Why radon is a winter problem in Finland
It isn't bureaucratic fussiness that radon is measured September to May. The building physics of a Finnish winter is the reason.
Radon enters a building because the air inside sits at a slightly lower pressure than the soil gas beneath the slab. That small, persistent pressure difference quietly draws radon up through cracks, joints, and service penetrations.
Enter the stack effect
In a Finnish winter the inside of a building is far warmer than the outside. Warm air rises and escapes near the top of the building; to replace it, the lower floors pull air in - and some of that make-up air is drawn from the ground. This “stack effect” is strongest when the indoor–outdoor temperature gap is largest, which is precisely the cold months, so indoor radon is typically much higher in winter than in summer. [4]
Which is why the calendar is fixed
STUK requires measurements to run between 1 September and 31 May, for at least two months, and then multiplies the result by a seasonal correction factor of 0.9 to estimate the true annual average. [5] A summer-only test would understate your real exposure, and it isn’t valid for compliance. [5]
The practical takeaway
Plan around the season. If you’re reading this in summer, the move is to book now so detectors go in the day the window opens - not to test in July and get a number that means nothing.