Exposure to Radon, a naturally occurring radioactive gas that is one of the leading causes of lung cancer, has been linked to an increased risk of stroke, according to a new study.

Published Wednesday in the online issue of Neurology, which is the medical journal of the American Academy of Neurology, the study examined the association between home radon exposure and the risk of stroke among middle-aged and older women in the U.S.

Radon, a radioactive gas with no smell, taste, or color, is naturally produced from the breakdown of metals like uranium or radium in rocks and soil. Radon can make its way into homes and buildings through small cracks or holes, construction joints and gaps around pipes. As occupants of the household breathe in radon over time, radioactive materials get trapped in the lungs.

Radon causes about 21,000 lung cancer deaths in the U.S. every year, according to U.S. Environmental Protection Agency (EPA) estimates.

Results of the study published Wednesday do not prove that radon exposure causes stroke but show an association between the two.

As part of the study, 158,910 female participants aged 50–79 years at baseline (1993–1998) were involved, and 6,979 strokes were identified among participants for about 13 years.

Researchers determined how much radon the female participants were exposed to by using radon concentration data from the U.S. Geological Survey and EPA and linking them with the participants' home addresses.

As per EPA recommendations, average indoor radon concentrations should not exceed four picocuries per liter (pCi/L), and if the concentration is this high, households are recommended to have a radon mitigation system to lower radon levels in the home.

"Radon is an indoor air pollutant that can only be detected through testing that measures concentrations of the gas in homes," said study author Eric A. Whitsel, MD, MPH, of the University of North Carolina in Chapel Hill.

"Our research found an increased risk of stroke among participants exposed to radon above —and as many as two picocuries per liter (pCi/L) below—concentrations that usually trigger Environmental Protection Agency recommendations to install a home radon mitigation system," Whitsel added.

The researchers divided the participants into three groups based on average radon exposure — the highest group lived in areas with average radon concentrations of more than four pCi/L, the middle group lived in areas with average radon concentrations between two and four pCi/L, and the lowest group that lived in areas with average concentrations of less than two pCi/L.

Researchers found that there were 349 strokes per 100,000 person-years compared to 343 strokes in the middle group and 333 strokes in the group with the lowest exposure. Person-years represent the number of people that were part of the study and the length of time each individual was involved.

Participants in the highest group had a 14% increased risk of stroke compared to those in the lowest group and those in the middle group had a 6% heightened risk, researchers concluded. This was determined after factors such as high blood pressure, smoking and diabetes were adjusted.

"It's important to note that we found an increased stroke risk among those exposed to radon concentrations as much as two pCi/L below the current lung cancer-based threshold for recommending radon mitigation," Whitsel said. "More studies are needed to confirm our findings. Confirmation would present an opportunity to improve public health by addressing an emerging risk factor for stroke."