Unidentified open-air factors (OAFs) found to be adverse to the survival of microorganisms suspended on microthreads were investigated for their effect on realistic aerosols of Francisella tularensis in an open-air environment. This organism was chosen because it is probably the most infectious organism known to be capable of infecting both animals and man via the respiratory route, hence its potential use as a bioterrorist agent. A direct correlation was found between an open-air adverse effect on viability and virulence of airborne particles of <3 μm via the respiratory route in guinea pigs. One viable organism was sufficient to initiate an infection that resulted in a fatal tularaemia infection. The lethal effect of OAFs on F. tularensis was found to vary from day to day and was related to the source of the air in the UK. The adverse effect on viability was associated with an inverse effect according to the size of the airborne particle.
