Toxic substances are materials that are poisonous to living organisms. There are hundreds of thousands of artificial and natural toxic substances, also known as toxins, that are found in solid, liquid, or gaseous form. Toxic substances damage living tissues or organs by interfering with specific functions of cells, membranes, or organs. Some destroy cell membranes, others prevent important cell processes from occurring. Many cause cells to mutate, or make mistakes when they replicate themselves. Some of the most common and dangerous anthropogenic (human-made) toxic substances in our environment are chlorinated hydrocarbons, including DDT (dichlorodiphenyl-trichloroethane) and polychlorinated biphenyl (PCB). Many of these are produced by pesticide manufacturers and other chemical industries specifically because of their ability to kill pests. Petroleum products, produced and used in oil refining, plastics manufacturing, industrial solvents, and household cleaning agents, are also widespread and highly toxic agents. Heavy metals, including cadmium, chromium...