The variability problem
Radon concentration inside a building is not a constant. It moves with barometric pressure, wind, temperature differential, soil moisture, frozen or thawed ground, whether windows have been opened, and how the heating and ventilation are running.
The swing is not subtle. The same house can read very differently across a week, and differently again between seasons. A measurement taken over two days captures whatever conditions happened to prevail across those two days.
That is why Health Canada recommends a long-term measurement of at least three months — long enough for the short-term noise to average out into something that represents the building rather than the weather.
What each test is for
A short-term result well above the guideline is a strong signal that a long-term test is warranted. A short-term result below the guideline is much weaker evidence, because the conditions during those few days may simply have been favourable.
Neither is a substitute for the other, and a professional will say which they are conducting and why.
- Long-term (three months or more, heating season): the measurement Health Canada recommends for deciding whether a home needs mitigation. This is the one that should drive spending.
- Short-term (typically two to seven days): a screening measurement. Useful for a quick indication, for post-mitigation verification, and in transaction timelines that cannot accommodate three months.
Where this bites in practice
Real estate transactions are the common case. A conditional period is measured in days, not months, so short-term testing is often the only option available inside the timeline. That is a legitimate use — provided everyone involved understands they are looking at a screening result rather than the measurement Health Canada recommends for a mitigation decision.
The reasonable outcome of an elevated short-term result during a transaction is usually not an immediate mitigation demand but an agreement about long-term testing afterwards, and about who pays for mitigation if it turns out to be needed.
Placement matters as much as duration
A correctly chosen device in the wrong place produces a number that means nothing. Devices go in the lowest level of the home used regularly, at breathing height, away from exterior walls, windows, doors, drafts, heat sources and areas of high humidity.
This is a substantial part of what certification is for, and it is the main argument against treating radon testing as a purely do-it-yourself exercise even though the devices are cheap and widely available.