Trihalomethanes (THMs) have been a known issue in Irish drinking water since a 2024 EU court ruling. Here’s where things stand now, and how to check if your own water is affected.
The EU’s THMs in Drinking Water Ruling
In early 2024, the European Court of Justice ruled that Ireland had failed to keep drinking water quality within safe limits for trihalomethanes (THMs). At the time, the Environmental Protection Agency (EPA) reported that the issue affected drinking water for roughly 240,000 people, across counties including Clare, Cork, Donegal, Kerry, Kilkenny, Limerick, Longford, Meath, Offaly, Roscommon, Tipperary and Wicklow.
Two years on, there’s been real progress. The EPA’s latest Drinking Water Report, published in July 2026, shows the number of water supplies on the Remedial Action List (RAL) has dropped to 35, down from 45 in 2024. Uisce Éireann completed remedial works at 10 supplies during 2025 alone, and a further 14 are forecast to be resolved or substantially complete by the end of 2026.
It’s not all good news, though: the same report noted that boil water notices and water restrictions affected around 200,000 people in 2025, up from 84 supplies the year before. So while THM-specific progress is real, wider resilience issues in the network persist.
What Are THMs?
Chlorine is essential for disinfecting drinking water; it’s what keeps harmful bacteria and viruses out of the water supply. But chlorine doesn’t just disinfect in isolation. Most of Ireland’s drinking water comes from surface sources like rivers, lakes and streams, which naturally carry small amounts of organic matter (think dirt, twigs and decaying leaves). When chlorine reacts with this organic matter during treatment, it can form THMs as a by-product.
The legal limit for THMs in Irish drinking water is 100 micrograms per litre (µg/L), in line with EU Drinking Water Regulations. Supplies exceeding this limit are placed on the EPA’s Remedial Action List until the issue is resolved.
Is Your Tap Water Safe to Drink? What About Cancer Risk?
For the vast majority of the country, yes, tap water is safe. The EPA’s 2026 report confirms Ireland’s public drinking water meets very high standards overall, with over 99.8% compliance with microbiological and chemical limits, serving more than four million people.
On the cancer question specifically: the HSE classifies THMs as “possibly carcinogenic” to humans, based on some evidence linking long-term, high-level exposure to increased risk of bladder and colon cancer. This is not the same as a confirmed causal link, and the HSE has been clear that short-term exposure to elevated THMs is very unlikely to cause harm.
Crucially, the HSE’s official position is that the benefits of using chlorine to treat drinking water are much greater than any possible health risk from THMs. Stopping or reducing chlorination to avoid THMs isn’t a safe alternative: untreated water carries a far more immediate risk from bacteria and pathogens.
If you’re on one of the 35 remaining RAL supplies, or simply want certainty about your own water (particularly relevant for private wells and group schemes, which fall outside Uisce Éireann’s public network), testing is the only reliable way to know your THM levels. THMs are odourless, colourless and tasteless, so you can’t detect them by taste or smell alone.
Health Concerns of THMs in Drinking Water
Independent research has linked THM exposure to:
- Cellular damage (cytotoxicity)
- DNA damage (genotoxicity)
- DNA mutations (mutagenicity), a potential precursor to cancer development
These effects are generally associated with long-term, elevated exposure, not typical day-to-day tap water use in a compliant supply. The HSE’s guidance remains that the risk from THMs is far outweighed by the risk of inadequately disinfected water.
How Uisce Éireann Is Tackling THMs
Uisce Éireann’s remedial programme targets water supply zones with the highest THM risk, focusing on two things:
- Removing organic material from source water before treatment
- Controlling chlorine concentration during disinfection
This approach has driven the reduction in RAL-listed supplies over the past two years, though the EPA has urged Uisce Éireann to move faster on the remaining 35 and to improve broader network resilience.
Check Your Own Water Quality
National progress is reassuring, but it doesn’t tell you what’s actually coming out of your own tap, especially if you’re on a private well or group water scheme, which aren’t covered by the same monitoring as the public network. Regular independent testing is the only way to know for certain.
Southern Scientific Ireland offers INAB-accredited water quality testing (ISO 17025, registration 194T), testing over 10,000 drinking water samples each year. Our dedicated THM in Water Test is a single-parameter test built specifically for this issue: you’ll get your results by email, along with consultancy advice on next steps if your levels come back elevated.
