Persistence of aerosols in a closed space

An aerosol particle suspended in a parcel of still air is acted upon by gravity and when the rate of fall is such that the gravitational forces acting on the 

In still air, particles with a diameter greater than 6 pm will settle out rapidly but particles below this size are readily kept in suspension by the smallest air currents. 

Problems associated with surface contamination were reviewed in Surface Contamination. Keruluk et al., who were contributors to the book, provided estimates of the airborne concentration arising from known contamination densities on the floor. They concluded that the air filtration systems in clear rooms considerably reduce the microbial contamination in the air over that present in non-environmental areas but the amount of microbial surface contamination, particularly at the bench level and on the floors, was in many instances as high as in non-environmental areas. 

Ventilation theory provides a simple means of estimating the consequences of the release of material inside a building. If a building of volume V m is ventilated with a volume of v m of fresh air per second, the ventilation rate or the number of air changes which occur in one hour in the space (N) = 3600v/y. The ventilation rate for typical structures is given below. 

For a given supply of air the ventilation rate depends mainly on the size of openings, but a change of external wind speed from 5 to 15 mph can increase the ventilation rate four-fold. A strong temperature gradient has a much smaller effect. 

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