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Chlorine and trihalomethanes

The addition of chlorine is an essential step to ensure harmful bacteria are eliminated from your drinking water. The formation of Trihalomethanes is a consequence of this process.

Disinfecting water with chlorine is an essential step to make your water safe to drink. When chlorine reacts with organic matter in water, trihalomethanes (THMs) can form.

How trihalomethanes (THMs) form

Most drinking water in Ireland comes from surface water sources, like rivers, lakes and streams. The water often contains tiny particles of organic matter, such as dirt, twigs and leaves. To make the water safe to drink, chlorine is added during the disinfection process.

THMs can form when chlorine reacts with organic matter in the water. There are 4 main factors which cause THMs to form.

There are 4 main factors which cause THMs to form.

Organic matter in source water

If there are high levels of organic matter, the treatment plant may not be able to remove it all.

Chlorine levels in treated water

The higher the concentration of chlorine, the more likely THMs are to form.

Length of time to reach your tap

The longer the time chlorine has to react with any organic matter, the more likely THMs will form. Most water supplies have at least one storage reservoir and several kilometres of water mains.

The time of year

THMs are more likely to form in autumn when dead and decaying leaves enter the source water. The temperature of the water is also higher then, which can affect THM formation.

Health and THMs

Health and THMs

There are concerns about possible health risks from drinking water with increased THM levels. The Health Service Executive (HSE) found no scientific study that conclusively linked THMs with serious illnesses, such as cancer. The HSE's report on Trihalomethanes states:

“The benefits of using chlorine to treat our drinking water are much greater than any possible health risk from THMs.“

Health Service Executive (HSE)

What we're doing

We are working to remove the risk of elevated levels of THMs forming in all public water supplies. We are doing this through a programme of investment to address all water treatment issues. The programme targets water supply zones where the risk of THMs is highest. The works will optimise and upgrade these water treatment plants to:

  • remove as much organic material as possible
  • control the concentration of chlorine in the water

We're also taking measures to reorganise networks and regularly flush storage reservoirs and pipelines.

Further information

For more about THMs and your drinking water, visit the Environmental Protection Agency. For more about THMs and your health, visit the Health Service Executive.