Posted By Ian Blenkharn Its a huge and highly complex problem. Aerobiology is often dismissed as a simple and straightforward issue but is really very complex. What if the source of the pathogens you anticipate? And why Salmonella? It's not a respiratory pathogen. If you are worried about fall-out from air onto foods, cover the food or protect the environment. If its a general health issue, consider also the 'normal' flora of air, and the problem of fungal material that can illicit allergic respiratory symptoms that are exacerbated by exposure to other respiratory irritants, including dusts. Dust particles fall out of still air at a rate that is dependent on their mass. Smaller particles remain suspended for far longer periods, particularly in turbulent air. Droplets rather than dry dusts are the most common source of micro-organisms in air - but really does depend on your circumstances. Droplets may carry micro-organisms and these can remain suspended almost indefinately in turbulent air as evaporation reduces them to micronuclei. Even with significant pathogens, dilution helps considerably. Bets approach is as thorough review of your situation, and most probably direct sampling of air. This would need to be done on a number of different occasions to take accout of sampling error, and background variation due to season and weather conditions. Usual approach is to sample across a site and the circumferentially to evaluate what you are shedding to the environment at large and what you pick up from upwind sources. Feel free to get in touch if you want to discuss in detail. Ian

Posted By Ian Blenkharn Geoff I very much doubt that the data you seek is available in the public domain, most of it will be in the relevant textbooks and journals, and as a microbiologist dealing on occasions with aerial dispersion of pathogens and methods of containment I must say its not a very exciting topic so much of the data is now quite old - funding for further research is scarce but the data is stil valid. For what its worth, an individual Salmonella cell is about 0.5 x 2 micron Still puzzled about your concerns of Salmonella. Are you worried about an animal rearing facility, or perhaps fallout from sewage treatment. If that is the case, then bacteria may be a convenient marker but the real concern will include virus particles that are at least an order of magnitude smaller and behave completely differently in air. Ian