Smoking is the leading cause of lung cancer in Americans, as highlighted in the latest State of Lung Cancer report from the American Lung Association. The second leading cause is radon exposure.

A colorless and odorless gas, radon can seep into houses without notice — and can lead to lung cancer for even those who have never smoked. Any home is at risk for elevated levels of radon, and the U.S. Environmental Protection Agency’s action level of 4 pCi/L marks the interior level at which residents are at risk. In New Jersey, 9.8% of test results were at or above the recommended action level, according to the EPA.

The state with the lowest percentage of tests at or above the EPA recommended level is Hawaii, followed by Louisiana, Mississippi, Texas, California, D.C. and Oklahoma. New Jersey ranks 8th lowest for radon levels in the country. On the bottom tier, Iowa, North Dakota and South Dakota face the most harmful levels of interior radon gas.

Radon gas forms when uranium in soil or rock breaks down to form radium. As the gas decays, it releases toxic, radioactive byproducts. It can build up in homes or schools. The only way to know is to test for it with a radon test kit, easy to use and inexpensive.