Association Between Rainfall and Ragweed Pollen Counts in the Midwest

Rationale: Ragweed is a common allergen and accounts for one of the most important causes of allergic rhinitis in North America.  It can produce about a billion grains of pollen per season.  We looked at the relationship between ragweed pollen count and rainfall over the last 14 years in the Midwest.

Methods: Using a Hirst spore trap, pollen was collected on glass slides coated with silicone grease from the roof of a 5 story building in Kansas City, Missouri.  Total ragweed counts for 14 successive ragweed seasons were collected and analyzed using Microsoft Excel.  Pollen counts were certified by the National Allergy Bureau.  Weather data was collected from an automated weather station located within 100 feet of the collector.  Measurements were taken every hour and stored in a database.  Rainfall was presented in thousandths of an inch.  Logistic regression analysis was performed to evaluate rainfall and ragweed counts.

Results: Our data showed that there was no consistent correlation between total yearly rainfall and ragweed counts.  There was also no correlation found between total rainfall from January to July and subsequent ragweed counts for that year.  However, total rainfall from August to October did decrease ragweed counts slightly.

Conclusions:   Previous studies have shown that heavy rainfall can reduce pollen concentrations by washing pollen from the air.  We found that rainfall during the ragweed season rather than prior to the ragweed season had the greatest impact on total ragweed counts.  This suggests that rainfall has more affect on pollen counts at one point in time.