You may not realize it, but there could be a “silent killer” seeping into your home.

Radon is a radioactive gas that comes from the natural breakdown of uranium in soil, rock, and water. It can seep into the home through cracks in the foundation. The consequences of undetected radon in the home can be deadly: Radon is the second leading cause of lung cancer after smoking, according to the American Cancer Institute, and causes an estimated 15,000 to 22,000 deaths in the U.S. each year.

Unfortunately, radon is invisible and odorless—much like carbon monoxide—and you won’t know it’s present. In addition, there’s no way to distinguish which homes may have it. “Any home may have a radon problem,” says the Environmental Protection Agency (EPA). “This means new and old homes, well-sealed and drafty homes, and homes with or without basements.”

The only way to know for sure what radon levels your home has is to test for it.

When and how should I test my home for radon?

Radon is present in any home across the country. Levels of radon vary within states, townships, and even neighborhoods. Testing is a simple way to educate yourself and those around you on what the radon levels are. It is important to remember that levels can fluctuate over time for a number of reasons, such as weather, temperature, or erosion, so testing every other year can ensure radon still is not present.

The EPA recommends conducting a radon test every two years, even if you have a mitigation system in place. One of the biggest misconceptions that homeowners have is that if they have a system they don’t have to worry about radon. Testing helps ensure the system is operating properly and mitigating the radon exposure.

It’s also important to remember that just because your neighbor’s home tests low for radon, yours may not. Because so many factors can play a role in radon getting into the house, from soil composition and ground permeability to how the home was constructed, levels may vary even from house to house. Every home in the community should be tested regularly.

Testing should be performed by a professional, such as a radon mitigator. The radon mitigator conducts the test using a charcoal kit to count the alpha particles that radon emits. The charcoal kit is then sent to a lab for evaluation, and the mitigator will share the results with you along with any recommendations, if needed.