The Effects of Chlorination and Chlorine on Drinking Water

Chlorine has been the most widely used disinfectant in the U.S. for over 60 years (1) and is the primary disinfectant for drinking water in the world. There is concern in the scientific and regulatory community over the use of chlorine compounds to disinfect drinking water. This stems from the potential adverse health effects of the chemical by-products found in water as a result of their use.

When chlorine enters water it reacts chemically with any organic matter found in the water. There is always some organic matter in natural waters. By-products of this reaction include trihalomethanes (THMs) such as chloroform. The chemistry of the production of trihalomethanes is very complex and the health risks are not fully understood. We do know that chloroform is a cancer causing agent (1). Some researchers suggest that ozonation after treatment of chlorine with ammonia reduce THM levels in water (2). Chloramine used in conjunction with chlorine stops the formation of halomethanes and reduces tastes and odors associated with the formation of chlorophenolic compounds. Chloramine has been used as an alternative to chlorine but is not as effective against viruses or Giardia cysts (1).

The EPA has set a maximum contaminant level (MCL) for THMs in drinking water at 0.10 milligrams per liter. The EPA also sets the criteria for disinfection of our drinking water supplies.

The benefits of pretreatment of our water supply with chlorine far outweigh the risks associated with no treatment. or less effective treatment. Without treating our drinking water we would continually be infected by disease causing microorganisms. Historically, many deaths, sickness and misery are associated with untreated water. There are no feasible alternatives which do not carry health risks (3).


1. Drinking Water and Health, Vol. 2. National Academy of Sciences, Washington, DC. 1980.

2. Singer, Philip C. 1995. Formation and Control of Disinfection By-Products. ESE Notes, Fall 95 Vol. 30, No. 2. Dept. Env.Sci and Engineer., Univ. No. Carolina at Chapel Hill.

3. Safe Drinking Water Act: Amendments, Regulations, and Standards. Ed by E.J. Calabrese, et al. Lewis Publishers, Chelsea, MI. 1989. pp61-62.

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This page was prepared by T.L. Pedersen, June 1997, Revised by B.T. Johnson, Dec. 1997, UCD EXTOXNET FAQ Team.