AIRBORNE BIOLO' KIALS 



sources, ragweed pollen concentra- 

 tions seldom exceed 200 grains per 

 cubic meter during the emission sea- 

 son. Other aeroallergens also vary 

 greatly in concentration, and certain 

 fungus spores are often present in 

 great numbers. These variations in 

 concentration lead to difficulties in 

 choice of sampling methods and pe- 

 riods. An efficient sampler may over- 

 load in the presence of high concen- 

 trations, while an inefficient one may 

 not take an adequate sample if con- 

 centrations are low. 



The Limitations of Sampling Meth- 

 ods — Obtaining a sample of air- 

 borne particles in the aeroallergen size 

 range may be accomplished by many 

 samplers, but obtaining an accurate 

 or representative sample over all size 

 ranges commonly present is a difficult 

 problem not solved by any sampler 

 in current use. In fact, it can be 

 stated categorically that no single 

 sampling method so far devised is 

 capable of obtaining a representative 

 sample of all aeroallergens from the 

 free atmosphere and that no perfect 

 sampling method exists for any. 



The principal cause of difficulty in 

 sampling particles in this size class is 

 the momentum that they acquire in 

 moving air as a result of their mass 

 and velocity. This inertia causes their 

 path to deviate from that of the sur- 

 rounding air if that air is forced to 

 change speed or direction, as by a 

 sampling device. 



With the exception of isokinetic 

 sampling, which has not yet been 

 perfected for use in the free atmos- 

 phere, sampling methods in which air 

 and, hopefully, its entrained particles 

 are drawn into an entrance or orifice 

 tend to be inefficient for large parti- 

 cles since these often fail to follow 

 the airstream into the entrance. Since 

 momentum increases with particle 

 size, particle density, and air speed, 

 it follows that such samplers are 

 not only size-selective but vary in 

 entrance efficiency with wind speed. 



In general, methods of removing 

 from the airstream those particles 



that do get into the entrance are satis- 

 factory. These methods include fil- 

 tration, impaction, liquid impinge- 

 ment, and electrostatic attraction. 

 Suction-type samplers are sometimes 

 used for sampling aeroallergens, but 

 cannot be recommended except for 

 the smaller fungus spores. 



The most common device for sam- 

 pling aeroallergens is a microscope 

 slide coated with adhesive and ex- 

 posed horizontally, usually between 

 rain shields. This "gravity-slide," or 

 "Durham" sampler, collects by tur- 

 bulent impingement and gravitational 

 settling, but is generally unsatisfac- 

 tory since the volume of air sampled 

 cannot be defined and the catch is a 

 function of wind speed, turbulence, 

 and wind direction relative to the 

 long axis of the slide as well as the 

 concentration of particles and their 

 characteristics. Although still in wide- 

 spread use and of some value for 

 qualitative purposes, it should be re- 

 placed by other samplers where quan- 

 titative measurements are desired. 



To date, the most satisfactory de- 

 vices for sampling aeroallergens are 

 those which collect by impaction. 

 Here, the momentum of the particle 

 is used to effect its capture; efficiency 

 increases as particle size, particle den- 

 sity, and wind speed increase. How- 

 ever, efficiency of wind impactors 

 does vary with particle parameters, 

 so that each particle of interest is 

 likely to be sampled with a different 

 efficiency and the efficiency for all 

 will vary with wind speed. In gen- 

 eral, samplers of this nature must 

 be accompanied by a sensitive ane- 

 mometer, and the catch corrected for 

 sampling efficiency. An advantage of 

 wind-impaction methods is that im- 

 paction efficiency can be computed 

 mathematically for certain simple 

 geometric shapes like cylinders and 

 spheres if impactor dimensions, par- 

 ticle parameters, and wind speed are 

 known. For a given collector and a 

 single particle type, impaction ef- 

 ficiency can be calculated and graphed 

 as a function of wind speed. (See Fig- 

 ure X-10) Total collection efficiency, 



however, depends on both 

 and retention efficiency and ad< 

 adhesive must be used on collecting 

 surfaces to insure good retention of 

 impacted particles. Wind-impaction 

 samplers are usually cylindrical in 

 shape and are commonly mounted on 

 a wind vane so that the sample is 

 taken only on one side. Such sam- 

 plers have normally been used only in 

 controlled research programs and are 

 not recommended for general use. 



The disadvantages of wind-impac- 

 tion samplers were largely overcome 

 and their advantages retained by the 

 development of powered impaction 

 devices such as the rotorod, rotobar, 

 and rotoslide samplers. In these, the 

 sampling surfaces are rotated through 

 the air at a high rate of speed, giving 

 virtually constant impaction efficiency 

 for any given particle type. Al- 

 though efficiency may still vary with 

 particle size and density, it is gen- 

 erally much higher than for wind- 

 impaction samplers. Adequate reten- 

 tion requires a thicker or better ad- 

 hesive, since particles impacting at a 

 high rate of speed tend to bounce 

 off. Since the efficiency of these 

 devices is high and their sampling 

 surfaces small, overloading becomes 

 a problem during prolonged sampling 

 periods at commonly encountered 

 concentrations. This problem is over- 

 come by sequential or intermittent 

 sampling, but the sampling surfaces 

 must be protected from wind im- 

 paction when not rotating. Several 

 methods have been devised for this 

 purpose. Rotating impaction sam- 

 plers are the most satisfactory sam- 

 pling devices now available for most 

 aeroallergens and are being used by 

 an increasing number of allergists, 

 public health agencies, universities, 

 and research groups. 



Aeroallergens collected on sam- 

 pling surfaces are commonly identi- 

 fied and counted using an optical 

 microscope. Routine counting of a 

 single particle type such as ragweed 

 pollen may be readily accomplished 

 by unskilled workers, but critical 

 identification of many pollens and 



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