Because particle capture is related to both particle diameter and to the speed at which the particle is moving, one can control the size of the particles collected by changing the speed with which particles approach the collection surface. The cascade impactors (e. g., Andersen, 1958) fractionate aerosols by accelerating air through smaller and smaller orifices.

The nature of the impaction surface will also influence the efficiency of particle collection. Unless a particle is traveling at exactly the right speed to stop at the collection surface, it may hit the surface and bounce back into the airstream. This bounce is a function both of particle inertia and of the stickiness of the collection surface. For culture plate impactors, bounce is probably minimal; particles tend to penetrate the agar surface rather than bounce off. For spore traps, however, the collection surface must be coated with adhesive, and it gradually becomes less sticky as more and more particles are trapped. Sampling times must therefore be short enough so that overloading does not occur. Spore traps not only collect spores but all kinds of other particles that might be in the air. In aerosols in which levels of nonbiological particulates far exceed spore concentrations, surfaces often overload before a significant number of biological particles have been collected.

The nature of the impaction surface often determines the kind of analysis that can be used. Adhesive (greased) surfaces are usually analyzed by light microscopy. Agar surfaces are most commonly used for cultural analysis, but they can also be homogenized and assayed using immunochemical or biochemical techniques (Tovey et al., 1981a; Yoshizawa et al., 1991).

Modifications of the Hirst spore trap (the Burkard, Kramer-Collins, and Lanzoni spore traps [Solomon et al., 1980b]) are increasingly used for evaluating outdoor allergen aerosols. These suction devices collect 10 liters of air per minute, impacting particles on a moving, greased tape (over seven days) or on to a microscope slide (over 24 hours). The orifice of the sampler is designed to collect particles as small as about 3 µm efficiently. The trap commonly used outdoors has a wind vane so that the orifice faces into the wind. Similar indoor traps have no wind vane, and the orifices face upward. The recording Burkard version collects particles continuously over 7 days or 24 hours. The 7-day and 24-hour slide samples can be analyzed as 24-hour averages or in time increments of as little as 1 hour. Other devices collect samples in discrete bands over 24 hours (Samplair) or on one spot over a few minutes (Burkard personal).

The rotorod has been used for many years to evaluate outdoor allergens, in spite of the fact that it grossly underestimates fungus spore levels. The particle collection efficiency of these rotating arm impactors depends on the rotational speed and the width of the collecting surfaces (the rods). The wide collection surfaces commonly used for allergen monitoring (1.59 mm) efficiently collect particles as large as 10 µm; however, small particles

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