High levels of radon in a home can create a long-term health risk for residents. When inhaled, radon gas particles remain in lung tissue and begin to decay. As the radon particles decay, they release bursts of radiation that can damage the lung tissue cells. Over time, the cell damage can lead to the development of lung cancer.
According to Health Canada in Ontario, where the population-weighted estimate was 4.6% of homes exceeding the guideline, 13 of 36 Health Regions (greater than one third of the Health Regions) had more than 10% of the homes test above the guideline.
To know more, How are homes tested for radon? etc. Please read A Homeowner’s Guide to Radon