There’s a chance your home contains elevated levels of an odorless, colorless radioactive gas that is the country’s second-leading cause of lung cancer.

The culprit? Radon. Experts in its mitigation recommend that homeowners test for it every two years. If you already own a radon-reduction system, it’s a good idea to periodically test that it’s working.

Radon develops underground when uranium and other radioactive elements decay. It can enter your home through cracks and crevices, the water supply, construction joints, gaps in suspended flooring and unsealed spaces around service pipes. All homes are susceptible, especially those with basements and water wells.

The U.S. Environmental Protection Agency says radon causes up to 20,000 lung cancer deaths a year, second only to smoking.

Ongoing exposure can also cause asthma, bronchitis, pneumonia and other illnesses. Smokers exposed to radon increase their already significant health risks.

The EPA estimates that one in 15 homes has an elevated radon level. Radon is measured by picocuries per liter of air (pCi/L). A picocurie is one-trillionth of a curie, which has the radioactivity of one gram of radon. The EPA says a typical home has a radon reading of 1.3 pCi/L, with a level of 4 pCi/L or above considered dangerous.

You can test for radon yourself, with kits available at hardware stores for around $10 to $30 or available for free from some local utility companies.

A short-term test occurs over several days. A long-term test runs over three to 12 months, which ultimately can be more accurate and useful, given that radon levels fluctuate over time.