Trihalomethanes (TTHMs)
Byproducts of chlorinating water. Linked to bladder cancer at chronic exposure.
The science, plainly.
Total trihalomethanes (TTHMs) — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — form when chlorine reacts with naturally occurring organic matter (decaying leaves, algae) in source water. They are an unavoidable trade-off of disinfection, but utilities with cleaner source water and better treatment can keep them well below the legal limit.
The pathways into the tap.
- Reaction of chlorine with organic matter in source water
- Higher in systems with surface water (rivers, reservoirs)
- Higher in warm months when organic matter is highest
What the evidence shows.
Chronic exposure linked to bladder cancer (most consistent finding), and possibly colorectal cancer. Some studies report associations with adverse birth outcomes, but the evidence is mixed.
Cancer
Most consistent evidence is for bladder cancer at chronic exposure above ~40 ppb, but the dose-response is not well-defined.
Pregnancy
Mixed evidence for small increases in stillbirth, miscarriage, and low birth weight.
What the law allows vs. what's actually safe.
EWG health guideline of 0.15 ppb is dramatically stricter than EPA.
Where exposure is highest.
Higher in systems with surface water sources, especially in the Southeast and lower Mississippi River basin.
Filtration that actually works.
- Activated carbon (point-of-use)
- Reverse osmosis
- Aeration / boiling (partially — TTHMs are volatile)
We don't recommend brands. The certification on the box matters more than the brand printed on it. Look for the actual NSF/ANSI standard number specific to the contaminant you're removing.
Sources
- National Primary Drinking Water Regulations — U.S. Environmental Protection Agency
- Chlorine in Drinking Water — U.S. Environmental Protection Agency