The bacteria referred to are implicitly ambient, or environmental bacteria. Complete air mixing will delay the removal of airborne pathogens in an exponential manner. This represents the limiting case for normal buildings, and is a reasonable and simple model to use for evaluating the removal rate of airborne pathogens. Given the assumption of complete air mixing, the primary factor determining the removal of airborne pathogens is the air change rate. Airborne pathogen hazards are dependent on the species of microbe.
Some microbes are extremely lethal at very low doses.
Some guidelines exist for levels of airborne fungi, and these are used as general indicators. The actual amount of outdoor air that can be economically brought in to a building can depend heavily on ambient conditions. In mild dry climates, large volumes of outdoor air can be used to purge a building continuously with little added cost. In hot, dry climates it is possible to use two stage evaporative coolers to recover a large fraction of exhaust cooling and thereby bring in outdoor air at possible high volumes for a much reduced cost. Toxicity of open air to a variety of microorganisms.