An improved volumetric impinger for pollen counting

Abstract

T HE first studies of atmospheric pollen concentrations were conducted by Scheppegrell in 1917. He attempted to derive a formula for converting the number of pollens on a square centimeter of slide to a volumetric equivalent by applying Stokes’s law for small falling bodies.2 Using this formula, Scheppegrel arrived at a factor of 1.8 for converting the number of ragweed pollens on a square centimeter of slide to a cubic yard equivalent. This remained the basis for counting and converting atmospheric equivalents until 1937, when Cocke3T 4 pointed out some difficulties in these formulas. In the first place, Stokes’s law pertained to smooth, spherical bodies in still air; pollens are, for the most part, spiculated and aspherical, and the air is usually in motion. Second, it is necessary to determine accurately the rate of fall, the specific gravity, and the size of each pollen before one is able to apply Stokes’s law. One would then obtain a separate factor for each pollen. Third, Cocke pointed out that Scheppegrel had erred in his original calculations and had used the diameter and not the radius of the pollen in calculating his factor for ragweed. Dahl and Ellis5 reported on pollen concentrations of the atmosphere in 1942. Durham+I2 did monumental work in conducting pollen surveys in this country and in developing a standard method of pollen counting. The major portion of this work was done by the gravity-slide method, utilizing pollen shelters of varying construction. The advantages of this method were its simplicity and the rapidity with which one could obtain the desired information. No further refinements were made in the procedure of pollen counting until 1946 when Durham, I1 dissatisfied with Scheppegrel’s formula, ran parallel gravity-slide and volumetric studies. This work formed the basis for the adoption of standard apparatus, counting procedure, and conversion factor by the Pollen Survey Committee of the American Academy of Allergy’:’ in 1947. The conversion factor adopted for ragweed was 3.6, just double th(s previous one. At that time it was the feeling of the members of the Committee that the gravimetric method, even with the revised conversion factor. was unsatisfactory and that a volumetric method, if available, would be more accurate and preferable. Among the factors which prevent an accurate conversion from the gravity slide to the volumetric equivalent are such va.riables as the location of the shelter, wind velocity, wind direction, wind direction in rela.tion to the source of the pollen, and convection and deflection currents.