Abstract
Multiple-jet impactors, typically with 200 or 400 holes, are used widely for collecting aerosols of living bacteria and fungi. In this type of impactor, the air jets impinge directly onto nutrient agar in a petri dish which is incubated after sampling until collected cells multiply into colonies. The observed number of colonies can be adjusted for the probability that more than one viable particle was collected through a sampling hole and merged with other microorganisms at an impaction site to produce a single colony. A ‘positive-hole’ correction table has been published for a 400-hole impactor, but none has been produced previously for the 200-hole impactor. The expected number of sampled particles required to fill each of 1 through 200 and 1 through 400 impaction sites and the standard deviations of these values were calculated from probability theory. The results were compared with a Monte Carlo simulation. By using correction tables (which include the standard deviation of an expected value) an investigator can report the most probable viable particle count and a 95% confidence interval (mean ±2 standard deviations). The range of collected particles that could have produced an observed number of colonies increases as the number of collected particles increases, and investigators should acknowledge the uncertainty associated with adjusted counts. It is advisable to use an impactor with the greatest practical number of sampling holes because this decreases the likelihood that multiple particles are deposited at the impaction sites. The assumption that all jets in a single-stage, multiple-jet sampler are equally likely to collect particles was tested experimentally with an aerosol of bacterial spores and found to be reasonable.