we used portable air cleaners in two day care units (A and B, number of children participating in the study n = 43) and compared infection incidents between the two intervention units to the rest of the units in city of Helsinki (n = 607). … At day care centre A the average reduction was 60% (range 52% − 88%) and at day care centre B 53% (range 14% − 59%). … On average, the parents were absent from work due to child’s illness in reference day care centers for 5.53 days and 3.77 days in intervention day care centers during the study period (p=0.009). In relative terms the reduction was approximately 32%.
Hard to tell because I can’t find a news article linking the study, but I think https://www.sciencedirect.com/science/article/pii/S2950362024000043 is the Helskinki study.