Trihalomethanes (THMs)
Disinfection byproducts formed when chlorine reacts with natural organic matter in water.
ADWG Guideline
0.25 mg/L
Trihalomethanes (THMs) are a group of chemical compounds that form when chlorine used for water disinfection reacts with naturally occurring organic matter (like decaying leaves and soil) in the source water.
Why it matters
The ADWG sets a guideline of 0.25 mg/L for total trihalomethanes. The most common THM is chloroform. Concerns include:
- Long-term exposure may be associated with increased bladder cancer risk
- THM levels tend to be higher in warmer months and in water sourced from surface catchments with high organic content
- The guideline represents a balance between disinfection benefits and byproduct risks
Reducing THMs
Water utilities manage THM levels by:
- Optimising chlorine dosing
- Using chloramine instead of free chlorine (produces fewer THMs)
- Improving source water quality through catchment management
- Removing organic matter before disinfection
At the household level, activated carbon filters effectively remove THMs.
Related Terms
Check Your Water
See how trihalomethanes (thms) levels compare in your suburb with real water quality data.
Disclaimer: Glossary definitions are provided for general educational purposes only and do not constitute medical advice or professional guidance. For specific health concerns about your drinking water, consult a qualified health professional. See our Terms of Use.