City water is closely regulated for some health hazards and is very unlikely to contain dangerous biological contamination. It is, however, almost always treated with chlorine to reduce the microbial count. Chlorine combines with such organic compounds as natural methane gas (from decomposed material) in water, to form organochlorine compounds. The most common are the THM’s (trihalomethanes), such as chloroform. These are carcinogens. They also give water an unpleasant taste and odor.