Role of Aerosols and Their Significance

Laboratory studies of potential sources of infection have focused on hazards associated with routine microbiological techniques.  Considerable information has been accumulated to indicate that nearly all routine bacteriological and virological procedures are capable of producing aerosols.

The aerosol generation and dispersion characteristics of laboratory procedures has been studied using concentrated bacterial suspensions. Basically a series of air samplers were positioned around an area where the bacterial cultures were poured, pipetted, transferred, mixed, etc., and the bacteria that were released to the air were collected.   From these studies the "spray factor" (the number of organisms released by the technique divided by the initial concentration of organisms) was determined. This spray factor was then used to characterize the aerosol production capacity of laboratory techniques. For example, if the starting concentration was 10⁸ organisms per milliliter, and a lab procedure, e.g., blending, liberated 1.5 x 10³, then the spray factor was 1.5 x 10 ^-5.  The larger the number of organisms released as an aerosol, the larger the spray factor.  Spray factor then could be used to characterize the biohazard potential associated with laboratory techniques

Typical data on the number of viable particles recovered within two feet of a work area are presented in the following Table.

Concentration and Particle Size of Aerosols Created During Representative Laboratory Techniques

These data are based on an extensive series of air sampling determinations.

The simple presence of organisms in the air is insufficient to cause disease.  To initiate a respiratory infection, infectious aerosols must be deposited and retained within the respiratory tract. 

Studies have shown that particles greater than 5 µm range are retained in the upper respiratory tract while particles less than 5 µm range deposit in both the upper and the lower tracts.

Droplets less than 5 µm in size lose their liquid content rapidly when airborne.  These dried particles are called droplet nuclei, and they can remain suspended in air for long periods and can be carried about the room and building by both convective air currents (39) and building ventilation systems. 

The information presented in Table above indicates that standard laboratory procedures generate aerosols that are respirable and therefore potentially hazardous to the investigator and to other personnel in the vicinity.