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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 relatives, humidity upon the number of airborne colony forming units per m3 (cfu/m ) in six schools. The airborne bacteria found were mainly non-pathogenic with a few pathogenic bacteria appearing in December, January and February. The number of cfu/m3 were mainly a function of occupancy. In non-occupied periods the number of cfu/m were in the range of 40 to 70, this increased 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 were effective in removing the bacteria from air and could be used to minimize airborne bacteria in buildings. The number of classroom airborne bacteria were 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 non-pathogenic and the pathogens may have a different survival pattern with relative humidity. In general, the level of airborne bacteria in the classroom were mainly a function of occupancy and air recirculation rate through a filter.