Because it's odorless and invisible and its effects only show up over a long period of exposure, the danger of radon is often underestimated. The highest residential radon concentrations occur when the house is “closed-up” (e.g., winter).

Radon, which enters homes through cracks in foundations, is the second leading cause of lung cancer overall and the primary cause of lung cancer in people who have never smoked, says Bill Field, a Professor in the Department of Occupational and Environmental Health & Epidemiology, College of Public Health, at the University of Iowa.

"Iowans need to be particularly aware of the health risk radon poses because Iowa leads the nation in the number of homes containing elevated levels of radon gas," he adds. While no level of radon exposure is completely safe, the United States Environmental Protection Agency (EPA - http://epa.gov/radon/) recommends reducing radon concentrations in homes that exceed a yearly average of 4 picocuries per liter of air (pCi/L) - the unit used to measure radon. If the radon levels exceed 200 pCi/L, contact the Environmental Protection Agency or your state health department immediately.

"If it is determined that you need to take steps to reduce radon levels in your home, reliable methods such as ventilating the soil under the home are generally recommended," Field says. "In most states, lists of certified radon mitigation specialists are available from the state health department."