It is generally conceded that aerosols are the primary means by which infectious diseases are contracted or spread in the microbiological laboratory, although many are known to have occurred from animal bites, needlesticks, and similar situations where direct contact can occur.
Laboratory-associated infections represent an occupational hazard for all personnel working in institutions where infectious disease agents are handled. Of the nearly 4,000 reported laboratory associated infections, just over 4 percent proved fatal. Less than 20 percent of the infections could be attributed to some known accident, such as accidental inoculation. Although not directly proven, aerosol production could account for a large part of those accidents listed as due to "unknown causes." The majority of laboratory-associated infections occurred at institutions engaged in research or clinical diagnostic work. It is interesting to note that the personnel involved were generally trained professionals, those individuals most knowledgeable about laboratory hazards.
There are many opportunities for aerosols to be generated through normal laboratory procedures. The number of organisms sufficient to cause infections in humans can be present in a single droplet. Studies have been conducted to determine the average number of droplets created by many typical operations, and it was found that some procedures are prolific aerosol generators. Some of the laboratory operations which release a substantial number of droplets seem almost trivial in nature, such as breaking bubbles on the surface of a culture as it is stirred, streaking a rough agar place with a loop, using a vortex to mix a liquid and then removing the cap too soon, a drop falling off the end of a pipette, inserting a hot loop into a culture, pulling a stopper or a cotton plug from a bottle or flask, taking a culture sample from a vaccine bottle, opening and closing a Petri dish, opening a lyophilized culture, etc. Most of these only take a few seconds and are often repeated many times a day. Other more complicated procedures might be considered more likely to release organisms into the air, such as grinding tissue with a mortar and pestle, conducting an autopsy on a small animal, harvesting infected tissue from animals or eggs, intranasal inoculation of small animals and opening a blender too quickly.
Some incidents have occurred by failing to take into account the possibility that accidents can happen, such as a tube breaking in a centrifuge. The possibility of aerosol production should always be considered while working with infectious organisms.