Health Canada wants homeowners in Thunder Bay to know many residences in the city could have high levels of radon.
The colourless, odourless, and tasteless gas is radioactive gas, and can cause lung cancer.
Radon was the focus of a workshop Thursday in the city.
"On average, seven percent of homes tested above the guideline,” said Kelley Bush, head of Health Canada's Radon education and awareness program.
“My understanding is Thunder Bay is significantly above that seven percent — in the 10-15 per cent range. So, it would be an area where there is a higher chance that you may have higher levels of radon."
Bush said radon testing is inexpensive, and should be completed by every homeowner.
She says it can creep into any home, in any part of the country.
A spokesperson with the Canadian Association of Radon Scientists and Technologists said people need to learn more about how air circulates and filters into a home.
"If I take Tim Hortons cup and I have a hold on the top of the cup and push it down into a bucket of water, but I don't let the lid go under, we all know the water's trying to get in, but most people don't think about the driving force,” Bob Wood said.
“There's pressure inside the cup, then the water around it. If I put a couple little holes in the cup, and we held it long enough, everyone would expect it to fill up.”
He said the same principle applies when a home is “stuck in the soil.”
“We know that soil gas is going to get into that home, if it has a basement,” Wood said.
“So, what happens is radon mitigation system - what we do is put in piping and fan system that changes those flows."