A scary confluence of hazards killed at least four people and sent thousands of people to emergency rooms in Australia on Monday night.

A strong thunderstorm rolled through Melbourne, which, in itself, may not have been all that remarkable. But the air was hot and dry before this cold front, and grass pollen was lying in wait for the slightest bit of moisture.

The Bureau of Meteorology ended up posting severe storm warnings for the entire state of Victoria as it plowed across southern Australia.

“The noise was unbelievable, it was truly unbelievable,” Melbourne resident Marie Clement told the Australian Broadcasting Corp. “The chairs were shooting past the family room window, the garden chairs — it was very frightening indeed.”

Chairs and shingles were the least of this storm’s threats, though. After it passed, people couldn’t breathe. The rain and wind combined with the pollen created a deadly situation that even affected people who don’t typically have respiratory issues.

Between 6 and 11 that night, there were about 2,000 calls to triple-zero — Australia’s version of 911 — which is nearly seven times the average number, the ABC reports.

The scene at hospitals was chaos, the ABC reported:

Hospitals were swamped with emergency patients, while firefighters and police were called on to help paramedics respond to thousands of calls after the conditions caused breathing problems for Victorians.
Jill Hennessy, the Health Minister, said it was like “a bomb going off”, with all ambulances, police, fire and nonemergency transport being used to deal with the crisis.
“Our health system was stretched to the limit,” she told 774 ABC Melbourne.
“We know that we’ve got some people in intensive care units as well. At one point we had 140 code one cases, while dealing with other patients, all of them were cardiac arrests or severe respiratory conditions.”

Four people had died as of Wednesday morning, Australian Eastern time, according to the ABC.

Melbourne is the thunderstorm asthma capital of the world, says the ABC. It’s brought on when rain combines with extreme pollen counts, and it can affect people who don’t usually suffer from asthma.

Robin Auld from Asthma Victoria spoke with the ABC and explained how something like this can happen.

“What we understand is the heavy rain causes the rye grass pollen to absorb moisture and they then burst and become much smaller,” Auld said. “And those smaller particles can be dispersed very easily by wind over quite a distance. It’s those smaller particles that can then get in through the nose, into the small bronchial tubes in the lungs and that’s what causes the allergic reaction.”

Auld said the first known thunderstorm asthma event occurred in Melbourne in 1987.