Granite and radon the counter is visible, the slab is the source
Granite sometimes contains a little uranium, and therefore a little radon, like many stones. In a ventilated kitchen, that contribution is small next to the gas entering through the slab. Health Canada treats building materials as a possible source, and the soil as the usual one.
A report of 400 Bq/m³ in a basement bedroom is not fixed by changing the main-floor counter. Measure the bedroom. If the number is high, pull under the slab. The granite story circulates because the stone is visible and the soil is not.
Reference points: Health Canada, what you need to know about radon and the 200 Bq/m³ guideline. The two-year timeline between 200 and 600, and one year above 600, comes from Health Canada’s guide for radon measurements in homes.
The 4-day test gives a first reading. The 91-day test covers the three-month minimum Health Canada asks for. Both are $59 before tax: shipping, return and the laboratory are included, taxes extra. The PDF report is sent within 2 business days after the laboratory receives the detector.
Above 200 Bq/m³, mitigation is confirmed with a quote. Gaz Radon is C-NRPP certified for measurement and mitigation.
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