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

Indoor and outdoor airborne bacterialconcentrations were examined in an occupiedoffice building equipped with an airconditioning (HVAC) system, in Tempe, Arizona,USA. A two-stage microbial air sampler wasused to collect bacteria both inside andoutside the office at fixed locations in space,and at regular time intervals from August toDecember, 2000. Simultaneous measurement ofbacterial concentrations, temperature, relativehumidity, light intensity and wind speed wereperformed to explore the effects ofenvironmental factors on bacteria levels. Tengenera and twenty species of airborne bacteriawere found in our samples. AirborneGram-positive bacteria were most abundant, withmore than 90% of the measured population. Theparticle sizes of most outdoor and indoorairborne bacterial aerosols were larger than8.0 μm. According to the measurements ofeither a typical day or three consecutive days,the concentrations of outdoor airborne bacteriawere highest in the morning, but declined inthe afternoon and reached the lowest point inthe evening. With regard to the effects offour environmental factors – temperature,relative humidity, light intensity, and windspeed, the relative humidity had the mostpronounced influence on the outdoor bacterialconcentration, with the number of bacteriaincreasing sharply on a day of high relativehumidity.