Air is probably more frequently regarded as a vector of microbiological contamination than as a primary source. It is not a nutritive environment; microorganisms do not grow and multiply in air. Many microorganisms die in air. Anaerobes die as a result of oxygen toxicity but, more generally, aerobes die as a result of desiccation. Photosensitivity may also play a part in inactivating certain bacteria in air.
All natural air is microbiologically contaminated. Most microbiological contamination is associated with nonviable particles in the air such as dust, skin flakes, etc. Nonviable particulate matter is both a source of microorganisms and also a means of protecting microorganisms from death by desiccation.
The most likely types of microorganisms traceable to air are those with mechanisms to resist desiccation such as Bacillus spp., Micrococcus spp. and fungal spores, which have evolved to be dispersed in the air.
The most likely sources of Bacillus spp. are from excavation or building work, where soil or dust is disturbed. Micrococcus spp. may also survive in soil or dust, though they are more likely to be of human or animal origin.
The primary means of controlling airborne contamination in pharmaceutical manufacture is by the use of clean rooms or, in more critical cases, isolation technology. The manner in which clean rooms must be designed and operated effectively also places restrictions on contamination from other sources such as people, materials and water. In clean rooms and in isolators, contamination from air is controlled by a number of different mechanisms — based on filtration, dilution, pressure differentials and air flows. Further detailed information on these may be obtained from standards such as IS 14644 (Cleanrooms and Associated Controlled Environments) and in general texts such as those of Whyte (1991) and Wagner and Akers (1995).
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