Omar Majoulled, 18, died two days before what would have been his high school graduation. A fourth victim, Clarence Leo, was reported deceased early Wednesday. Several more remained in Melbourne’s intensive care units.

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The outbreak was severe even for those whose symptoms were mild under usual circumstances. “It felt like an elephant had his foot on my chest for about four hours,” said David McGann, from the Melbourne suburb of Preston, to the Australian Broadcasting Corporation. McGann said his asthma attacks were not normally worrisome, but during the storm they were crushing.

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By late Monday night, Melbourne pharmacies had depleted their stocks of bronchodilator medication. Emergency calls flooded in. Carnaveli’s relatives said they waited for more than a half an hour for an ambulance to arrive. “We would have taken her straight there, we wouldn’t have waited,” her uncle, John Carnaveli, told Melbourne’s 3AW Radio.

Between 6 p.m. and 11 p.m. on Monday, Ambulance Victoria fielded about 1,900 calls, nearly six times the usual volume.

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“We essentially had a day’s workload within five hours,” said Ambulance Victoria’s executive director, Mick Stephenson, in a statement. “This includes 200 cases for asthma, and we were seeing asthma in people who had not experienced breathing issues before.”

At their peak, 200 calls came within a span of 15 minutes. “That’s a call every 4.5 seconds,” Stephenson said.

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Ambulance Victoria put 50 extra ambulances into service, and police and firefighters responded to two dozen calls as well. The state’s Inspector General for Emergency Management announced it would review the spike in emergency demand and Ambulance Victoria’s response.

Since the first such events were recognized in the 1980s, there have been scattered reports of asthma attack outbreaks during thunderstorms around the globe, including Napoli, Italy and Atlanta. The largest confirmed episode to date was in London in June 1994. Six hundred-forty Londoners visited emergency departments with complaints of asthma or respiratory problems, of which more than a hundred were hospitalized, according to a 2016 review published in the journal Clinical & Experimental Allergy.

A 2000 case study of hospitalizations in Wagga Wagga, southern Australia, concluded that people with medical histories of rye grass and hay fever allergies had worsened asthma during October thunderstorms.

Allergen experts posit that, during the spring season, storms increase pollen’s ability to penetrate deep into the lungs. The storms concentrate pollen grains near the ground. There, the grains swell with so much water they rupture into tinier pieces, according to the prevailing hypothesis. Dispersed in the air, these particles form what is essentially a fine allergenic mist. If inhaled, the irritating matter winds up in bronchial crevasses normally too small for unbroken grains to enter.

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That hypothesis was consistent with observations from Monday’s storm. Robin Ould, head of Asthma Foundation Victoria, told the BBC that the rye particle count on Monday was 102, more than twice the pollen level typically rated as high.

“It’s quite rare and we’ve seen two incidents of this kind this century in Melbourne,” Ould told the BBC.

Fungal spores, too, can exacerbate asthma during thunderstorms. “When it rains, it spores,” as microbiologist Susan Kosisky told The Washington Post in 2014.

To minimize the effects of thunderstorm asthma, which can be most severe during the first 20 to 30 minutes of the storm, it is recommended to stay indoors to limit exposure to pollen, as The Post reported. Keeping windows closed with a running air conditioner can filter air, and showering will remove particles as well. For those susceptible to asthma, carry a rescue inhaler.

Respiratory disease scientists predict that thunderstorm asthma outbreaks are likely to happen with increased frequency, as an overall warmer globe brings about more severe weather.

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People who are “affected by pollen allergy, as well as subjects affected by seasonal rhinitis without asthma symptoms, should be alerted to the danger of being outdoors during a thunderstorm in the pollen season, as such events may be an important cause of severe exacerbation of asthma,” concluded the team of Italian and French researchers in their 2016 clinical review of thunderstorm asthma. “Such a risk is likely to increase in relation to climate change and related extreme events.”