AIR SAMPLING TECHNIQUE 



scanned transversely at intervals of 4 mm. to get a reading corresponding 

 to every two hours throughout the day and night. 



The Hirst trap is ideal for survey work, and it is a useful compromise 

 over many conflicting requirements. It is power-driven; sampling is 

 approximately isokinetic; identification is visual under the microscope, 

 and time-discrimination is obtained to i i hour or better. The advan- 

 tages of the Hirst trap are its robustness, simplicity, and continuous 

 operation with a few minutes attention every 24 hours. Its disadvantages 

 are capital cost, power requirement, and its unsuitability both for identi- 

 fication in culture and for trapping splash-dispersed spores. Pady (1959) 

 has described a modified Hirst trap with intermittent hourly movement 

 replacing continuous movement of the slide. Panzer et al. (1957) devised 

 a battery-driven spore collector for continuous operation, but its descrip- 

 tion suggests that its collection and retention efficiencies are probably 

 small. 



Scanning slides under the microscope is tedious unless the deposit 

 is dense. Hirst (1959) reports a comparison of mean density of spore 

 deposit on horizontal slides, vertical cylinders, and Hirst trap slides 

 (Table XVI), exposed for four summer months in the open air. Small 

 spores, which almost entirely escape the vertical cylinder and horizontal 

 slide, are caught in rewarding numbers in the Hirst trap deposits. 



TABLE XVI 



AVERAGE DENSITY OF SPORE DEPOSIT FROM THREE TYPES OF TRAP 2 m. 

 ABOVE GROUND, HARPENDEN, ENGLAND, JUNE TO SEPTEMBER 1951. 



(Hirst, 1959) 

 Spores per sq. cm. Ratio 



Examples of deposits obtained with the Hirst trap are shown in Plate 

 I (Frontispiece), and some results of its use are described in the next 

 chapter. 



(x) The Portable Volumetric Spore-trap (Gregory, 1954) samples 10 

 litres of air per minute, w ith suction obtained by turning a light, sliding- 

 vane pump by hand. The apparatus weighs about 10-12 lb. Its advantages 



lOI 



