Public health officials in Thunder Bay, Ont., say they hope expanding a health unit initiative to get people to test their homes for a radioactive gas in locales other than the city will shed more light on exactly where radon is most concentrated.

Last month, the Thunder Bay District Health Unit made more free radon testing kits available to homeowners in Oliver-Paipoonge and Marathon. That followed a study done by the health unit in 2014 that looked for the presence of the gas in homes in Thunder Bay.

"Some parts of the city, like Westfort for example, we found no homes with high radon levels, and then others, like McIntyre ward, nearly half the homes had high radon levels," said Lee Sieswerda, the health unit's manager of environmental health told CBC News. "Radon risk varies quite a lot."

With the new rural study — that people can sign up for on the health unit's website — Sieswerda said officials are hoping to paint a more localized picture of where the gas accumulates.

Marathon was chosen because it sits at the far east of the health unit's coverage area, Sieswerda said, while Oliver-Paipoonge was selected due to its proximity to Thunder Bay's McIntyre ward — where high concentrations of the gas were found — and extends south closer to where low radon levels were detected.

"If everything goes well and we get a good distribution within Oliver-Paipoonge and people return [the kits] and the study goes well, we'll try to do two estimates," he said.

Radon is a naturally-occurring, but cancer-causing gas which forms due to the breakdown of uranium in the ground. Sieswerda said radon is invisible and can't be detected by smell or taste. He added it is the leading cause of lung cancer among non-smokers.

150 kits have been allocated for Marathon, while 220 are available for Oliver-Paipoonge.