The quality of municipal drinking water supplies is regulated under the Safe Drinking Water Act, passed by Congress in 1974. Under this law, the U.S. Environmental Protection Agency sets Maximum Contaminant Levels (MCLs) for inorganic chemicals such as arsenic, cadmium and mercury and synthetic organic chemicals such as benzene (an industrial chemical), PCBs, atrazine (an herbicide). Municipal drinking water supplies cannot contain any microorganisms which may originate in fecal matter (e.g. Escherichia coli , a species of bacteria found in the guts of humans and other warm blooded animals) because fecal contamination may carry pathogens and spread disease. Recent concerns relating to drinking water include the presence of protozoan pathogens (e.g. Giardia) and the creation of cancer-causing chemicals (trihalomethanes, THMs) when drinking water is chlorinated.