Disinfection byproducts · in your tap water
Cancer-linked disinfection byproducts
Trihalomethanes form when chlorine — used to disinfect drinking water — reacts with naturally-occurring organic matter in the source water (leaves, soil runoff, etc.). The four main TTHMs are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. EPA limits them at 80 ppb; EWG sets its health guideline at 0.15 ppb — over 500× stricter.
If your water tastes like a swimming pool, you're already noticing the disinfection treatment. The TTHMs you can't taste are the byproducts of that treatment.
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Every source below was checked to make sure the link works and backs the claim it's next to. These are the primary regulators and peer-reviewed studies — not our opinion.
The federal rules that set the TTHM (80 ppb) and HAA5 (60 ppb) drinking-water limits.
Lists the TTHM MCL of 0.080 mg/L (80 ppb) and HAA5 MCL of 0.060 mg/L (60 ppb).
EWG health guideline of 0.15 ppb; the four THMs and their cancer/fetal-harm endpoints.
Pooled analysis: higher bladder-cancer risk with long-term THM exposure (OR ~1.44 in men).
Estimates ~6,500 EU bladder-cancer cases a year attributable to THMs in tap water.
Recent meta-analysis: bladder-cancer risk rises with THM dose; weaker colorectal signal.
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Get my free water report Book a free checkupThis page is general water-quality education, not medical advice. Health classifications and limits are attributed to the EPA, EWG, IARC, ATSDR/CDC, WHO and the cited studies. Contaminant levels vary by water system and home — the only way to know what's in your water is to test it. Prepared by SwiftPro Heating, Cooling & Plumbing.