the microbiology of the atmosphere 



Intra-mural Sources 

 Microbes in indoor air may come from the outdoor air-spora by 

 ventilation, or they may originate within the enclosure — in which case 

 they are probably limited in variety but may occur in high concentration. 



Defective timber attacked by fungi may be an important source of 

 spores in dwelling houses. A. W. Frankland & Hay (1951) showed that 

 some asthmatics are sensitive to the spores of the dry-rot fungus {Merulius 

 lacrymans)^ and spore concentrations ranging from 1,630 to 360,000 spores 

 per cubic metre have been recorded in buildings with active fructifications 

 of this fungus (Gregory et al^ 1953). Timber in mines is particularly 

 liable to fungal decay and may also have superficial moulds growing on it. 

 Extensive growth of Sporotrichum beurmanni (the pathogen of human 

 sporotrichosis) was found on fresh timber of mines in Transvaal by Brown 

 et al. (1947). The fungus was isolated from the air, and ventilating currents 

 of I metre per sec. could detach spores from wood provided its moisture 

 content was less than 80 per cent. 



Processes by which infectious diseases are transmitted through the 

 air have been matters of vigorous controversy in medicine, and the answers 

 given have influenced social habits and prophylactic measures. Cornet 

 (1889) held that pulmonary tuberculosis is normally acquired by inhaling 

 dust of dried sputum, but Fliigge (1897) believed that infection was from 

 germs expelled from the mouth and nose when coughing. G. S. Wilson 

 & Miles (1955) conclude that both processes occur, dust infection being 

 commoner in drier countries whereas droplet infection is the rule in 

 moister climates and in crowded places. 



Air exhaled from the lungs in normal breathing is optically clean and 

 almost sterile; but in coughing and sneezing, large numbers of droplets 

 of mucus and saliva are propelled with explosive violence into the 

 atmosphere. 



Jennison (1942) obtained photographic evidence of 20,000 droplets 

 being put into the air from a single sneeze. The largest number observed 

 was 40,000, and a weak, stifled sneeze gave only 4,600 droplets. A cough 

 produced a few hundred droplets and the enunciation of consonants was 

 also productive. Sneeze droplets, ranging in diameter from a lower limit 

 of 5-10 /Lt, and with 20-40 per cent smaller than 50 /x, could evaporate 

 instantaneously to 'droplet nuclei'. 



The concept of droplet nuclei, developed by Wells (e.g. 1955), has 

 proved fruitful. 'Droplet nuclei' are the particles formed from the smallest 

 droplets, which evaporate before falling to the ground and so remain sus- 

 pended in air. They consist of the solid residue of the evaporated droplet, 

 together with any bacteria or virus particles, and may be coated with semi- 

 dried-up mucus which tends to preserve activity and viability. Few 

 droplets are actually propelled more than 2 or 3 ft. ; but, when evaporated, 

 the resulting droplet nuclei, with any bacterial cells or virus particles, 

 would remain in suspension almost indefinitely. The droplet nuclei have 



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