Hourly variation of airborne ragweed pollen in Kansas City

Abstract

Introduction: Ragweed pollen is one of the major causes of allergic rhinitis in the midwest United States. Previous studies have demonstrated that ragweed pollen production begins after sunrise and airborne pollen levels peak several hours later. Variations in this pattern that may be of clinical importance within a small region and the effect of weather on these variations have not been investigated.

Methods: Volumetric pollen collectors were stationed at four sites around the metropolitan area. Ten-minute grab samples were taken at each location every 2 hours for continuous 24-hour periods during the 1997 ragweed season. The downtown location had a weather station that logged meteorological conditions at hourly intervals during this time. Ragweed pollen grains were counted 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 occur at noon for both suburban and urban sites. Rainfall was seen to be the most influential weather-related factor. Significant rainfall events effectively removed pollen grains from the air.

Conclusions: Though ragweed pollen emission begins at 6:00 AM, peak pollen exposure 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.