 A study from 2001, which included 436 non-smokers and a control group of 1649 non-smokers, showed that exposure to radon increased the risk of lung cancer in non-smokers.The group that had been exposed to tobacco smoke in the home appeared to have a much higher risk, while those who were not exposed to passive smoking did not show any increased risk with increasing radon exposure.The effects of radon if ingested are unknown, although studies have found that its biological half-life ranges from 30–70 minutes, with 90% removal at 100 minutes.