Due to scheduled mechanical ventilation, the radon concentration at workplaces can be significantly lower during working hours than in the evening or at night. This is further affected by the level of air exchange and any separate exhausts - for example, in toilets and staircases. If the building has a scheduled ventilation system and the result of a long-term screening measurement exceeds the reference value, it is advisable to measure the radon concentration at the workplace during working hours to find out the variation between night and day.

The figure below depicts a significant variation between day and night time radon concentrations at a workplace with scheduled mechanical ventilation. The radon concentration measured in a screening measurement of two months was 530 becquerels in one cubic metre (Bq/m³). After this, the radon concentration during working hours was measured for one week. During that week, the radon concentration during working hours was 80 Bq/m³ and the average radon concentration for the whole week was 480 Bq/m³. The highest radon concentration measured was 870 Bq/m³.

Variation in radon concentration at the workplace during night and day time caused by scheduled ventilation.

The STUK regulation on practice that causes exposure to natural radiation provides a formula for calculating the annual average radon concentration during working hours:

where C_TK is the average radon concentration during working hours within the evaluation period, C_VK is the average radon concentration within the evaluation period and C_RV is the annual average radon concentration calculated by multiplying the box screening measurement (C_p) times 0.9. The aim of this formula is to account for the possible variation in the radon concentration between different weeks.

Radon concentration during working hours must be measured with a method approved by STUK (found on this page in Finnish).