CHATTANOOGA (WRCB) -- You can't smell or see it, but it's a real threat in many homes: radon. It's a naturally occurring gas that, as it decomposes, becomes dangerous to your health.

"Radon is basically a gaseous breakdown product of uranium and radium, possibly some iron ore," says Dr. Daniel Smith, a pulmonologist in Chattanooga.

In other words, radon becomes radioactive and can damage your lungs if inhaled, even in small amounts over a long period of time.

"Radon can damage the lining of the smaller bronchial tubes," explains Dr. Smith.

The result is lung cancer and radon can easily find its way into your home emanating from ground soil through cracks in walls, floors, or foundations.

The latest numbers from the Environmental Protection Agency (EPA) show radon is responsible for more than 20,000 lung cancer deaths per year in the United States. This ranks higher than other potentially lethal hazards at home such as drowning, fires, or falls.

"It's the second leading cause of lung cancer," says Shirley Cudabac of the Chattanooga office of the American Lung Association.

According to the Tennessee Department of Environment and Conservation (TDEC), about six percent of homes surveyed across the U.S. had elevated levels of radon.  In contrast, 16 percent of Tennessee homes surveyed had elevated levels and in some counties 33 to 75 percent of homes being tested have elevated levels of radon.

"We've seen a higher rate of lung cancer in 'never-smokers'," says Dr. Smith.

This is what led to the search for other risk factors outside of smoking or asbestos exposure. Radon was found to be one of them.

Cudabac knew someone in the Chattanooga area who exercised and didn't smoke but died of lung cancer. High levels of radon were later found in his home.

"I think everybody was just shocked. Stunned. He was so very healthy," recalls Cudabac.

Home radon testing kits are available at local hardware and home improvement stores, so an ounce of prevention is worth a pound of cure.

Cudabac suggests folks in the Tennessee Valley take this simple precaution.

"If anybody can keep that from happening to a member of their family, they need to get that test," urges Cudabac.