SYDNEY, Australia — When David McGann left his office in Melbourne just after 5 p.m. to cycle home, a stifling heat had settled across the city, and the temperature was peaking at 95 degrees.

A hot, gusty northerly wind picked up. Rain clouds had gathered across the skyline, but there was little relief. “It was the hottest day of the season,” said Mr. McGann, 35, who manages accounts at a law practice. “By the time I got home and had a swim, my chest had started to tighten.”

Mr. McGann’s partner, Kelli Morris, kissed him goodbye at their apartment to join teammates playing in a nighttime netball competition. When she returned, he was on the couch, sitting quietly, struggling for breath. The inhaler he found after rummaging through drawers was five years past its expiration date.

Mr. McGann was one of thousands of people in Melbourne having an attack of thunderstorm asthma. They flooded the city’s emergency rooms, swamped ambulance call lines and joined lines around pharmacies during six hours on Nov. 21. All were struggling for breath. About 8,500 people went to hospitals. Eight have died, and one remains in intensive care more than a week after a thunderstorm surged across Melbourne, carrying pollen that strong winds and rain broke into tiny fragments.