Trihalomethanes in Drinking Water
About trihalomethanes
Trihalomethanes (TTHMs) are chemicals that form in drinking water when chlorine or chloramine reacts with natural organic matter, such as leaves and soil washed into a river. In an independent accredited lab test of a real Arlington, Virginia tap, total trihalomethanes measured 58 ppb; after the Sanavia Hydra reverse osmosis system, 2.79 ppb, a 95% cut. The group has four members: chloroform, bromodichloromethane, dibromochloromethane and bromoform.
Health effects
Long-term exposure to trihalomethanes is linked to bladder cancer and to harm to fetal growth and development. The EPA's legal limit for total trihalomethanes is 80 ppb, averaged over a year. EWG's health guideline is far stricter, at 0.15 ppb, which is why water can be fully legal and still sit hundreds of times above it.
In DC-metro tap water
Trihalomethanes form in the pipes after treatment, so they are highest far out in a distribution network and in warm months. Across the DC-area water systems on our map, total trihalomethanes are above EWG's guideline in nearly every one, from DC Water and WSSC Water to Fairfax Water. Check your own area with the zip code lookup.
How to remove it at the tap
Activated carbon and reverse osmosis both reduce trihalomethanes at the tap. A reverse osmosis system with carbon stages covers them along with the other disinfection byproducts and dissolved contaminants. Boiling is not a fix: it drives off some chloroform but concentrates other contaminants. The Arlington lab test measured the result on the Hydra: 58 ppb in, 2.79 ppb out.
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Where trihalomethanes show up locally
Local water reports where this contaminant class is documented in the utility's data or watershed. Open your area to see the specifics and look up your exact zip code.
Frequently asked
What are trihalomethanes in water?
Trihalomethanes are disinfection byproducts that form when the chlorine or chloramine used to treat water reacts with natural organic matter. The four are chloroform, bromodichloromethane, dibromochloromethane and bromoform, reported together as total trihalomethanes (TTHMs).
Are trihalomethanes in drinking water dangerous?
Long-term exposure to trihalomethanes is linked to bladder cancer and to harm during pregnancy, which is why the EPA limits them to 80 ppb. EWG's health guideline is 0.15 ppb, so most chlorinated supplies are legal but well above it.
Does reverse osmosis remove trihalomethanes?
Yes. A reverse osmosis system with carbon stages reduces trihalomethanes; in our independent Arlington lab test, the Hydra took total trihalomethanes from 58 ppb to 2.79 ppb.
Does boiling water remove trihalomethanes?
Only partly. Boiling drives off some chloroform, but it does not reliably remove the other trihalomethanes and it concentrates contaminants like lead and nitrate, so a carbon or reverse osmosis filter is the dependable fix.
Why are trihalomethanes higher in summer?
Trihalomethanes form faster in warm water and when there is more organic matter in the source, so levels usually peak in summer and at the far ends of a distribution system.
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