Disinfection of water supplies in the United States is almost always accomplished by using chlorine. Disinfection with chlorine, combined with the other surface water treatment processes has greatly reduced the incidence of water-borne disease among humans in the United States. It is this proven record and the familiarity with chlorine that makes chlorine the disinfecting agent used at most systems. There are three basic reasons that chlorine is usually the disinfectant of choice.

1. Chlorine is the most cost-effective disinfectant available considering its disinfecting power.
2. Chlorine is easily obtained through a variety of sources.
3. Chlorine produces a disinfecting residual. However, it should also be said that there are also two clear disadvantages or drawbacks to the use of chlorine.
4. Chlorine must be used and handled very carefully to prevent serious hazards to operators and the public.
5. Chlorine can sometimes form trihalomethanes (THMs) in water supples. Concentrations of THMs above the maximum contaminant levels (MCLs) are suspected of causing cancer.