Basements read highest because they are closest to the source and furthest from fresh air. That does not make a basement unusable, and it does not mean the rest of the house is fine.
Radon comes out of the ground. The basement is the part of the house in contact with the ground, so it is where soil gas arrives first and where concentrations are highest. It is also the least ventilated part of most houses, so what arrives tends to stay.
On top of that, warm air rising through the house creates a slight suction at the bottom, which actively pulls soil gas in through cracks in the slab, gaps around pipes, sump pits and unfinished floor sections.
Finishing a basement does not create radon, but it can change things in two ways. It often reduces air movement, and it covers up the entry points, which makes them harder to find and to seal later. It also, crucially, means people now spend hours a day in the part of the house with the highest concentration, which is what turns a measurement into an exposure.
If you are about to finish a basement, test first. It is far cheaper to run a mitigation pipe before the walls go up.
Upstairs will usually read lower, often much lower. That does not make the basement reading irrelevant, because what matters is where you actually spend time. A basement bedroom or home office at 400 Bq/m³ is a real exposure even if the main floor reads 120.
Test the lowest level you use regularly, and judge it on that.
Sealing cracks and covering a sump pit reduces entry a little and is worth doing, but on its own it rarely brings a high reading down to where you want it. The reliable fix is sub-slab depressurization, which vents the soil gas before it can enter at all.
Basement dehumidifiers, air purifiers and more insulation do not reduce radon.
No, but do get it fixed. Radon risk builds over years, so the sensible response is to arrange mitigation rather than to abandon a room. If the basement is a bedroom, that is a reason to move it up the priority list rather than a reason to panic.
Similar, and often worse, because a dirt-floor crawlspace has no slab at all between the soil and the house. Mitigation for a crawlspace usually involves sealing a membrane over the floor and venting beneath it, which is a slightly different job.
An open sump pit is a direct opening into the ground and is a common entry point. Sealing it with a proper airtight lid is one of the standard things a mitigation system includes.
No. Radon is a gas and is unrelated to humidity. A dehumidifier makes a basement more pleasant and does nothing to the radon in it.
Sources. Health Canada radon guidance and published measurement data; the 2024 Cross-Canada Survey of Radon; World Health Organization Handbook on Indoor Radon; United States Environmental Protection Agency; Canadian National Radon Proficiency Program. Costs described are general ranges, not quotes.