Scientists call it the curse of the Celts. Baffled at the extraordinarily high number of Irish babies born with brain defects every year compared to other nationalities, they explain it away as genetic, geographic misfortune, a cruel accident of birth.
But a major new study could be about to turn that theory on its head.
In the largest ever piece of research of its kind, a chilling link has been made between some of the most devastating diseases of childhood and our single most precious natural resource: the water that runs from our taps.
Undertaken by the National Institute of Public Health in Norway, the study provides new evidence to show that drinking water treated with chlorine is associated with higher levels of birth defects, in particular spina bifida, a fault in spinal cord development.
Published in the American journal Epidemiology, it examined 141 000 births over three years and found that children living in areas where tap water is chlorinated had a 14 percent higher chance of having birth defects than those in non-chlorinated regions.
The risk of urinary tract abnormalities was shown to double.
In Ireland, the country with the second highest levels of spina bifida in the world, chlorine is added to aboout 90 percent of the national water supply, excluded only from some private group water schemes and dwellings with their own wells.
Extremely toxic
The most widespread disinfectant of water in the world, it was first introduced here in the early part of the last century as a powerful weapon against a host of waterborne pathogens such as salmonella, E-coli and viral diseases like polio and hepatitis.
In the 60s and 70s, however, concerns began to stir about the safety of chlorine when it came to light that even very low levels of the chemical and its by-products were extremely toxic to a wide variety of fresh water and marine animals and plants.
Exposure to minimal amounts of chlorine was shown to kill fish, oysters and clams outright and also cause more subtle effects such as reproductive abnormalities and infertility.
In 1974, an American study revealed that virtually every chlorinated drinking water supply was contaminated with chloroform, a carcinogen which targets the liver and kidneys.
Chlorine works as a disinfectant because it is extremely chemically reactive. When it combines with organic matter in water, especially colour, it forms trihalomethanes (THMs), toxic chemicals which are associated with stillbirth and cancer.
While levels of fluoride and aluminium, the other main chemicals added to the Irish water supply, are regulated under EU and Irish Drinking Water legislation, no such upper limit exists for chlorine.
Even more convincing
Instead, it is left to the discretion of individual treatment plants to operate a code of practice whereby minimum doses of chlorine are added consistent with effective disinfection. This dose is known to vary greatly from plant to plant.
Although this is the first time scientists have made a link between chlorinated water and brain damage, other recent epidemiological studies have made the case against chlorine even more convincing.
Last year, Canadian chemists revealed that high levels of THMs caused a significant rise in the number of stillbirths.
Chlorine is also associated with an increased risk of bowel and bladder cancer, diseases whose incidence in Ireland is among the highest in the world.
A further Canadian study in 1995 found a 60 percent increase in the risk of bladder cancer for people exposed to high levels of chlorinated by-products for 35 years or more.
Resistant to chlorine
While the World Health Organisation maintains that the advantages of chlorine still outweigh the disadvantages, growing numbers of scientists and water authorities are beginning to question the reliance on chlorine as a primary disinfectant and are concerned that the disease-countering benefits it offered in the last century are becoming increasingly redundant.
During the early 90s, evidence emerged that a number of organisms, including strains of E.coli, cholera and the bacteria associated with Legionnaire's Disease, had become resistant to chlorine.
In July 1993, some 35 000 residents of New York City had to switch over to boiled water when it was discovered that E.coli 0157 had survived chlorination and made its way into the public water supply.
As confidence in chlorine begins to dwindle, countries like the US are considering changing to less harmful methods of disinfecting tap water.
In February 1994, President Clinton signed an order stating that chlorinated water and all its by-products would be banned throughout the United States as soon as a safe alternative could be put in place.
Despite the availability of such alternatives, this ban has not yet been introduced.
Birth defects and cancer
There are very effective, efficient and non-toxic alternatives to chlorine available now such as treating water with ultra violet or gas ozone.
Such is its concern at the findings of the new report, the British government has ordered an independent study into the use of chlorinated water and its links to birth defects and cancer.
In Ireland, officials at the Department of the Environment admit the accumulation of data on the dangers of chlorine is worrying and that the implications of this latest report will be examined with urgency.
The Spina Bifida and Hydrocephalus Association has expressed grave concern at the new study and, having received medical advice on the matter, is considering making representations to the Irish government.
"This research, given its huge scope, must be taken seriously," says spokesman Tony Britton.
"The evidence seems so convincing. We still do not know what causes spina bifida but this might go some way to providing an explanation."