Thunderstorms bring wind, rain, loud rumbles, and flashes of lightning. But researchers have known since the 1980s that they can also bring on asthma attacks. In one extreme case during a 2016 thunderstorm in Australia, thousands of people experienced asthma attacks, and at least nine people died. Thunderstorm asthma events have been reported in Australia, Europe, North America, and the Middle East. But researchers are only now coming to understand how one leads to the other — and how to set up warning systems. It’s thought that pollen is the likely trigger for thunderstorm asthma. But pollen grains are too large to be directly inhaled into lung tissue to cause an asthma attack. A thunderstorm can change that though, by sweeping up pollen grains kilometers into the sky. There the grains encounter high humidity that scientists believe causes the large grains to swell and rupture into hundreds of smaller pieces. Riding downdrafts in the storm, the pollen shrapnel comes crashing back to Earth — and it’s small enough to enter the lower respiratory tract and cause an inflammatory response like an asthma attack. In Melbourne, Australia, thunderstorm asthma has been reported seven times since 1984. The city appears to have all the right conditions for an epidemic of this sort: nearby fields of pollen-producing rye grass, regular thunderstorms, and a large population. Analyzing hospital and meteorological records, researchers found that all of the thunderstorm asthma cases occurred in the month of November, which coincided with when pollen levels were high. Also, the thunderstorms typically included multiple storm cells, which may have helped spread pollen shrapnel. Victoria state, of which Melbourne is the capital, last year rolled out one of the world’s first thunderstorm asthma warning systems. It combines thunderstorm predictions from weather agencies with pollen counts from 8 counting stations across the state. This gives people time to plan for coming storms. Currently, the team is able to forecast 3 days before a thunderstorm asthma event occurs.