Trihalomethane (TTHM)
TTHM·Affects liver, kidneys, heart, reproductive
The EPA's regulatory grouping for the four most common chlorinated/brominated THMs. Capped at 80 ppb combined in every US municipal supply.
TTHM
EPA limit
0.080 mg/L (TTHM combined)
Most at risk
Long-term consumers, pregnant women, frequent shower users
Found in
City water
AquaOx filters
YesActivated coconut shell carbon
What it does to you
Health effects
Total Trihalomethanes (TTHM) is the EPA's grouping of four disinfection byproducts: chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform. They are regulated together at 80 ppb because they form in the same reaction, occur together, and have overlapping toxicity.
As a group, THMs are classified as probable human carcinogens. Long-term epidemiology associates THM exposure with elevated bladder and colon cancer rates and with adverse reproductive outcomes including low birth weight and miscarriage. The liver and kidneys are the primary target organs for chronic exposure, and a cardiovascular signal appears at sustained moderate exposures.
Crucially, THMs are volatile. Hot showers vaporize them into bathroom air, and warm bath water enables dermal absorption. For frequent showerers in chlorinated cities, inhaled and dermal THMs often exceed the dose from drinking the same water.
Where it comes from
How it gets into your water
THMs form inside the water-treatment process itself. When chlorine added by your utility reacts with naturally-occurring organic matter (leaves, soil, algae) in the source water, it produces the THM family. The specific mix of chloroform vs. brominated species depends on how much bromide is in the source water.
The reaction continues in distribution pipes on the way to your tap, so TTHM levels are often higher at the end of the line than near the treatment plant. Warmer temperatures accelerate the chemistry, which is why levels peak in late summer and early fall.
Chlorinated treatment plants
Chlorine reacts with organic matter
Distribution pipes
Reaction continues during transit
Hot showers
Volatile, vaporize into bathroom air
Find out for yourself
Is it in your water?
If your home is on a chlorinated municipal supply (the vast majority of US cities), TTHM is in your water. Every utility tests TTHM and reports the running annual average in its Consumer Confidence Report (CCR). The 80 ppb MCL is a regulatory ceiling; well-run utilities operate well below it, but levels vary by season and by your position in the distribution network.
Households at the periphery of long networks, and households served in late summer / early fall, typically see the highest residuals. Well-water households generally do not have THMs unless they shock-chlorinate periodically.
The fix
How AquaOx filters it
THMs are filtered efficiently by activated carbon. Both the Centaur catalytic carbon and the coconut-shell activated carbon stages in AquaOx systems capture the full TTHM family as water passes through, with independent testing showing 95%+ reduction.
Because AquaOx is whole-house, it filters THMs from your shower water too. That matters: the inhalation and dermal exposure routes during a hot shower are the part of TTHM exposure that pitcher filters cannot address.
Explore more
Related contaminants
Disinfection byproducts
Chloroform
Disinfection byproducts
Bromodichloromethane
Disinfection byproducts
Dibromochloromethane
Disinfection byproducts
Bromoform
Disinfection byproducts
Haloacetic Acids (HAAS)
Treatment additives