Canada

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.