AIR SAMPLING TECHNIQUE 



with spore-aggregates as well as inorganic particles, whereas cultural 

 methods allow the most accurate taxonomic determinations — but they 

 will only show viable micro-organisms that can grow on the media chosen. 

 Visual pollen identification can be carried to species level in many genera ; 

 but whereas spores of a few pathogenic fungi can be recognized with 

 certainty, the identification of spores and cells of bacteria, actinomycetes, 

 and most of the smaller fungi, is tentative in the extreme (see Appendix I, 

 p. 207). 



To economize description in the following account, the various kinds 

 of apparatus for sampling the air-spora are grouped according to the 

 physical processes by which they remove particles from the air and deposit 

 them on surfaces for examination. Many of the techniques can be adapted 

 to give either a deposit for visual examination under the microscope or 

 cultures on artificial media. 



Gravity Sedimentation Methods 

 sedimentation from still air 



A simple box, developed by Alvarez & Castro (1952) for the study of 

 airborne fungi, had two hinged sides and a covered tray at the bottom for 

 inserting a microscope slide or Petri dish. To take an air sample, the two 

 hinged sides were raised horizontally and wind was allowed to blow 

 through the box. Closing the hinged sides trapped a box-full of air and 

 the entrapped spores then sedimented under gravity. 



In theory the result is not affected by wind velocity or particle size. 

 Sampling is discontinuous, only a small volume of air being sampled at a 

 time, and there will be losses on the walls and roof of the box due to 

 convection and diffusion {see Tyndall, 1881 ; Green & Lane, 1957, p. 229). 



SEDIMENTATION FROM WIND 



The method of examining the deposits on a freely exposed, horizontal 

 surface such as a glass microscope slide was used by Pouchet in his 

 controversy with Pasteur, but can be traced back to van Leeuwenhoek. 



(i). The 'gravity slide'' has been the routine method for investigating 

 the pollen and spore content of the air since the early days of hay-fever 

 studies. Scheppegrell (1922) used ordinary 76 X 25 mm. microscope 

 slides exposed without protection from rain. Most workers, however, e.g. 

 Blackley (1873), Wodehouse (1945), Durham (1946), Hyde & Williams 

 (1950), and Hyre (1950), have exposed the slides horizontally, with the 

 sticky slide facing upwards in some form of shelter — open to the wind but 

 giving protection from rain. The adhesive coating of the slide is usually 

 glycerine jelly or petroleum jelly. A slide is commonly exposed for a period 

 of 24 hours, and slides are changed daily throughout a season. 



The gravity slide is cheap, simple, and operates continuously, but 

 has serious defects as a quantitative method of air sampling out-of-doors, 



91 



