THE AIR-SPORA NEAR THE EARTH S SURFACE 



TABLE XVIII 



MEANS OF MONTHLY MEAN NUMBERS OF BACTERIA AND MOULDS PER CUBIC 

 METRE OF OUTDOOR AIR IN PARIS (Miquel, 1 899), IN CULTURE IN NEUTRAL 



BEEF BROTH 



The numbers of bacteria in summer averaged several hundred per 

 cubic metre, and were reduced in a few hours by rain to a mere 20-30 

 per cubic metre, but they increased again as the ground dried out. Sur- 

 prisingly enough, bacterial numbers often increased after snowfall. The 

 numbers of bacteria in the air increased with increasing wind-speed and 

 remained high during a drought, unless it was prolonged. To Miquel 

 it was clear that rain had a complex action: air that contained many 

 bacteria after a fine, dry spell of weather was rapidly purified by rain, but 

 often during a spell of humid weather the fall of rain would contaminate 

 the air more than it purified it — possibly because raindrops collected 

 bacteria in their fall towards the ground and, by evaporating before 

 reaching the soil, added their collection to the air near ground-level. We 

 may also suspect that bacteria were put into the air by rain-splash. 

 The first rain after drought might contain 200,000 bacteria per litre 

 instead of the average number of 3,380 per litre (see Chapter XI). 



Slowly Miquel came to the conclusion that the source of most outdoor 

 airborne bacteria is the surface of the ground, whence they are picked up 

 with dry soil particles by wind — a conclusion still acceptable 80 years later. 



Moulds reacted differently to the fall of rain. With the onset of rain, 

 the air was at first purified ; but when rainy periods lasted for some days, 

 the numbers of mould spores in the air at Montsouris often increased 

 remarkably, even reaching 95,000 to 120,000 per cubic metre. In dry 



III 



