The toxins, known as trihalomethanes, are compounds including chloroform which occur in drinking water as a result of reaction between organic materials, such as peaty soil, and chlorine, which is added as a disinfectant.

Levels of trihalomethanes in drinking water are limited by the EU drinking water directive and World Health Organisation guidelines.

Correspondence with Europe

This week Irish Water confirmed excess levels of trihalomethanes had been detected in supplies. It said its latest figures showed the water supplies of approximately 380,000 people “have exceeded the parametric limit for trihalomethanes”.

It said current and future capital investment will address “long-standing trihalomethanes non-compliances” by introducing the appropriate treatment technology”. Irish Water said it was “currently in the process of developing a national trihalomethanes plan”.

Irish Water said removal of trihalomethanes from its supplies was a priority for it and for the Environmental Protection Agency, which oversees the quality of water supplies. It also said its business plan up to 2021 set out a clear commitment to reduce the number of all schemes on the agency’s remedial action list, including those affected by trihalomethanes, “to zero”.

The utility said it was “committed to providing long-term solutions that will reliably deliver compliant drinking water across the full range of drinking water parameters”.

Irish Water supplied information on trihalomethanes to the Government, which passed it on to the EU, on foot of a complaint from Friends of the Irish Environment. Friends of the Irish Environment said it was seeking to highlight the ongoing presence of toxins in water supplied to a large sector of the population, which it noted was in breach of EU and World Health Organisation limits.

The campaign also sought unsuccessfully to compel Irish Water to publish the levels of trihalomethanes on customers’ bills, to enable consumers to see when their supply exceeded human health guidelines.

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The organisation said the research has allowed it to establish safety guidelines for human consumption.