Hourly variation of airborne ragweed pollen in Kansas City

Introduction: Ragweed pollen is one of the major causes of allergic rhiniti s in the midwest United States. Previous studies have demonstrated that rag weed pollen production begins after sunrise and airborne pollen levels peak several hours later. Variations in this pattern that may be of clinical im portance within a small region and the effect of weather on these variation s have not been investigated. Methods:Volumetric pollen collectors were stationed at four sites around th e metropolitan area. Ten-minute grab samples were taken at each location ev ery 2 hours for continuous 24-hour periods during the 1997 ragweed season. The downtown location had a weather station that logged meteorological cond itions at hourly intervals during this time. Ragweed pollen grains were cou nted microscopically at 400 X. Uninterrupted data covering at least 20 days during the season were evaluated for each collection site. Results:The lowest ragweed pollen counts occur at 6:00 AM and the highest o ccur at noon for both suburban and urban sites. Rainfall was seen to be the most influential weather-related factor. Significant rainfall events effec tively removed pollen grains from the air. Conclusions:Though ragweed pollen emission begins at 6:00 AM, peak pollen e xposure occurs at midday. This observation is in agreement with many other studies. The myth that highest ragweed exposure occurs in the early morning hours is not supported by this or previous studies.