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Are there THMs in my drinking water? Why?

Yes. THMs are found to varying degrees in all chlorinated drinking water. They form when chlorine reacts with naturally-occurring organic matter in water. Chlorine is necessary for disinfection and required by regulations to protect public health. It has been used to disinfect water for almost a century due to its effectiveness at killing bacteria and viruses in water, reducing the incidence of intestinal illness and other health problems caused by waterborne pathogens.