Trees in the birch family, including alder, hazel, and birch, have wind-pollinated flowers that produce copious amounts of pollen and are a major cause of allergic rhinitis. How their pollen allergens cause asthma has been a mystery because pollen grains are too large to be inhaled into the airways where asthma occurs. Over the past 20 years, studies showed that episodes of pollen-induced asthma occurred during the flowering season, especially after periods of rainfall or thunderstorm. Immunologic studies revealed that abundant quantities of micronic particles occur in the outdoor air, especially after rainfall, and they are loaded with birch pollen allergens. How these respirable allergens were released into the air remained unknown. Recently, we have shown that for flowering grasses, pollen remains on the open anthers in the absence of wind or other disturbances. If wetted, pollen can rupture within minutes.1 Fragmented cytoplasm is emitted through the pore region of the pollen grain. Drying winds release this cytoplasmic debris as a respirable allergen-loaded aerosol.