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Explainer: What are THMs and what risks do they pose in Irish water?

Risk from drinking untreated water outweighs any possible risks from long-term exposure to trihalomethanes, HSE says

The use of chlorine is essential to ensure safe drinking water, but when organic matter is present it can react with the chlorine to form trihalomethanes (THMs). This is often the case in Ireland, as most of our drinking water comes from surface-water sources, such as rivers, lakes and streams, that often have high levels of vegetation or organic matter.

Uisce Éireann has said it is taking advice on whether it needs to warn more than 220,000 consumers that their drinking water contains a level of toxic chemicals called trihalomethanes in excess of that permitted under European Union limits. This follows a European Court of Justice ruling last month that Ireland had failed to meet obligations to rid public and private water supplies of THMs.

The concentration of THMs in drinking water varies according to the level of organic matter in the water, the amount of chlorine used to treat the water and the temperature of the water.

Some studies have suggested a link between cancer and long-term exposure (over years) to THMs, and also that THMs can affect reproduction. There is some evidence that THMs cause cancer in animals. As a result, they are classified as “possibly carcinogenic” to humans.


The problem with THMs in Irish drinking water supplies has been known for many years.

The Environmental Protection Agency (EPA) said 91 per cent of public water supplies were THM-compliant in 2015, with 59 supplies showing excessive amounts. Data from the final quarter of 2023 shows that THM levels remained above European Union safety levels in more than 20 locations across 15 counties. More than 220,000 people are reliant on such supplies – roughly half of them (114,764) in the Limerick city area.

Regarding private water schemes, 16 such schemes supplying 14,000 people failed the standard for THMs in 2022, most of them in Border or western counties.

A national plan has been drawn up to deal with the problem, but it is dependent on a significant injection of funds to update reservoir and treatment infrastructure.

The Health Service Executive (HSE) says excessive THMs in drinking water over the short term is unlikely to result in any health risk. It also says the potential risk from drinking untreated water far outweighs any possible risks from long-term exposure to THMs.

Information on drinking water parameters for all public supplies is available on the Uisce Éireann website. Users can check the results of tests on their supplies by entering their address and scrolling through the table on the linked page.

If you are concerned about THMs in your drinking water, the advice is to either boil water for a minute, store it in an open jug in the fridge for 24 hours, or use filters or bottled water.

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