Tap water treated with chlorine can double the risk of serious heart and brain abnormalities in unborn babies, according to Birmingham scientists.
Researchers who studied almost 400,000 infants born in Taiwan found high levels of chlorine by-products in tap water could affect the development of babies in the womb.
A significant impact was seen on the likelihood of babies being born with three common but serious types of abnormality.
They were ventricular septal, or “hole-in-the-heart” defects, cleft palate, and anencephalus - a condition that causes much of the brain, skull and scalp to be missing at birth.
The findings, from a team led by Professor Jouni Jaakkola at the University of Birmingham, were published in the journal Environmental Health.
All public-supply tap water is chlorinated in the UK to get rid of bugs that can cause diseases such as cholera and typhoid. The Drinking Water Inspectorate (DWI), which acts as the guardian of water quality on behalf of the Department for Environment, Food and Rural Affairs (Defra), said its own research had been unable to confirm a link between trihalomethanes and birth defects.
Chlorine is added to tap water in the final stages of treatment to kill germs. In England and Wales, two thirds of drinking water comes from surface sources, including reservoirs, lakes and rivers, and a third from underground aquifers.
Before being piped to the consumer, the water is clarified, filtered and disinfected. Although the amounts of chlorine used are very small, they can leave a lingering taste or smell.
Other methods of disinfection include adding ozone to water or exposing it to ultraviolet light, but neither has the long-lasting effect of chlorine.
Prof Jaakkola’s team looked at what effect women drinking water containing high, medium and low levels of chlorine by-products had on the risk of 11 common birth defects.
Prof Jaakkola, from the Institute of Occupational and Environmental Medicine at the University of Birmingham, said: “The biological mechanisms for how these disinfection by-products may cause defects are still unknown.
“However, our findings don’t just add to the evidence that water chlorination may cause birth defects, but suggest that exposure to chlorination by-products may be responsible for some specific and common defects.
“Whilst the benefits of water chlorination are quite evident, more research needs to be carried out to determine these side-effects.”
