Britannica Sources used in this answer

How can chlorination produce chloroform and other trihalomethanes in water?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

During the process of chlorination, which is used to disinfect public water supplies, chlorine can react with naturally occurring organic compounds present in the water.12

This chemical reaction leads to the formation of chloroform and other related by-products known as trihalomethanes. Trihalomethanes are derivatives of methane that contain three halogen atoms per molecule. While these by-products are considered potentially harmful, the risk is minimized in modern water treatment facilities by applying chlorine only after other treatment steps, such as coagulation, sedimentation, and filtration, have been completed.32

To further reduce the formation of these by-products, some water systems have adopted chloramination, a method that uses a combination of chlorine and ammonia for disinfection.2