The microbiology of air inside enclosed spaces

Microorganisms normally found in the outside air will also be present inside buildings where they are introduced by air currents, but the main sources of contamination arise from within the buildings [1]. 

A variety of indoor sources of microorganisms have been observed. The following data were taken from the Gregory s book [3], where references may be found. 

Penicillium dominates the air inside most houses, in contrast to Cladosporium outside. Bacteria tend to be more abundant indoors in winter than in summer. In densely populated rooms in schools and mills, the stirring up of dust increased the ratio of bacteria to moulds in the air. Shaking beds, brushing carpets, and repair work increase the mould-spore content of the air up to 17 times, but it rapidly returns to normal when the activity ceases. 

Recommendations for the maximum tolerable number of particles carrying bacteria in operating theatres are 700 per cubic metre for minor operations, and down to 70 or even 15 per cubic metre for dressing burns 

Special attention has been given to microorganisms present in the air of underground railways in London and New York in the first quarter of the century [3], and more recently in the Hungarian capital of Budapest [10]. 

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