THE AIR-SPORA OF ENCLOSED SPACES 



no trajectory but move with the slightest air currents, and are emitted 

 in large numbers. Thus akhough most airborne bacteria seem to be carried 

 on rafts of dust particles which settle rapidly, they appear to be relatively 

 innocuous saprophytes; the pathogens are present only in special en- 

 vironments, being carried in much smaller and more insidious droplet 

 nuclei which are small enough to be capable of entering, and being 

 retained by, the alveoli of the lungs. 



The Air of Different Environments 



dwelling houses 



In spite of ventilation, Penicillium dominates the air inside most 

 houses, in contrast to Cladosporium outside, and bacteria tend to be more 

 abundant indoors in winter than in summer. 



Microbial concentration indoors varies greatly with mechanical and 

 human activity. Carnelley et al. (1887), using Hesse's tubes in schools and 

 mills in Dundee, Scotland, observed that, in densely-populated rooms, 

 stirring up dust increased the total air-load and increased the ratio of 

 bacteria to moulds. When air in rooms is left undisturbed the bacteria 

 (or particles to which they are attached) settle out rapidly, but the moulds 

 do so much more slowly. 



Maunsell (1954, 1954'^') used the slit sampler in bedrooms and found 

 that shaking beds, brushing carpets, and any building repairs, increased 

 the mould-spore content of the air up to 17 times, but that it rapidly 

 returned to normal when activity ceased. 



Other studies of the air of dwelling houses are discussed by Miquel 

 (1879, 1883), Rostrup (1909), Winslow & Browne (1914), Flensborg 

 & Samsoe-Jensen (1948), Nilsby (1949), Wallace et al. (1950), and 

 Swaebly & Christensen (1952), among others. 



Tests with the portable volumetric spore-trap (Gregory, unpublished) 

 show that the airborne dust in inhabited rooms is commonly dominated 

 by what appear to be fragments of human skin in the form of minute, 

 flattened scales from the stratum corneum of the epidermis. Concen- 

 trations of several thousand of these potential bacterial 'rafts' per cubic 

 metre are common indoors, and 390,000 per cubic metre have been noted 

 after bed-making. These epidermal scales probably carry a large propor- 

 tion of the airborne bacteria of indoor air. 



HOSPITALS 



In studies of hospital air over a period of 15 months, Miquel (1883) 

 found a mean value of 11,100 bacteria per cubic metre in the crowded 

 wards of the Hopital La Pitie, Paris, the counts varying from 5,100 in 

 June to 23,100 in December. The general improvement in hospital 

 hygiene since that time is illustrated for example by Colebrook & Cawston 



159 



