Estimation of Real-Time Viable Concentration With a Real-Time Optical Particle Counter

Kline J, Dabisch PA

Oral presentation and accompanying poster presented at the Aerobiology in Biodefense IV Conference, Glen Allen, VA.

Knowledge of the viable aerosol concentration of a microorganism within an exposure chamber is key to
a dynamic dosing system. Concentration growth and decay within a chamber ideally follows a binary
model, where the steady state concentration and the decay back to zero are reached instantaneously.
Unfortunately, this is difficult to achieve for larger volume chambers and those with relatively low
airflow. This study will explore the use of electronic particle samplers for modeling non-ideal aerosol
profiles, and calibrating these profiles to represent viable particle concentration.

For this study a 19-L dynamic airflow head-only exposure chamber was used. Bioaerosol concentration
was determined using 25-mm gelatin filters, and all-glass impingers (AGIs). Particle concentration data
were obtained using an APS Model 3321. Viable concentration samples for two different bacteria –
Bacillus anthracis and Burkholderia pseudomallei - were collected for the aerosol generation period, and
were used to normalize the average APS particle concentration to viable concentration.

Results and Conclusions
Starting and sampling concentration ratios were used to normalize the APS particle concentration to
viable particle concentration. The degree of variability was largely a result of variability in the bacterial
assay (average CV=26.9%) as opposed to variability in the APS profile (average CV =10.8%). These data
suggest that there is variability intrinsic to either the aerosol sampler or the bacterial assay, and not the
aerosol generator and subsequent atmosphere. The sources of this variability need to be identified
before the particle concentration data from the APS can be accurately calibrated to reflect the viable
concentration present.