What is a THM and how is it formed?

A THM, or trihalomethane, is a chemical compound that often occurs in drinking water as a result of the reaction of chlorine treatment for disinfection purposes and natural organic matter present in the water. Often formed in areas of heavy rainfall, especially after a drought or extended period where sediment and debris have not been washed from the earth, THMs are colorless, volatile and readily dissolve in water. The four common THMs are chloroform, dibromochloromethane, dichlorobromomethane, and bromoform. A new water quality regulation (DBPR-II, or Stage II of the Disinfection Byproduct Rule) requires our water to comply with a Total THM maximum contaminant level of 80 ug/l, based on a running annual average at each monitoring location. All locations within the City of Napa Water System are meeting the regulatory standard and have been in full compliance since April 2016.

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1. I’ve never heard of disinfection byproducts before, so why am I hearing about this now?
2. Why do we need to add chlorine to the water?
3. What is a THM and how is it formed?
4. How much is a ug/l?
5. What about other water systems?
6. What can I do to lower the THMs at my tap?
7. Are personal home filters effective?
8. Should I drink bottled water instead?