(CNN) An unusual combination of weather conditions leading to a freak illness known as thunderstorm asthma has left four people dead in Australia.

Thousands of people were rushed to hospital Monday with breathing problems in the southern Australian state of Victoria as emergency services struggled to cope.

Three were still in a critical condition Thursday, a Victorian Department of Health spokesman told CNN.

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.

Freak incident

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.

"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.

Though grass pollen is the most common known cause of thunderstorm asthma, attacks can also be triggered by excessive levels of tree pollen and fungal spores in the atmosphere. "This will vary by geography," said Aziz Sheikh, Professor of Primary Care Research and Development at the University of Edinburgh, adding that pollen from olive trees, for example, was reported in a previous thunderstorm asthma event in Italy in 2010.

Levels of fungal spores in the atmosphere typically peak during harvest, which can also be drawn up and broken down during large thunderstorms due to the rise in atmospheric pressure, according to Sheikh.

Edward Newbigin, a professor of biosciences at the University of Melbourne, said that many of those affected in Australia this week may never have had an asthma attack before.

"I imagine it was absolutely terrifying," he said.

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.

'Absolute shock'

One of the dead, 18-year-old Omar Moujalled, had just finished his final year of studies at Melbourne's Australian International Academy.

18-year-old Omar Moujalled was one of the victims of a "thunderstorm asthma" incident in Victoria, Australia on November 21, 2016.

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."

The fourth victim was father-of-two Clarence Leo, 7 News Australia said , who died after an asthma attack early on Tuesday morning.

Apollo Papadopoulos and Hope Carnevali died of breathing complications due to "thunderstorm asthma."

Rare illness

Thunderstorm asthma last struck Victoria in May 2010, having previously "had several reports," according to Sheikh.

But the condition is not limited to Australia. Experts say incidents of thunderstorm asthma have been seen in the UK, US, Italy and Canada, but the condition is "very under reported," said Newbigin.

Sheikh pointed out, however, that "English-speaking countries are also countries with the highest prevalence of asthma."

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.

"Anybody with severe or brittle asthma (a less common form involving severe but irregular attacks) is most likely to experience severe symptoms and need rapid treatment," said Sheikh, adding that smog days, pollution and smoking as other environmental triggers for an attack. "They are much less likely to trigger it if there is good underlying asthma control."