Thunderstorms have often been linked to epidemics of asthma, especially during the grass flowering season; however, the precise mechanisms explaining this phenomenon are unknown. Evidence of high respirable allergen loadings in the air associated with specific meteorologic events combined with an analysis of pollen physiology suggests that rupture of airborne pollen can occur. Strong downdrafts and dry, cold outflows distinguish thunderstorm rain from frontal rain. The weather system of a mature thunderstorm likely entrains grass pollen into the cloud base, where pollen rupture would be enhanced, then transports the respirable-sized fragments of pollen debris to ground level where outflows distribute them ahead of the rain. The conditions occurring at the onset of a thunderstorm might expose susceptible people to a rapid increase in concentrations of pollen allergens in the air that can readily deposit in the lower airways and initiate asthmatic reactions.