AEROBIOLOGY 



dust horizon — convection will take it higher. The concentration of spore- 

 clouds approximates to the expected logarithmic decrease with height, 

 but equilibrium is never reached in the atmosphere. Often there is a zone 

 of increased concentration at an altitude of two or three thousand metres. 



Dilution of the spore-cloud as it travels horizontally downwind is the 

 result of eddy diffusion. Of the various meteorologists and physicists who 

 have formulated eddy diffusion, O. G. Sutton has provided the most use- 

 ful method. Experimental values obtained in microbiological work for the 

 parameters in Sutton's diffusion equations are useful in predicting spore 

 concentrations at various distances from the source. We await still better 

 methods; but they may be long in coming, for the problem appears to be 

 particularly intractable. 



Concentration of the spore-cloud decreases not only by diffusion but 

 also because particles are lost by various deposition processes — of which 

 impaction, turbulent deposition, and sedimentation under the influence 

 of gravity, are the most important. Wind speed, the area and orientation 

 of the surface, and the size of the particle, all have great effects on im- 

 paction efficiency, both in the wind-tunnel and out-of-doors. 



Deposition from the atmosphere under natural conditions outdoors 

 has been studied experimentally. Under uniform conditions, particle size 

 has an important effect on the amount deposited from a spore-cloud of 

 given concentration at ground-level. When liberation is near ground-level, 

 loss by deposition is great. We cannot yet choose between two theories 

 of deposition to the ground, one of which depends on the wind-speed and 

 the other on the distance traversed by the cloud. Near the source, unex- 

 plained high deposition values have been observed. Elevating the point 

 of liberation reduces the amount of deposition near the source. Topo- 

 graphical features which affect wind-speed also affect the number of spores 

 deposited, and may play a part in development of a plant disease epidemic 

 — complicated by the fact that they may simultaneously modify the 

 ecoclimate in a direction favouring or inhibiting infection. 



Estimation of the microbial content of the air is particularly difficult 

 because, although microscopic, the particles are often large enough to 

 demand attention to the aerod}Tiamic design of the sampling equipment. 

 Choice of the correct sampling equipment must be determined by the 

 range of organisms to be sampled. Throughout this book emphasis has 

 been placed on methods, because methods commonly determine results. 

 Single bacterial cells in aerosols are small enough to be handled in the 

 manner of a gas, without regard to their inertia; but larger organisms 

 (and bacteria on 'rafts') impact on surfaces, stick on corners, slip out of 

 streamlines, and settle under the influence of gravity. These aerodynamic 

 effects must be allowed for in the design of apparatus for reasonably 

 accurate sampling. 



The basic study of the air-spora must be by visual methods under the 

 microscope. Crude as visual identification is, it is based on the only common 



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