THE AIR-SPORA NEAR THE EARTH's SURFACE 



also in rainy weather so long as not more than 2-3 mm. of rain fell in 24 

 hours. Miquel also showed that the outside changes soon penetrated 

 into the rooms of buildings unless they were exceptionally well sealed. 



(ii) Diurnal periodicity of ttwuld spore numbers resembled that of 

 bacteria. Trapping by impaction on a moving slide which gave him hourly 

 readings, Miquel found that moulds had two maxima at o8-oo and 20-00 

 hours. These maxima were independent of wind velocity and fluctuated 

 much more than did bacterial counts. Miquel then tried 15-minute 

 sampling periods and discovered the important principle that hourly 

 values are merely a smoothing of still more rapid fluctuations; he says 

 (transl.) 'what I wish to establish by all these examples is the variability 

 of the nature of the organisms living in the atmosphere'. 



RELATIVE NUMBERS OF BACTERIA AND MOULDS 



Miquel had started with the aim of describing the cryptogamic flora 

 of the atmosphere, and in the earlier years of his work he reported much 

 larger numbers of moulds than bacteria. 



In 1879 Miquel was assessing mould spores visually by a continuously 

 operated, aspirated aeroscope at 2 metres above ground-level at the centre 

 of a lawn in the Pare. He caught microbes at 100 times the rate of the non- 

 aspirated aeroscopes of Maddox and Cunningham, and he concluded that 

 less than 10 per cent of the organism seen visually would grow in culture. 

 The numbers of germs (principally mould spores as showTi by his draw- 

 ings) in the air at the Pare Montsouris during continuous sampling in 1878 

 averaged 28,500 per cubic metre. In rainy periods in June they rose to 

 100,000 or even 200,000 per cubic metre. In winter the numbers were as 

 low as 1,000 per cubic metre during snow, though they might be 14,000 per 

 cubic metre when the wind came from over the centre of Paris. Numbers 

 increased again in spring ; they remained high in summer and diminished 

 again in autumn. 



By the 1890's Miquel had lost interest in airborne moulds, and for 

 sampling air he constantly recommends the use of sugar-free media which 

 discourage moulds but enhance bacterial counts. Henceforth the mould 

 counts which he reported fell to the level of the bacteria and, by deliber- 

 ately using a selective medium, he could forget the rich fungus spora that 

 had embarrassed him in the earlier years. The values already given (Table 

 XVIII) are for media which favour bacteria but repress moulds, and are 

 certainly underestimates for the latter. 



Knowledge of the broad features of the bacterial flora of the outdoor 

 air near the ground remains to this day substantially as Miquel left it at 

 the beginning of this century. Further, comparable measurements in this 

 century include those by: Forbes (1924), Wells & Wells (1936), Buch- 

 binder et al. (1945), and Colebrook & Cawston (1948). On the whole the 

 topic has been neglected and, significantly, the American Association for 



H 113 



