The Effect of Sampling Time on the Overall Performance of Portable Microbial Impactors

In this study we analyzed the sampling time effect on the overall performance of seven portable impactors (SMA MicroPortable, BioCulture, Microflow, MAS-100, Millipore Air Tester, SAS Super 180, and RCS High Flow) as well as a stationary impactor (BioStage) when collecting airborne bacteria and fungi from 2 to 30 minutes indoors and outdoors. Their performance was compared against the reference BioStage impactor collecting samples for 2 minutes. When bacteria were collected outdoors, the average concentration ratio of all test samplers relative to the reference sampler was 0.64, but it decreased to 0.04 when bacteria were collected for 2 min first and subsequently exposed to particle-free air for 28 min (30 min total sampling time). For fungi, the average concentration ratio decreased from 0.35 to 0.03 under the same conditions. When the test impactors sampled particle-free air first and then collected bioaerosols outdoors for 2 min, the concentration ratio for bacteria averaged for all test samplers decreased from 0.77 (agar plates not exposed to particle-free air and particles collected for 2 min) to 0.04 (agar plates exposed to particle-free air for 28 min first and then particles collected for 2 min). For fungi, the relative recovery averaged for all samplers decreased from 0.45 to 0.03 under the same conditions. The effects of sampling time and impactor model were statistically significant for all experimental conditions. Thus, the data obtained in this study suggest that when impactors are used for the collection of airborne bacteria and fungi, sampling times should be as short as reasonably possible to minimize the under-representation of airborne microorganism concentration which could be a factor of 10 or higher for prolonged sampling times.