There isn't one. Every authority that sets a number says so plainly. What they publish are action levels, and Canada, the United States and the World Health Organization have each drawn that line in a different place.
The World Health Organization's handbook on indoor radon sets a reference level of 100 Bq/m³, and says that where national conditions make that unachievable it should still not exceed 300. Canada's guideline of 200 sits between those two figures. The United States Environmental Protection Agency acts at 4 picocuries per litre, which converts to about 148 Bq/m³, and suggests homeowners consider acting between 2 and 4 pCi/L, roughly 74 to 148.
Applied to the same Calgary homes Health Canada long-term tested, the three thresholds do not describe the same city:
Same houses, same measurements. The share of homes with a radon problem moves from one in seven to nearly three in five depending purely on whose line you apply. The Calgary map and the underlying numbers are here.
Radon causes lung cancer, and the risk rises with how much you are exposed to and for how long. There is no measured threshold below which the risk becomes zero, which is why every authority above describes its figure as an action level rather than a safety guarantee. An action level is the point at which fixing the problem is considered clearly worthwhile, not the point at which harm begins.
Health Canada's own guidance is that you should consider acting between 200 and 600 Bq/m³ within two years, and above 600 within one year. Lower is better at any reading.
Canada and most of the world measure radon in becquerels per cubic metre. The United States uses picocuries per litre. 1 pCi/L is about 37 Bq/m³. So 2 pCi/L is roughly 74, 4 pCi/L is roughly 148, and Canada's 200 Bq/m³ is roughly 5.4 pCi/L.
A long-term test, at least three months and ideally over the heating season, in the lowest lived-in level of the home. Short-term tests swing with weather, ventilation and how often the doors have been open, which is why every guideline above is written in terms of long-term averages. No map, including ours, can tell you about your own house, because radon varies enormously between neighbouring homes depending on how each building sits on and draws air from the ground beneath it.
Sources. World Health Organization, Handbook on Indoor Radon: A Public Health Perspective. United States Environmental Protection Agency action level of 4 pCi/L. Health Canada radon guideline of 200 Bq/m³. Calgary percentages calculated from Health Canada's published measurements, see the Calgary page for the method and sample sizes.