Hundreds of people were rushed to hospital Monday with breathing problems in the southern Australian state of Victoria as emergency services struggled to cope.
During a four hour period Monday, Ambulance Victoria received more than 1,900 calls, or one call every four to five seconds. An extra 60 ambulances were deployed, as well as police and firefighters.
Thunderstorm asthma occurs when a storm hits during a period of unusually high rye grass pollen, said Robin Ould, chief executive of the Asthma Foundation of Australia.
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"When you have a perfect storm coming together (of) a very high pollen day, high humidity, and a thunderstorm, the grains of rye grass absorb water with the humidity and they break up into thousands of pieces," Ould said.
"Normally with rye grass the pollen would be trapped by nose hairs. When it breaks up it goes straight to the lungs."
The pollen irritates the lungs' bronchial tubes, causing them to become inflamed and filled with mucus and making it hard for people to breathe.
Pollen levels peak in late spring. When this combines with strong winds, rain and high temperatures, as it did in Victoria this week, it can lead to incidents of thunderstorm asthma.
Edward Newbigin, a professor of biosciences at the University of Melbourne, said that many of those affected may never have had an asthma attack before.
"I imagine it was absolutely terrifying," he said.
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Of more than 2,500 people surveyed by the university, 74% said they experienced asthma during the storm. Of those, 32% had never experienced an asthma attack before.
One of the dead, 18-year-old Omar Moujalled, had just finished his final year of studies at Melbourne's Australian International Academy.
His friend Shuayb Talic told CNN the news left him in "absolute shock, denial, then horror."
"Omar was the fittest of the group," Talic said. "We were discussing a gym meet up just a few days earlier. However, he did have asthma, and apparently it was so severe during the storm he could barely stand up to treat it."
Apollo Papadopoulos, 35, and Hope Carnevali, 20, also died during the storm. Carnevali passed away in the arms of her family members after waiting more than 40 minutes for an ambulance, according to CNN affiliate 7 News Australia.
Thunderstorm asthma last struck Victoria in May 2010, but the condition is not limited to Australia.
Newbigin said incidents of thunderstorm asthma have been seen in the UK, Italy and Canada, but the condition is "very under reported."
People with hay fever are particularly at risk, Newbigin said. He advised them to "better manage your hay fever during the pollen season," by using antihistamines, eye drops and other medications.
Though normally hay fever occurs in the nasal area, the freak weather conditions which cause thunderstorm asthma can drive the allergens deep into the lungs, causing a far more severe asthmatic attack.
CNN's Pedram Javaheri contributed to this report.