Operation time of the ventilation system as a cause of microbial contamination of the infiltration filter

The study monitored two similar ventilation systems with fine filters during a few months, one operating continuously, one only during day time. Air, surface and filter material samples were taken to evaluate the extent of microbial contamination with viable fungi and bacteria. Filter and air of the permanent system,with higher relative humidity, were less contaminated by microbes and the contamination was more evenly spread in the filter.