Experimental study of indoor and outdoor airborne bacterial concentrations in Tempe, Arizona, USA

Indoor and outdoor airborne bacterial concentrations were examined in an occupied office building equipped with an air conditioning (HVAC) system, in Tempe, Arizona, USA. A two-stage microbial air sampler was used to collect bacteria both inside and outside the office at fixed locations in space, and at regular time intervals from August to December, 2000. Simultaneous measurement of bacterial concentrations, temperature, relative humidity, light intensity and wind speed were performed to explore the effects of environmental factors on bacteria levels. Ten genera and twenty species of airborne bacteria were found in our samples. Airborne Gram-positive bacteria were most abundant, with more than 90% of the measured population. The particle sizes of most outdoor and indoor airborne bacterial aerosols were larger than 8.0 μm. According to the measurements of either a typical day or three consecutive days, the concentrations of outdoor airborne bacteria were highest in the morning, but declined in the afternoon and reached the lowest point in the evening. With regard to the effects of four environmental factors - temperature, relative humidity, light intensity, and wind speed, the relative humidity had the most pronounced influence on the outdoor bacterial concentration, with the number of bacteria increasing sharply on a day of high relative humidity.