Is there a risk of epidemic thunderstorm asthma every time there is a thunderstorm?

No. Epidemic thunderstorm asthma events are thought to be triggered by a unique combination of high grass pollen levels and a certain type of thunderstorm (rather than all thunderstorms). Rye grass pollen levels are thought to be an essential factor in determining whether an epidemic thunderstorm asthma event may occur. They typically increase in October and taper off toward the end of December.