I wouldn't call this a statistically significant result, and the fact that the buildings in question had good mechanical air ventilation could very well explain the results:
> "Utilizing the model, 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). The intervention buildings had mechanical supply and exhaust air ventilation."
You also have to consider external air quality, as ventilating a building with polluted air would have negative effects like increased asthma. In that case perhaps a sealed building with air purifiers is a better option, but then CO2 buildup is a concern, so you'd need CO2 scrubbers, which are expensive.
That's why clean air regulations matter, and getting off fossil fuel combustion as an energy source (and limiting pesticide/herbicide use in agricultural zones) is the easiest route forward.
> "Utilizing the model, 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). The intervention buildings had mechanical supply and exhaust air ventilation."
https://www.sciencedirect.com/science/article/pii/S295036202...
You also have to consider external air quality, as ventilating a building with polluted air would have negative effects like increased asthma. In that case perhaps a sealed building with air purifiers is a better option, but then CO2 buildup is a concern, so you'd need CO2 scrubbers, which are expensive.
That's why clean air regulations matter, and getting off fossil fuel combustion as an energy source (and limiting pesticide/herbicide use in agricultural zones) is the easiest route forward.