Thunderstorm Asthma: Why Storms Break Pollen Into Fine Particles
Thunderstorm asthma happens because a storm's outflow concentrates pollen at ground level and moisture ruptures the grains into fragments small enough to reach the lower airways — particles that would normally be filtered out by the nose instead go straight into the bronchi. A whole pollen grain is roughly 20 to 40 microns across and gets trapped in the upper airway. The starch granules released when it bursts are 0.6 to 2.5 microns, which is small enough to be inhaled deep into the lung. That size change is the entire story.
The effect can be dramatic. In Melbourne in November 2016, a single thunderstorm during grass pollen season produced thousands of emergency presentations within hours and overwhelmed the city's ambulance service. It is the most severe documented event of its kind, and it made a phenomenon that had been described in scattered case reports into a recognised public health hazard.