Most thunderstorm horror stories involve lightning strikes. But a wild storm that struck Melbourne, Australia, last week turned deadly in a different way—by inducing asthma attacks.

According to the Associated Press, "Monday's storm caused rain-sodden ryegrass pollen grains to explode and disperse over the city, with tiny pollen particles penetrating deep into lungs." Some 8,500 people ended up in the hospital for treatment of breathing problems; and so far six people have died

But how could an ordinary weather event trigger asthma attacks in so many people?

It's not the thunder or the lightning, says Clifford W. Bassett, MD, the founder and medical director of Allergy and Asthma Care of New York. It's the airflow patterns in a storm that lead to so-called "thunderstorm asthma."  

“Allergy-inducing grass grain particles get swept up into the air and break down, releasing pollen that can cause an asthma attack,” says Dr. Bassett. Rain and big gusts of wind help spread the pollen.

For anyone who's genetically predisposed to asthma, exposure to such a high concentration of allergens can lead to a severe reaction, he says, even in those who've never had an attack before. The AP reported that about a third of the patients in Melbourne said this was their first asthma attack.

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While thunderstorm asthma is a rare phenomenon, cases have been reported in other countries as well, including Italy, the United Kingdom, Canada, and the United States (in Atlanta). Dr. Bassett expects it to become more common as global warming leads to longer warm-weather seasons, which will allow for more exposure to pollen—and therefore lead to greater risk in general of allergic reactions and asthma symptoms.

If you know you have asthma, there are ways to protect against this freakish phenomenon, says Dr. Bassett. He suggests consulting an allergist and coming up with a plan to manage your symptoms in case of a severe attack.