The standard radon test is a 90-day passive cassette. You leave it on a shelf for a season, post it to a lab, and receive a number on a PDF.
The number is a season-long arithmetic mean. It is the regulatory output, but it is rarely the operationally useful one.
What the average hides.
- A bedroom that runs 60 Bq/m³ during the day and 480 Bq/m³ from midnight to 6 AM averages around 165. That figure reads as acceptable. The eight hours of sleep at 480 Bq/m³ are not.
- A ground-floor home office at 280 Bq/m³ during the working day and 90 at night averages 165 too. Same number. Different problem. Different fix.
- A loft conversion at 30 Bq/m³ pulls down a downstairs reading of 320 to a household average of 175. The cassette placed in the wrong room is reassuring; the actual exposure is not.
Why this matters.
UKHSA recommends action above 200 Bq/m³ averaged across the year. The risk model behind the threshold is dose-time — total exposure equals concentration multiplied by hours. Eight high-concentration hours during sleep can dominate annual dose without ever showing up on a daytime cassette.
What a continuous monitor sees.
- The morning peak in a closed bedroom. Radon accumulates while you sleep with the door shut.
- The drop the moment the door opens.
- The slow climb in unused rooms — guest bedrooms, store rooms, basements.
- The seasonal pattern. Radon ingress correlates with cold outdoor temperatures and the stack effect, so winter readings dwarf summer.
A passive cassette gives you a number to put on a form. A continuous reading gives you a curve to act on. Both are useful. Only one tells you which window to crack at 5 AM.