You’ve installed smoke detectors and tested for carbon monoxide. But could another dangerous gas be sneaking into your home?

Radon is a radioactive gas that forms naturally as uranium decays in soil. It seeps into homes from the ground, or from well water through gaps in the foundation or cracks in the building.

There is no safe level of exposure to radon, which is known to cause lung cancer. The Environmental Protection Agency estimates that radon trapped in indoor air causes about 21,000 deaths a year from lung cancer. Radon is second only to smoking as a cause of this fatal disease.

Because it occurs in almost all types of soil, radon can build up in nearly every type of home, old or new. But some areas are more at risk than others, and recent evidence suggests that homes located close to natural gas fracking operations are at greater risk. Click here to view a map of high radon areas.

In 2015, 11 national organizations, led by the American Lung Association, came together to announce a new action plan for reducing radon exposures. Called the National Radon Action Plan – A Strategy for Saving Lives, it calls for precautionary measures aimed at saving 3,200 lives a year by 2020. It’s a sure step in the right direction.

The EPA estimates that nearly one in every 15 American homes has elevated radon. What can you do to protect yours?

Test your home. Like carbon monoxide, radon has no taste or smell, so it’s nearly impossible to detect without a test kit. If you’ve never tested your home, do so. You should also re-test your home for radon if you begin using a previously unoccupied lower level or basement. If you’re buying a new home, request results of tests done within the past two years.

Luckily, testing is easy and inexpensive. To find a test kit, contact your local home improvement store or National Radon Program Services at Kansas State University, which sells discounted kits online. Click here to learn about radon requirements in your state, and whether your state provides free or discounted kits to residents.

Know your limits. Although the EPA’s threshold level for taking action to reduce radon is 4 picocuries per liter or higher, your family may still be at risk even if your number is below that. The EPA recommends that homeowners consider making modifications to their homes if the radon level is anywhere between 2 and 4 picocuries per liter. The good news: Effective measures to lower radon can reduce your home’s level by as much as 99 percent, and there are a variety of options to match your budget.

Consult a professional. To learn more about the risks of radon in your home or how to fix high levels, contact a trained professional. Good sources include the National Environmental Health Association National Radon Proficiency Program and the National Radon Safety Board. You can also call the National Radon Information Line at (800) SOS-RADON.