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Radon is an odorless, colorless radioactive gas that, over time, with repeated exposure, can damage DNA in a way that causes cancer. Radon is released from the ground, and can enter your home through cracks in the foundation, through openings around sump pumps and drains, and the spaces around pipes. Radon is sometimes found in the water supply of homes that have well water. With today's building construction designed to keep heat or cooling in, the air-tight design often locks radon gas in, making it easier to reach higher levels.
There are test kits available, and Radon Inspection services that can test the air and water (if you have a well) in your home for the presence of radon.
The government determines a safe level of radon (as of this writing, the US EPA has set an action level of 4 pCi/L), but the ideal is to try to have the minimum level possible. Some would argue that there is no safe level of radon. Checkout radon.com for the latest information on safe levels of radon.
There is some good news. If you detect radon in your house through an inspection, there are ways to mitigate. Cracks in the foundation can be sealed. Water filtration systems can be installed.
But also keep in mind, that part of the problem is that homes are air-tight contributing to the build-up of radon. If you frequently go in and out your doors, or open windows, have the flu open in the fireplace, then you are ventilating your home and improving radon levels.
Radon accumulates in the basement (as it seeps up from the ground), so finding ways to improve ventilation in your basement may be a good first step while you are waiting for inspections or mitigation, or just to improve air quality in your home.