Canada

To my knowledge it's global, not just specific areas, however the levels vary. In the Midwest (USA) it's quite common to perform radon tests during a home sale, and homes with basements will frequently have a 'radon mitigation' system installed; basically a large fan draws the air out of the space beneath your concrete slab and air-tight plastic is installed in crawl spaces to form a negative pressure so the airborne radon is pulled out before it enters the home, and then exhausted up above the roof line to blow away like other exhaust from HVAC and appliances.

Have a system installed in my home to keep levels as low as possible, even though it tested below the 'dangerous' threshold when I purchased. (Better safe than sorry).

It is not canada specific. There are maps online like https://radonmap.com/ found another one here https://radonkit.ca/blog/radon-gas-map-for-canada-potential-radon-levels-across-canada/

I didn't think my area was high but all the maps I find say it is. Maybe time for a test...

Edit: fixed link.

I work for a health charity that does some work in the radon field. I won't rehash what others have said other than it's legit and relatively unknown. The only thing I'll add is that modern home construction makes it worse since buildings are much more sealed than they used to be which causes the radon to build up instead of escaping into the surrounding atmosphere.

Some (maybe all at this point) provincial building codes now require a rough in pipe opening in your foundation so you can install a mitigation system if levels become a problem in your house.

Yep, exactly.

I found this article that explains it well.

Radon gas can be found throughout the United States – literally from coast to coast. In fact, there is radon in every single state, although at different levels and not all amounts are considered dangerous.

Not to get too complicated, but radon gas is a byproduct of the element radium which ultimately comes from uranium. According to a report by the U.S. Geological Survey, uranium is the parent element and is found deep within the earth’s crust. It’s billions of years old. As time passes, it breaks down forming radium, the daughter element. When solid radium decays, it releases radon gas. All three are radioactive substances.

The reason radon gas is found everywhere is because uranium is everywhere – all rocks are composed of at least a trace amount of it. Over time, the rocks beneath the earth’s surface break down and turn into soil. So, if the deep rocks in the area contain uranium, it stands to reason that the soil at the surface will too.

The U.S. Geological Survey explains that radon moves easily and quickly through porous soils, like sand and gravel, and slower through more solid soils, clay being one such example. Moisture has a lot to do with this. In wet soil, like clay, it’s harder for the radon to permeate it, so the radon often becomes trapped and deteriorates before it reaches the surface. In dry, porous soils, radon gas is able to quickly escape into the air.

It can happen everywhere, but there is at least one case in Canada where the radon is coming from uranium mine tailings rather than just being there naturally. It's been in the news in northeastern Ontario on and off for months now.

Most parts of eastern Canada have comparatively high levels of Uranium in the ground - the radioactive decay process turns Uranium into Radon, which finds its way into buildings, and collects in basements because it's heavier than air. It's been made worse by the whole "R2000" building standard which means homes are practically air tight.

The worst part is that federal and provincial building codes don't make the installation of radon remediation equipment standard -- and it costs practically nothing to install at construction time -- whereas cutting trenches in your foundation to install PVC pipe is extraordinarily disruptive.