Radon - 226
Some groundwater supplies may contain elevated concentrations of radon. High radon concentrations are seldom found in surface drinking-water supplies. Radon dissolved in drinking-water can be released into indoor air. Normally, a higher radon dose is received from inhaling the radon and radon progeny compared with their ingestion.
Radon present in surface waters, such as lakes and rivers, is readily released into outdoor air by agitation as it passes over rocks and soils. Groundwater from wells and boreholes usually contains higher radon concentrations than surface waters. In some extreme circumstances, very high radon concentrations can be found in drinking-water supplies from these sources.
The percentage of radon present in drinking-water that is released into indoor air will depend on local conditions, such as the total consumption of water in the house, the volume of the house and its ventilation rate, and is likely to be highly variable. It has been estimated that a radon concentration of 1000 Bq/l in drinking-water discharged from a tap or shower will, on average, increase the radon concentration by 100 Bq/m3 in indoor air.
Epidemiological studies have clearly shown that long-term exposure to high radon concentrations in indoor air increases the risk of lung cancer. Radon ingested in drinking-water will give a radiation dose to the lining of the stomach. Scientific studies have not shown a definitive link between consumption of drinking-water containing radon and an increased risk of stomach cancer.