A buildup of radioactive gas could be going undetected in Oklahoma buildings. Radon is found nearly everywhere and is released from decaying uranium through rock, soil and water.

This month, residents of Oklahoma can order a free kit through the Department of Environmental Quality to test their homes for high amounts of radon. The U.S. EPA suggests making repairs to homes with levels 4 picocuries per liter or higher.

Although the gas poses little threat outdoors, radon can accumulate in houses and lead to significant health risks. According to the EPA, high levels of radon account for more than 20,000 lung-cancer-related deaths per year. But sealing foundation cracks or installing a ventilation system can reduce exposure.

An Oklahoma report from the American Lung Association using data from the Centers for Disease Control and Prevention shows Canadian County had the highest average levels of radon from 2008 to 2017. The data came from counties that completed more than 10 tests.

During the surveyed years, Oklahoma reported 814 tests in the state and ranked 25th for its radon levels.

The EPA classifies the state as having relatively low radon concentrations compared to other parts of the country. Most counties in Oklahoma are considered in the agency’s “Zone 3,” which represents the lowest levels.