Two different sampling methods were employed to measure a biological aerosol disseminated from a contaminated letter: slit samplers and a microfibre filter medium located at the inhalation port of a respirator canister worn by the individual who opened the letter. Measurements obtained from slit samplers may only be interpreted in terms of numbers of viable particles. Particles may be comprised of single or multiple organisms. Microfibre filter data are presented in terms of the number of colony forming units, which refer to individual viable organisms. We observed a significant difference between the number of viable particles and the number of colony forming units measured by these two techniques. The former was calculated to be 30975 Bacillus atrophaeus (BG) containing particles and the latter was determined to be 1072113 CFUs, assuming similar air flow sampling rates. At the simplest level, this suggests that each particle may contain 37 viable BG spores. It is far more likely however, that aerosolised BG consists of a distribution of particles of various sizes which, depending on the volume of the particle, would contain different numbers of spores. A model was developed to define a relationship between numbers of BG spore containing particle-aggregates measured by the slit samplers and numbers of individual viable spores comprising particles measured by entrapment on the microfibre filter medium. From this an estimate was derived of the total number of individual viable
