Seasonal characteristics of ragweed pollen dispersal in the United States.

BACKGROUND: The most definitive reports concerning the seasonal characteristics of ragweed (Ambrosia spp) pollen dispersal in the United States date back 60 years to work conducted with gravity slide samples. OBJECTIVE: Volumetric pollen data from 23 sampling sites were used to study the ragweed pollen season. METHODS: The date of first and final pollen capture and the date with maximum airborne pollen concentration (the "peak date") were used to compare the pollen seasons at locations ranging from approximately 30 degrees N to 45 degrees N latitude. RESULTS: Sixteen cities located north of 38 degrees N possessed similar peak dates (P = 1), generally achieving maximum airborne pollen concentration in late August or early September. Four cities located south of 38 degrees N experienced later peak dates, with the most southerly city reaching maximum pollen levels in mid-October. Three cities located in the Northwest did not recover ragweed pollen in air samples. CONCLUSION: This study confirms earlier reports and suggests what time of year source avoidance is most critical for ragweed sensitive individuals in various locations.