At least four people have died and hundreds were sent to the hospital this week in Melbourne, Australia after an outbreak of "thunderstorm asthma" struck late Monday.

The rare condition is thought to occur when high levels of pollen become saturated by rain and disintegrate into teeny fragments that are easier to inhale. Those minuscule particles can invade the deep crevices of the lungs and get lodged there. In people who are allergic to pollen and/or already suffer from asthma, the scenario can easily become life-threatening.

On Monday, the city’s rye grass pollen count was reported to be double the level normally considered "high."

After a spring thunderstorm hit the city Monday evening, emergency responders were inundated with calls; in the span of five hours, they received more than 1,870 calls, six times the number for an average day. Many of the calls were due to asthma attacks and breathing problems. Some victims said they had never had such respiratory problems before.

Initial reports indicated that two people had died during the storm. According to the BBC, one woman died while waiting 30 minutes for an ambulance after having an asthma attack. All the while, her family was desperately performing CPR. Another woman died before she could reach the hospital.

As of late Wednesday, two more deaths have been confirmed, with one victim succumbing early Wednesday, bringing the toll to four. The Associated Press reported that additional victims had been transferred to intensive care.

Robin Ould, head of Asthma Foundation Victoria, told the BBC that although devastating, such outbreaks are quite rare. "We've seen two incidents of this kind this century in Melbourne," she said.

Still, experts recommend that people who suffer from allergies or asthma be aware of the potential for "thunderstorm asthma," particularly as global warming stands to increase the frequency of the phenomenon. In the event of such an attack, sufferers are advised to close windows, run an air conditioner with a filter, shower to remove particles, and keep a rescue inhaler on hand.