AIR SAMPLING TECHNIQUE 



are charged by ions after entering the apparatus is not yet known. The 

 apparatus was used in aerobiological studies by Kelly et al. (1951), and 

 later by Pady and his colleagues in Kansas. 



Other electrostatic sampling methods include those of Rack (1959), 

 and O'Connell et al. (i960). 



Comparison of Methods 



Under simple conditions it is not difficult to define an absolute standard 

 for air-sampling. With non-aggregated spores of one species liberated in a 

 wind-tunnel, isokinetic sampling through a feathered orifice facing up- 

 wind (using a suitable membrane filter, a Cascade Impactor, or a liquid 

 impinger, with precautions against overloading) should give a reliable 

 visual estimate of the number of particles in a measured volume of air. 

 The Cascade Impactor tends to reveal any spore clumps intact, and, if this 

 feature is undesirable, the liquid inpinger should be used to break the 

 aggregates. The more varied the population in species, size, and state of 

 aggregation, the harder it becomes to devise equipment to measure 

 microbial concentration in the air. Particles over lOju, in diameter must 

 be sampled directly and cannot be ducted around corners on the way to 

 the apparatus without heavy wall-losses. 



Air hygiene in bacteriology has been mainly a study of the air within 

 buildings, and its equipment has therefore been developed for sampling 

 still or slowly moving air. Aerodynamic effects have been neglected, 

 despite the fact that bacteria are carried on 'rafts' or spray droplets of 

 greatly varying size, so that efficiency of retention has been achieved 

 but efficiency of collection has been neglected. Most devices, such as the 

 slit sampler and the electrostatic sampler, avoid this difficulty by pointing 

 the orifice upwards — but this makes them unsuitable for use in the open 

 air. 



In outdoor aerobiology the sizes of pollen grains and fungus spores 

 {see Appendix I), and the variability of wind-speeds, has focused atten- 

 tion on collection efficiency. Results from the various 'surface' traps, 

 depending on natural deposition processes, are usually difficult to translate 

 into volumetric results. Over short periods while wind-velocity is constant 

 out-of-doors, or in a wind-tunnel, a vertical strip or cylinder can be 

 used to estimate concentration provided the wind-speed and the deposition 

 efficiencies of the particles concerned are known. If the particle's terminal 

 velocity is known, fairly close estimates can also be made by using theor- 

 etical formulae (e.g. of C. N. Davies & Peetz, 1956). Most data from sur- 

 face traps, such as gravity slide and Petri dish counts, cannot be translated 

 into concentration but merely measure surface deposition. Only the vast 

 diffisrences in natural concentrations, that occur at different times and 

 places, make it possible to infer changes from deposition records. Never- 

 theless Hyde (1959^) shows that in general and over a long period, gravity 



105 



