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It is hypothesized that the increase in respiratory diseases in winter is caused by the reduced indoor relative humidity increasing the survival time of airborne bacteria and viruses. This paper reports the effect of ventilation and relative humidity upon the number of airborne colony-forming units per m (cfu/m3 ) in six schools . The airborne bacteria found were mainly nonpathogenic witha few pathogenic bacteria appearing in December, January, and February. The number of cfu/m3 were mainly a function of occupancy: in nonoccupied periods, the numbers of cfu/m3 were in the range of 40 to 70, which increasesd to 300 to 700 as soon as the students entered the classroom. The average level of the cfu/m3 was largely a function of the air recirculation rate, because the filter removed 90% of the airborne bacteria. Ordinary filters are very effective in removing bacteria from air and could be used as a protection from sources disseminating pathogenic bacteria in ventilating systems. The number of classroom airborne bacteria was reduced slightly as the relative humidity decreased, which does not support the hypothesis proposed for the increase of respiratory illnesses in winter. They were, however, mainly nonpathogenic, and the pathogens may have a different survival pattern with relative humidity. In general, the level of airborne bacteria in the classroom was mainly a function of occupancy and air recirculation rate through a filter.

Units: SI