LOUISVILLE, Ky. (WDRB) -- Louisville's health department is offering free radon testing kits.

Radon is a gas you can't smell, taste or see, but it's the second leading cause of lung cancer in the U.S, behind smoking.

According to a press release from the city's health department, radon "forms naturally when uranium, radium and thorium break down in rocks, soil and groundwater. People can be exposed to radon primarily from breathing radon in air that comes through cracks and gaps in homes and other buildings. Radon can cause lung cancer through prolonged exposure."

The entire state of Kentucky is at a high risk for radon exposure, with about 40 percent of homes estimated to have unsafe levels.

The only way to know if you have radon in your home is to test for it.

"Radon can exist at dangerous levels in homes in Louisville, but people may not know because it's a gas that you can't smell or see," said Dr. Sarah Moyer, director of the Louisville Metro Department of Public Health and Wellness, in a statement. "We encourage people to take advantage of the free test kits to test their homes for radon."

The free radon kits will be available while supplies last. To request a kit, call 502-574-6650, or CLICK HERE.

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