Jon J. Calomiris, Water Research Program Manager at the United States Air Force Research Laboratory, and Keith A. Christman, Director, Disinfection and Government Relations at the Chlorine Chemistry Council, collaborated on this answer.

While quenching your thirst with a glass of tap water, enjoying your morning shower or swimming in a pool, you most likely are, at one time or another, aware of the chlorine used to disinfect your municipal water. Although its distinctive aroma may be unpleasant to some, it is an indication that your water supply is being adequately treated to stave off harmful or deadly microorganisms.

Chlorine effectively kills a large variety of microbial waterborne pathogens, including those that can cause typhoid fever, dysentery, cholera and Legionnaires' disease. Chlorine is widely credited with virtually eliminating outbreaks of waterborne disease in the United States and other developed countries. And Life magazine recently cited the filtration of drinking water and use of chlorine as "probably the most significant public health advance of the millennium."

Image: Chlorine Chemistry Council TYPHOID FEVER has been virtually eliminated through the chlorination of water.

Health officials began treating drinking water with chlorine in 1908. Previously, typhoid fever had killed about 25 out of 100,000 people in the U.S. annually, a death rate close to that now associated with automobile accidents. Today, typhoid fever has been virtually eliminated.

Chlorine is currently employed by over 98 percent of all U.S. water utilities that disinfect drinking water. It has proved to be a powerful barrier in restricting pathogens from reaching your faucet and making you ill. Chlorine and chlorine-based compounds are the only disinfectants that can efficiently kill microorganisms during water treatment, and maintain the quality of the water as it flows from the treatment plant to the consumer's tap.

Although chlorine's value has been known for nearly a century, the mechanism by which the compound kills or inactivates microorganisms is not clearly understood. The bulk of chlorine disinfection research, conducted from the 1940s to the 1970s, focused on bacteria. Though limited, this work gave rise to some speculation. Researchers postulated that chlorine, which exists in water as hypochlorite and hypochlorous acid, reacts with biomolecules in the bacterial cell to destroy the organism.

Further work led to the so-called "multiple hit" theory of chlorine inactivation. It asserted that bacterial death probably results from chlorine attacking a variety of bacterial molecules or targets, including enzymes, nucleic acids and membrane lipids.

Early research efforts focused on how chlorine attacks enzymes. The disinfectant was able to inactivate extracts of various enzymes because it is highly reactive with sulfur-containing and aromatic amino acids. But it had no effect on cytoplasmic enzymes, suggesting that it might not reach biomolecules within the bacterium. Thus, researchers redirected their attention to the molecules on the surface of the bacterial cell.

A new hypothesis proclaimed that perhaps chlorine acted by attacking the bacterial cell wall. Proponents of this idea suggested that chlorine exposure might destroy the cell wall--by altering it physically, chemically and biochemically--and so terminate the cell's vital functions, killing the microorganism.

A possible sequence of events during chlorination would be:

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