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Disinfection Byproducts
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What are trihalomethanes (THMs)?
Trihalomethanes, also known as THMs, are formed along with other disinfection byproducts when chlorine is added to drinking water during the water treatment process. The chlorine, in addition to destroying disease-causing organisms, reacts with naturally occurring organic matter (dissolved leaves and other vegetation) to produce THMs. THMs include the compounds Chloroform, Bromodichloromethane, Dibromochloromethane and Bromoform.
In our local water supplies, individual THMs are formed at low levels, generally in the range of a few parts per billion (ppb) up to 40 ppb. The total of all THMs may not exceed 80 ppb in accordance with USEPA and State Drinking Water Standards. The 80 ppb standard is based on a running average of quarterly samples collected throughout the water system. Currently, the average total THMs in the San Jose Water Company system is 35 to 45 ppb, well below the maximum allowable level.
The current drinking water standard for THMs was developed in response to studies indicating that certain THMs cause cancer in laboratory animals and may pose a similar risk in humans.
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