The volumetric incidence of atmospheric allergens: II. Simultaneous measurements by volumeric and gravity slide methods. Results with ragweed pollen and alternaria spores

Abstract

1. 1. In obtaining representative samples of the pollen and fungus spore content of the air under the best conditions I could provide, two volumetric devices of totally different design, one of which sampled more than ten times as much air as the other, recovered practically the same number of pollen grains and Alternaria spores per cubic yard of air on a given day. Simultaneously exposed gravity slides caught amounts of pollen and spores varying greatly day by day in comparison to the amounts gathered by the volumetric apparatus. 2. 2. In forty-seven tests over a total period of 337 hours, the average excess of the gravity catch of ragweed pollen over the theoretical standard was more than 100 per cent. 3. 3. The greatest excess catch of ragweed on gravity slides usually occurred on days of highest wind velocity, but the degree of this inaccuracy could not be predicted with any certainty. 4. 4. Rates of fall for various types of pollen grains cannot be calculated from the rate of deposit of air-borne pollen on outdoor gravity slides because of the unavoidable impinging effect of frequent descending wind currents. Therefore the Scheppegrell-Cocke-Dahl tables are of no practical value in converting gravity slide figures to accurate volumetric equivalents. 5. 5. This study shows that over a long period of time, under the special conditions described in this paper, and with average wind velocities of about 10 miles per hour, the average ragweed pollen catch on a gravity slide (1 sq. cm., 24 hours) is approximately one-sixth of the average number of ragweed pollen grains in 1 cubic yard of air. If subsequent experiments in other places with comparable wind velocity should show an average similar proportion, it may be possible to use the following arbitrary conversion factors: 5.97 when the unit area counted on the gravity slide is 1 sq. cm., 3.2 when the unit area is 1.8 sq. cm. 6. 6. Under the conditions enumerated above the average catch of Alternaria spores on a gravity slide (1 sq. cm., 24 hours) is 1 26 as much as the actual average alternaria spore content per cubic yard of air. If subsequent experiments in other places with comparable wind velocity should show an average similar proportion, it may be possible to use the following arbitrary conversion factors: 26.08 when the unit area counted on the gravity slide is 1 sq. cm., 14.5 when the unit area is 1.8 sq. cm. 7. 7. Since no simple volumetric device is yet available for routine atmospheric tests, it may be necessary for some time to continue to rely on gravity slides, but they must be regarded henceforth as being much more inaccurate than many workers have assumed. It is possible that the gravity sampling method can be improved. 8. 8. For volumetric testing the Hawes device is to be recommended for accuracy and simplicity.