UK legal limit: 100 µg/L (annual average). Find out what trihalomethanes is, its health effects, and how to check and reduce it in your tap water.
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Trihalomethanes (THMs) are a group of disinfection by-products formed when chlorine — used to disinfect tap water — reacts with naturally occurring organic matter in the water. The four main THMs measured in UK tap water are chloroform (trichloromethane), bromodichloromethane, dibromochloromethane, and bromoform. THMs are an unavoidable consequence of chlorination and are found in virtually all chlorinated water supplies worldwide.
In the UK, the legal maximum for the annual average of total THMs is 100 µg/L. Most UK water zones achieve levels well below this — typically 20–60 µg/L. However, in some zones — particularly those serving areas with high natural organic carbon in the source water — THM levels can approach the regulatory limit.
THMs are classified by the International Agency for Research on Cancer (IARC). Bromodichloromethane is classified as Group 2B (possibly carcinogenic to humans). Chloroform is Group 2A (probably carcinogenic). However, these classifications are based on high-dose animal studies. At the concentrations found in UK tap water, the risk is considered negligible by the DWI and the World Health Organization (WHO).
THMs are weak oestrogen mimics at very high concentrations, but the levels found in tap water are far below those that would produce any hormonal effect. The WHO's assessment is that the health risk from not disinfecting water (i.e., leaving microbiological contamination) greatly outweighs any theoretical risk from THMs at regulatory concentrations.
THM concentrations in tap water are highest when: source water contains high organic matter (moorland or river sources after rainfall), water has been in distribution for longer (end-of-pipe zones), water temperature is higher (summer peaks), and chlorine dose is higher. Yorkshire Water's moorland-sourced zones and some South West Water zones tend to see higher THM readings. London and southern chalk aquifer sources tend to have lower organic matter and lower THMs.
Activated carbon filtration effectively removes THMs from tap water. A standard carbon jug filter (Brita, ZeroWater) or countertop carbon filter will reduce THM levels significantly. Boiling also volatilises chloroform and reduces THM concentrations, but does not eliminate them. Running the tap for 30 seconds before filling a glass also helps, as water that has been sitting in pipes tends to have higher THM concentrations from prolonged chlorine contact.
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THMs are disinfection by-products formed when chlorine reacts with organic matter in water. The four main types in UK water are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. They are found in virtually all chlorinated water supplies.
At UK tap water concentrations, THMs are not considered a significant health risk. The WHO's view is that the risk from not disinfecting (microbiological contamination) far outweighs any theoretical THM risk. However, some THMs are classified as possible carcinogens at high doses.
The UK legal limit for total trihalomethanes is 100 µg/L as an annual average. Most UK zones achieve 20–60 µg/L. Enter your postcode to check your zone's THM level.
Yes. Activated carbon filters (jug filters, under-sink carbon filters) effectively remove THMs from tap water. Reverse osmosis also removes them. Boiling reduces but does not eliminate THMs.
THM levels tend to be highest in areas with high organic matter in source water — moorland reservoirs in Yorkshire and the South West, and some river-sourced areas. Chalk aquifer sources (London, Cambridge) typically have lower THMs.
If your zone's THM level is above 50 µg/L, a carbon filter is a sensible precaution. A standard jug filter will reduce THMs significantly. For the lowest THM levels, reverse osmosis removes virtually all THMs.