AIR SAMPLING TECHNIQUE 



now be bought with pores from o-8 to 50 ju- in diameter which allow 

 flow-rates adequate for some air sampling (First & Silverman, 1953; 

 Goetz, 1953; Haas, 1956). After exposure these membranes can be placed 

 directly on the surface of solid media in a Petri dish to allow growth on 

 the membrane ; or they can be shaken in water and the suspension plated 

 out; or the membrane can be mounted as a transparency and examined 

 directly under the microscope. 



(ii). Impaction filters differ from sieving filters in consisting of a deep 

 layer of fibres or granules separated by relatively wide air-spaces. Air- 

 borne particles are subjected to repeated encounters and are impacted 

 in the foremost layers of the filter substance. Pasteur's air filter {see p. 5) 

 was packed with a nitrocellulose plug which, after sampling, was dis- 

 solved in an alcohol-ether mixture and the particles were examined 

 microscopically. As this treatment killed the trapped organisms, P. F. 

 Frankland (1887) substituted powdered glass which was washed, diluted, 

 and plated out after exposure. Because of the difficulty of washing organ- 

 isms off a solid filter, many workers have preferred a completely soluble 

 filter medium such as sodium sulphate (Miquel, 1890). Duller & Lowe 

 (191 1) substituted a plug of powdered sugar; Richards (1955) introduced 

 the sodium alginate-wool filter, and others have used ammonium alginate 

 (Hammond, 1958), which is soluble in water, or calcium alginate, which 

 is soluble in Ringer's solution or in sodium hexametaphosphate. 



For many years the standard technique for trapping airborne microbes 

 was that of Petri (1888) or modifications of it (e.g. Weinzerl & Fos, 1910). 

 Air was drawn through a sterilized tube 9 cm. long and i-6 cm. wide, con- 

 taining two columns of sand separated by wire gauze. These two sand 

 filters were plated-out separately, the rear portion acting as a control. 

 Equipment for exposing a set of five such tubes from aircraft during 

 flight is described by Overeem (1936). 



A simplified, wind-operated filtration method has been developed by 

 Rishbeth (1958, 1959) to study fungus pathogens of coniferous forests. 

 Butter-muslin squares (20-cm. sides), dry-sterilized in paper envelopes 

 at ioo°C., are put on a wire frame exposed either at right-angles to the 

 wind (with the frame clipped to a stick, commonly at 2 metres above 

 ground-level) or mounted on a moving vehicle. Exposures may last from 

 minutes to hours; when they have been completed, a lo-cm. square of 

 muslin is cut out, shaken in water, and the suspension plated out. 'Plating' 

 on freshly-cut slices of pine trunks gives a sensitive and extremely 

 selective method of detecting spores of two species of fungi, Fomes 

 annosus and Peniophora gigantea. 



(iii) Liquid scrubbing devices remove particles suspended in air, as it 

 bubbles through a liquid, by various combinations of impaction, sedi- 

 mentation, and diffusion. A simple 'aeroscope' bubbler devised by Rettger 

 (1910), consisting of an inlet tube ending in a submerged perforated 

 bulb, appears to be the prototype of a practicable method. Another 

 G 97 



