How the continuous measurement works
Instead of a passive canister that integrates continuously for two months, a working-hours measurement uses a continuous, electronically-logging instrument placed for at least seven days. It records the radon concentration through the day and separates the hours the space is genuinely in use from nights and weekends. [6] STUK defines a formula that turns those readings into an annual working-hours average - the number that actually reflects occupational exposure. [6]
Why ventilation can distort a canister result
Many workplaces run ventilation only during the day. A passive canister integrates over nights and weekends too, when the ventilation is off and radon builds up, so it can overstate the exposure workers actually receive - a passive result can fail even when the air is fine while people are actually present. [6]
Is it accepted by STUK?
Yes. STUK accepts a continuous working-hours measurement of at least one week, together with its defined formula, as valid evidence of the annual working-hours average. [6] Because of that, a space that failed the two-month passive test can often demonstrate compliance without a structural fix: if the concentration during actual working hours stays below 300 Bq/m³, no remediation is required - and in roughly half of failing workplaces with timed ventilation, that is exactly what the working-hours measurement shows. [6] It does not replace the obligatory statutory measurement, though: the passive alpha-track canister remains the legal baseline that is filed for compliance. The working-hours measurement complements it.
What you receive
A clear report: the working-hours concentration set against the off-hours readings, the annual working-hours average computed to STUK's method, and a plain-language read on where you stand against the 300 Bq/m³ reference level. [5] If the number is over the limit, we advise the cheapest legitimate next step - not automatically a remediation.