exposure of workers to these chemicals can cause several types of cancers. Heavy metal poisoning through paint, food, and drinking water can affect the CNS and various other organs.