Most people know the importance of having a working smoke detector and many might have a carbon monoxide detector.

But there could be another silent killer lurking that's much more likely to strike than fires and carbon monoxide .

Radon -- a colorless, odorless gas directly linked to 21,000 lung cancer deaths in the U.S. each year -- could be seeping into homes more often than we know.

Leona Drenning said she was surprised and frightened when radon was discovered on a plot of land where her family was building a new home near Saxton, Bedford County.

"The inspector during one of the visits ... mentioned to the builder that he should close off the rough tin plumbing in the basement due to the fact that there's radon around."

After learning about the threat, she ordered a radon meter. Her first reading was a 7.4 -- more than three points higher than what's considered a safe level.

After it rained and the ground settled even more, Drenning said the level went up to about 11. At that point, the couple hired a company to install an air ventilation system in their home.

"It is a gas that's emitting into a home that has now been definitively proven to cause lung cancer," said Dr. Matt Massiello, of Memorial Medical Center.

Massiello said radon seeps into homes through cracks, soil and the water supply.

"It's how the soil breaks down. The uranium emits this gas into the home," he said.

To remedy the problem, a Breezewood-based company ran a vent through the Drenning's roof with a fan to pull the radon out of the home.

Massiello said, "In a rather inexpensive manner, (the procedure) could eliminate 99 percent of the radon."

Drenning said most people know very little about radon and how prevalent it is. And even though a majority of local counties are considered at high risk for indoor radon levels, school districts are not required by the state to test for radon.

"It's a nationwide problem. Some areas are more prevalent than others. We don't know exactly the reason for that, but it's an environmental issue that everyone's exposed to," Massiello said.