Radon is an odorless, colorless, chemically inert but radioactive gas that occurs naturally all across the U.S at varying concentrations. It is a decay product of uranium and radium present in bedrock, and its own decay products ("radon daughters") are also radioactive. Because radon is a gas, it moves easily up through soil. It also can be dissolved in and carried by groundwater and natural gas. Outdoors there is no problem because the gas gets diluted and blown away before it can decay. In your home or business environments, the gas can build up and that's when the trouble starts.
When radon decays, its RDPs, or radon decay products, can become attached to dust in the air you breathe. These RDPs then decay in your lungs, releasing alpha radiation that damages your cells and increases the risk of lung cancer. The effect was first seen in uranium miners, but today the threat goes far beyond just them. Government reports estimate that radon may cause nearly 30,000 lung cancer deaths a year among ordinary Americans.
Radon enters buildings many ways. It comes in through joints, cracks, in concrete walls or floors, openings in drains or sump pits, and gaps in plumbing passageways - it can be found even in well water or natural gas (shale gas). The more sealed up your windows and doors are (as with today's energy constructions), the more radon gets concentrated in the inside air.
4pCi/L or higher concentrations of Radon are considered to be above the safe level. The EPA recomends that even if your home tests below 4pCi/L that the home be retested every two years. This is due to the fact that radon levels fluctuate seasonally as well as even diurnally. Some points of the year have a higher concentration of radon than others, namely the colder months, when the home stays closed more often than the warmer months.
If 1000 people who smoked were exposed to these levels over a normal lifetime:
At 20pCi/L - About 135 people could get lung cancer - This is 100 times the risk of drowning
At 10pCi/L - About 71people could get lung cancer - 100 times the risk of dying in a home fire.
At 4pCi/L -About 29 people could get lung cancer - 100 times the risk of dying in a plane crash.
At 2.0pCi/L - About 15 people could get lung cancer - 2 times the risk of dying in a car crash.
At 1.3pCi/L - About 9 people could get lung cancer -
At a level of 0.4pCi/L - 3 could get lung cancer.
If 1000 people who never smoked were exposed to this level over a lifetime.
At 20pCi/L - About 8 could get lung cancer - The same risk as being killed in a violent crime
At 8pCi/L - About 3 could get lung cancer - 10 times the risk of dying in an airplane crash
At 4pCi/L - About 2 people could get lung cancer - About the same risk as drowning
At 2pCi/L - About 1 person could get lung cancer - About the same risk as dying in a home fire