How much radon is there in the atmosphere?

Dear Tom,
I know that radon seeps from the ground, but how much radon is there in the atmosphere?
Bob Kozlik, Riverwoods
Dear Bob,
Radon is a radioactive gas that cannot be seen, smelled or tasted and it is the second leading cause of lung cancer in this country. About 21,000 lung-cancer deaths per year are related to radon. Radon gas concentration in the open atmosphere is minimal, at a density of about 2,474 atoms per cubic inch. However, it is a heavy, dense gas: eight times the density of air. It escapes from the ground and, because of its density, accumulates in the lowest portion of houses, especially basements. Testing is the only way to know the concentration of radon gas in your home because there are no immediate symptoms that will alert you to its presence. It takes years of exposure before any health problems appear.

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