Total trihalomethanes (TTHMs)
Health & what it is
Trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) form when chlorine used to disinfect water reacts with naturally occurring organic material. They are a routine tradeoff of safe disinfection, not contamination from an outside source.
Long-term exposure above the EPA limit has been associated in some studies with increased cancer risk and, less consistently, with reproductive effects. Disinfection itself remains essential — the goal is to minimize byproducts, not stop disinfecting.
EPA’s maximum contaminant level (MCL) for total trihalomethanes is 80 parts per billion (ppb), measured as a running annual average.
Where it comes from
Formed inside the distribution system when chlorine reacts with organic matter; higher in surface-water systems.
How to reduce TTHMs at home
Look for a filter certified to NSF/ANSI 53 — Activated carbon.
- ✓Activated carbon (point-of-use or whole-house)
- ✓Reverse osmosis
- ✓Letting cold water run; avoiding boiling for long periods (TTHMs concentrate)
Frequently asked questions
What is TTHMs? +
A byproduct formed when chlorine disinfectant reacts with natural organic matter in water. Formed inside the distribution system when chlorine reacts with organic matter; higher in surface-water systems.
What is the EPA limit for TTHMs in drinking water? +
The EPA limit is 80 ppb (MCL). To remove it, look for a filter certified to NSF/ANSI 53.
What are the health effects of TTHMs? +
Trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) form when chlorine used to disinfect water reacts with naturally occurring organic material. They are a routine tradeoff of safe disinfection, not contamination from an outside source. Long-term exposure above the EPA limit has been associated in some studies with increased cancer risk and, less consistently, with reproductive effects. Disinfection itself remains essential — the goal is to minimize byproducts, not stop disinfecting. EPA’s maximum contaminant level (MCL) for total trihalomethanes is 80 parts per billion (ppb), measured as a running annual average.
How do you remove TTHMs from drinking water? +
Activated carbon (point-of-use or whole-house) Reverse osmosis Letting cold water run; avoiding boiling for long periods (TTHMs concentrate)
Is TTHMs in your water?
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