Trihalomethanes (TTHM)
A group of chemicals (chloroform, bromoform, and others) that form when chlorine used to disinfect water reacts with natural organic matter.
Source
Created during the water treatment process when chlorine is added.
Health Risk
Long-term exposure linked to increased risk of bladder cancer and liver, kidney, and central nervous system problems.
Filter Tip
An activated carbon filter (pitcher, faucet-mount, or whole-house) effectively reduces trihalomethanes.
Total Trihalomethanes (TTHM) are a group of four chemicals — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — that form when chlorine or chloramine used to disinfect drinking water reacts with naturally occurring organic matter like leaves and algae.
The EPA has set a Maximum Contaminant Level of 80 parts per billion (ppb) for total trihalomethanes, measured as a running annual average across the distribution system.
Why Are Trihalomethanes a Concern?
TTHMs are among the most common contaminants found in US drinking water. Key health concerns include:
- Increased risk of bladder cancer — the most consistently associated health effect
- Possible links to colorectal cancer
- Liver, kidney, and central nervous system damage
- Possible reproductive effects, including miscarriage risk at high exposure levels
What Causes High TTHM Levels?
TTHM levels tend to be higher when source water has more organic matter (rivers, lakes), during warmer months when organic matter increases, in areas further from the treatment plant (longer time for reactions), and when higher chlorine doses are used.
How to Reduce Trihalomethanes at Home
- Activated Carbon Filters: Very effective — even basic pitcher filters reduce TTHMs significantly
- Whole-House Carbon Filters: Also reduce exposure through showering and bathing
- Reverse Osmosis: Effective but usually unnecessary for TTHMs alone
- Letting Water Sit: TTHMs are volatile — leaving water in an open pitcher for a few hours can reduce levels
Trihalomethanes (TTHM) Levels in US Cities
Based on verified municipal water quality reports (Consumer Confidence Reports) from 17 cities. 1 city exceeds the EPA limit. 16 additional cities have detectable levels within limits.
Exceeds EPA Limit (1)
Detected Within Limits (16)
Frequently Asked Questions
What is a safe level of trihalomethanes in water?
The EPA MCL is 80 ppb measured as a running annual average. Some health organizations suggest levels below 40 ppb are preferable. TTHMs are nearly impossible to fully avoid in chlorinated water systems.
Do Brita filters remove trihalomethanes?
Yes, activated carbon filters like Brita can reduce trihalomethanes. The Brita Elite and Longlast filters are more effective than the basic Standard filter. For best results, use a faucet-mounted or under-sink carbon filter.
Are trihalomethanes dangerous in shower water?
TTHMs are volatile and can be inhaled during hot showers. Some research suggests inhalation and skin absorption during showering may contribute as much to total exposure as drinking water. Whole-house filters address this.