If your question is about general locations in Ontario where there is less potential for radon the answer is “no-one knows for sure”.
Radon Environmental Management Corp., based out of British Columbia, is an environmental health and building sciences company and recognised leader in raising radon awareness and education. They are a provider of radon maps and mapping services and using the data collected from the Government of Canada radon surveys and mining studies, have produced a map of potential for radon. Their mapping services cover the whole of Canada, but you can find their radon potential map of Ontario at their website here.
For educational purposes, we have provided a smaller version of the Radon Environmental Management Corps, map here.
The problem with this map is the data it’s based upon. While the areas that are potentially high in radon can be considered fairly accurate, based upon the soil and subsoil makeup, areas that are mapped as having a lower potential are mapped that way because of the lack of relevant scientific data either about the geological structure of the earth’s crust or from relevant radon measurements.
The maps will become more accurate as testing increases across the province, but until then, it is safer to assume that choosing to move to a specific area of the province will not significantly reduce your risks.
If your question is about choosing a home, then the answer is somewhat different. If the home you are selecting to purchase has been tested for radon and either found to have a low level or has been fitted with a radon mitigation system, then these would be considered “safer” than a home that is completely unknown.
If the home you are looking at does not have a radon mitigation system installed or has not been recently tested, then we strongly recommend that as part of any purchase agreement you request that an OntarioACHI Inspector provided, OCRMI short-term measurement is carried out.
If the measurement returns with an unfavourable result, take steps to have it made safer by mitigation from a C-NRPP certified professional remediator. This will ensure the home is not just safer for you and your family but adds value to the property others in the area may not have.