UK Legal Limit
0.1 mg/L (total THMs)
WHO Guideline
Varies by compound
EU Standard
0.1 mg/L (total)
Primary Sources
Chlorine + organic matter
Trihalomethanes (THMs) in UK drinking water is regulated at 0.1 mg/L (total THMs). The WHO guideline is Varies by compound and the EU standard is 0.1 mg/L (total). THMs form when chlorine used to disinfect water reacts with organic matter. Levels are typically higher in summer and in areas using surface water sources.
Long-term exposure to elevated trihalomethane levels has been associated with an increased risk of bladder cancer and potentially other cancers. Some studies have also suggested links to adverse reproductive outcomes, including miscarriage and low birth weight, though evidence is still evolving. The International Agency for Research on Cancer (IARC) classifies chloroform as 'possibly carcinogenic to humans' (Group 2B).
THMs form when chlorine reacts with humic and fulvic acids — natural organic compounds from decomposing plant material that dissolve into rivers, reservoirs, and lakes. THM levels are typically higher in summer (more organic matter, higher temperatures increase reaction rates) and in water systems that rely heavily on surface water sources.
| Jurisdiction | Limit / Guideline | Notes |
|---|---|---|
| UK (DWI) | 0.1 mg/L (total THMs) | Regulated under the Water Supply (Water Quality) Regulations 2016 |
| WHO | Varies by compound | World Health Organization Guidelines for Drinking-water Quality |
| EU | 0.1 mg/L (total) | EU Drinking Water Directive (2020/2184). The UK no longer automatically mirrors EU standards post-Brexit. |
Activated carbon
Porous carbon material (from charcoal or coconut shell) that adsorbs contaminants as water passes through. Best for organic compounds and some heavy metals.
Reverse osmosis
A membrane filtration process that removes up to 99% of contaminants by forcing water through a semi-permeable membrane under pressure. Highly effective but produces some wastewater.
Carbon block filters
Dense blocks of activated carbon that physically block particles and adsorb chemicals. More effective than granular carbon for lead and other heavy metals.
These list trihalomethanes (thms) among what they reduce. Check your own report first — a filter is only worth buying for a contaminant your supply actually has.
Listed to reduce Trihalomethanes (THMs)
Waterdrop G3P600 Reverse Osmosis
Reverse Osmosis · NSF/ANSI 58, NSF/ANSI 372
Typical price
£550approx.
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Listed to reduce Trihalomethanes (THMs)
Frizzlife PD600 Under Sink RO
Reverse Osmosis · NSF/ANSI 58
Typical price
£329approx.
Affiliate link: we may earn a commission at no extra cost to you.
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