Trihalomethanes
Disinfection by-products formed when chlorine reacts with natural organic matter in water. They embody the central trade-off in drinking water treatment: the disinfection that prevents waterborne disease also creates by-products of its own.
Key facts
- Contaminant group
- Disinfection by-product
- World Health Organization — guideline
- Individual guideline values per compoundWHO sets values per compound rather than a single total, with a fractional sum approach for mixtures.
- European Union — legal limit
- 100 µg/l total
- United Kingdom — legal limit
- 100 µg/l total
- United States — legal limit
- 80 µg/l totalAssessed as a locational running annual average.
Where it comes from
- Reaction of chlorine with humic and fulvic acids from soil, peat and vegetation
- Higher in surface waters, especially upland catchments draining peat
- Formation continues in the distribution network, so concentrations rise with residence time
How it behaves in water
Formation increases with organic carbon concentration, chlorine dose, contact time, temperature and pH. Bromide in the source water shifts the mix toward brominated forms, which are of greater toxicological concern than chloroform.
How it is measured
Measured in micrograms per litre as the sum of four compounds: chloroform, bromodichloromethane, dibromochloromethane and bromoform. Sampling point matters, because concentrations are lowest at the works and highest at the network extremity.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| World Health Organization | Individual guideline values per compound | guideline | WHO sets values per compound rather than a single total, with a fractional sum approach for mixtures. |
| European Union | 100 µg/l total | legal limit | — |
| United Kingdom | 100 µg/l total | legal limit | — |
| United States | 80 µg/l total | legal limit | Assessed as a locational running annual average. |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
Epidemiological studies report associations with bladder cancer at long-term exposure, though confounding is difficult to exclude. Every regulator that has assessed the balance has concluded that the risk from inadequate disinfection greatly exceeds the risk from by-products — which is why the response is to remove organic precursors before chlorination rather than to reduce disinfection.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Removing organic precursors before chlorination, through enhanced coagulation or activated carbon — the primary utility strategy | Sediment filtration |
| 2 | Activated carbon at the point of use | Water softening |
| 3 | Switching the residual disinfectant from chlorine to chloramine, which forms fewer THMs | Boiling, which drives some off but concentrates non-volatile substances |
| 4 | Letting water stand or aerating, since THMs are volatile | — |
Relationships
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted