Two people have died in Melbourne, Australia, from a rare phenomenon known as "thunderstorm asthma", after a storm hit the city late on Monday local time.
Paramedics and hospitals were stretched to their limits as thousands phoned to report breathing problems.
The trigger was an extreme level of rye grass pollen in the air, whipped up by strong winds.
Ambulance Victoria took more than 1,870 calls between 18:00 (07:00 GMT) and 23:00 on Monday.
The figure is more than six times the daily average, and at least 30 people are in intensive care.
The massive call volume affected waiting times and forced 60 reserve ambulances, police and fire-fighters to respond to medical emergencies.
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"It was extraordinarily busy, it was unprecedented," said Mick Stephenson, Ambulance Victoria's executive director of emergency operations.
He said about 200 calls were directly linked to asthma, but 600 more reported respiratory issues.
"What we do know is that a lot of people who called last night had never had asthma before, so this was their first experience," he said.
One of those killed by the freak weather event was named by her family as 20-year-old Hope Carnevali.
Local media said the young woman had suffered a horrific asthma attack, and lay on her lawn for more than 30 minutes waiting for an ambulance to arrive, while her family desperately performed CPR.
Eighteen-year-old Omar Moujalled, a high school student from Greenvale, died at a doctor's surgery before he could reach hospital.
A spokesman for Ambulance Victoria said: "Our thoughts are with the families of those patients. We will carry out a full clinical review into these cases."
Victorian Health Minister Jill Hennessy said the challenging conditions had forced private hospitals to open to the public.
"No doubt there will be lessons to learn... but every part of our emergency management surge occurred correctly," she said.
What is 'thunderstorm asthma'?
Melbourne's thunderstorm asthma event was caused by very high levels of rye grass pollen.
About one in 10 people have asthma in Australia, with about 80% of those sufferers experiencing allergies, particularly to rye pollen.
When saturated in rain, the pollen breaks down into smaller particles which become easier to inhale and trigger an asthma attack.
"A person who has an allergic response to that particular pollen, because it's so small, it will get into the lungs," Asthma Victoria chief Robin Ould told the BBC.
He said Monday's rye particle count of 102 was more than double what would be considered high on a normal day.
Melbourne's current spring season has been particularly wet, creating havoc for asthma and hay fever sufferers.
"It's quite rare and we've seen two incidents of this kind this century in Melbourne," Mr Ould said.
"It's the south-east corner of Australia that has this high humidity, high thunderstorm activity at this time of the year."
But he said the scale of Monday's emergency could not have been predicted.
"The key message of this event is that, if you have asthma, you should be using your preventative medication as described," he said.
Reporting by the BBC's Jay Savage.
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