Explainer: What are THMs in Kilkenny water supplies?

Sian Moloughney


Sian Moloughney

Kilkenny water THMs

What are THMs?

Irish Water warns of THMs in Kilkenny water supplies.

What are THMs?

THM stands for trihalomethanes.

What are trihalomethanes?

According to the HSE THMs are chemicals which may be found in water treated with chlorine.  The concentration of THMs in drinking water varies according to the level of organic material in the water, the amount of chlorine required to treat the water, and the temperature of the water that is being treated.
Some water supplies have reported levels of THMs which are higher than the recommended levels.  Irish Water is developing a National THM plan, in order to deal with this issue.

How might I be exposed to THMs in water?

THMs can be ingested (swallowed), if present in water.
THMs may be inhaled (breathed in), if present in air, as most THMs in water evaporate.
THMs can be inhaled or absorbed though the skin while showering or bathing.  Swimming in a chlorinated pool is another way that people may be exposed to THMs.

Can THMs affect my health?

Short-term health effects of THMs in drinking water are rare.  Therefore temporary raised levels of THMs in drinking water are unlikely to result in any risk to health.
Some studies suggest a link between long term exposure to THMs (i.e. many years) and cancer and reproductive effects but the evidence is not conclusive.
THMs are classified as ‘possibly carcinogenic’ to humans.
This means that scientific evidence cannot conclude definitively whether or not THMs cause certain cancers in humans.  There is some evidence that THMs cause cancer in animals.
Some studies suggest that very long-term exposure (e.g. 35 years or more) to high levels of THMs may be linked to a slightly increased risk of some types of cancer in humans, in particular bladder and colon cancer.  The evidence however, is not conclusive.
Study findings on risks of miscarriage and low birth weight, associated with drinking water containing THMs, are inconsistent.
Any such possible risks are, however, much lower than the risk of serious illness that could result from drinking water that has not been properly disinfected.

What is the HSE’s health advice on THMs in drinking water?

Short-term exceedances in THMs are unlikely to result in any risk to health.
Potential risks from drinking untreated water far outweigh any possible risks of long-term exposure to THMs.

You can check the quality of drinking water in your area on the Irish Water website click here.