Due to it's relatively low cost, Chlorine is the most common disinfection agent used by  municipalities to ensure potable water is free of bacteria and viruses. This doent mean the water is "safe or "healthy" for use by unsuspecting or uninformed consumers. 

The use of chlorine for disinfection of drinking water, produces low levels of various by-products; with the trihalomethanes and haloacetic acids being the most common. Trihalomethanes and haloacetic acids have been identified as carcinogenic in laboratory studies.