The discovery of high levels of a naturally occurring radioactive gas in Co Wicklow groundwater supplies represents an unknown risk to human health.The radiation does, however, deliver an extra dose to individuals exposed to it.
The main health risk of radon in water was cancer of the stomach, according to Dr Tony Colgan, principal scientific officer with the Radiological Protection Institute of Ireland, which conducted the Co Wicklow groundwater study.
"There have been some studies in the and they haven't been able to show any link between radon in drinking-water and cancer risk," he stated.
Exposure to high radon gas levels in the air, however, are linked to lung cancers. The institute in previous studies has estimated that perhaps one in 10 of the 1,500 annual lung cancer deaths here could be attributable to exposure to high radon levels in the home, with smokers at particular risk.
The goal in radiation protection was always to reduce exposures to as low as possible, Dr Colgan said, so the discovery of high radon levels in the drinking-water of some homes was a particular concern.
Radon levels in drinking-water above the "action level" specified by the EU could cause an annual doubling of radiation exposure, according to the institute's findings. A person living in Ireland might expect to receive 3.62 milliSieverts (mSv, a measure of radiation dose) a year from all natural and man-made sources. Four of the Co Wicklow wells would have given those drinking at least one litre of tap-water a day radiation exposures in excess of this annual dose, according to the pilot study results.
Irish tap-water use was typically higher than one litre a day, Dr Colgan said, and at two litres per day the most contaminated well would have delivered an annual dose of 43 mSv to those drinking it. This, he said, was eight times the Sellafield nuclear worker annual limit.
The institute's report pointed out that radiation exposures of one mSv per annum carried a lifetime risk of one in 20,000 of contracting a fatal cancer.
The findings were highly significant because of the potential number of householders using private wells who could be exposed to excessive radon levels. About a quarter of households in Co Wicklow rely on private groundwater supplies, Dr Colgan said.
If more extensive testing matched the pilot study results this would mean that about 1 per cent of all Co Wicklow homes using private wells would be over the EU limit for radon in water, Dr Colgan added.
The institute looked at Co Wicklow first because its geology is known to deliver high radon levels in the soil, the source for radiation both in the home and in drinking-water.
The findings raise questions about the suitability of groundwater supplies in other high radon areas pinpointed by earlier studies by the institute. These include counties Carlow, Galway, Louth, Mayo, Sligo, Waterford and Wexford, along with Wicklow. Significant levels were also found in Clare and Kilkenny.
Information on radon is available on the institute's website: http://www.rpii.ie