Gov. Jared Polis has designated January to be National Radon Action Month, amid estimates that the odorless, colorless gas is responsible for approximately 500 lung cancer deaths in the state each year.

“Testing your home for radon is simple and should be done when all your doors and windows are closed,” said Chrystine Kelley, radon program manager for the Colorado Department of Public Health and Environment. “That’s why January is a great time to test.”

Radon gas is naturally occurring and radioactive. While radon is generally harmless outdoors, it can become trapped indoors, where long-term exposure increases cancer risk.

According to the state’s health department, approximately half of Colorado homes have radon levels that exceed recommendations of the U.S. Environmental Protection Agency. The department has a list of resources for people interested in testing their homes.

Last month, CDPHE announced the winners of a radon poster contest, with all three winners being from Fowler Junior High School in Otero County. A student identified as Rylie took first place, winning $300. Students Madison and Devine won second and third place, respectively.