Estimation of viable spores in Bacillus atrophaeus (BG) particles of 1 to 9 μm size range

This study examined the viable organism content in a bioaerosol disseminated from a letter contaminated with Bacillus atrophaeus (BG) (anthrax simulant). In addition to using nutrient medium based slit samplers to sample the bioaerosol in the air space where the dispersion occurred, the individual who opened the contaminated letter was also used as a bioaerosol sampler by incorporating an external filter trap positioned over the inlet of the respirator filter canister worn by the individual. The reasons for this latter approach should be clear: it would provide actual quantitative information on the inhalation exposure hazard to an individual faced with this scenario. Where bioaerosol particles may be comprised of aggregates of many smaller particles, organism viability within the aggregate is important and may have an impact on the current definition of 'infectious dose' or 'lethal dose'. Both terms assume single organism counts. Results from two different methods of measuring aerosols due to opening contaminated letters are presented: slit samplers and glass microfibre filter medium placed on the inlet port of a respirator cartridge. The total number of viable bioaerosol particles measured at the inhalation port of the filter canister was 40 times greater than measured by the slit samplers. Accordingly, this may be accounted for by a distribution of bioaerosol particles comprised of aggregates of single BG spores. A model has been proposed that estimates the number of individual viable spores in a distribution of aerosol particles through a relatively simple probability density function distribution for particle size, and which incorporates realistic assumptions regarding particle diameters, packing density and spore viability.