XII 



THE AIR-SPORA OF ENCLOSED SPACES 



A SMALL but important fraction of the atmosphere is walled-in and pro- 

 vides microbes with an environment different from the outdoor world. 

 Indoor air-hygiene is an aspect of medical science with a voluminous 

 literature which can be approached through such works as: Aerobiology 

 (Moulton, 1942), Studies in Air Hygiene (Bourdillon et ciL, 1948), Airborne 

 Contagion and Air Hygiene (Wells, 1955), and Mould Fungi and Bronchial 

 Asthma (Werff, 1958). The brief treatment given here of 'intra-mural 

 aerobiology' presents an ecological instead of a medical viewpoint. 



Outdoor air moves as wind flowing bodily over surfaces, and a point 

 near the ground is immersed in a continually flowing stream of fresh air. 

 Rooms, on the other hand, are ventilated, and fresh air is assumed to mix 

 thoroughly with the existing air instead of displacing it bodily. By one 

 'air-change' ('ventilation turn-over') is meant the introduction of a 

 volume of fresh air equal to the volume of the room; an equal volume of 

 mixed stale and fresh air is displaced during the process, leaving a mixture 

 of stale and fresh air in the room. Unless continually renewed, any micro- 

 bial concentration in the air of an enclosed space will tend to diminish 

 with time as a result of ventilation and deposition. Concentration of viable 

 organisms will also decrease with time — following the natural death-rate, 

 or because of any disinfection that may have been applied. 



Die-away of Concentration 



Die-away of concentration is a phenomenon seen most clearly under 

 intra-mural conditions, because out-of-doors a concentration is carried 

 away bodily by wind. So far it has not been feasible to trace concentration 

 changes out-of-doors in one air mass during its travels. 



Decrease of concentration with time is caused by: (i) exchange with 

 outside air (i.e. ventilation); (2) deposition on walls, ceiling, and floor, 

 by various processes including sedimentation; and (3) reduction in the 

 viable count through death. Ventilation does not immediately sweep away 

 the whole microbial load, but progressively dilutes it exponentially. 

 Then 'n' air changes will reduce concentration in the ratio: i/e". Decreases 

 in concentration due to deposition, death, or disinfection, may also follow 

 a logarithmic law, and these can then be expressed in units of equivalent 

 ventilation turn-overs for ease of comparison. 



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