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.