Peak thunderstorm asthma season is upon us.
October to the end of December is the main time for asthmatics to be on alert, experts say.
Australia has one of the highest rates of asthma in the world, with one in 10 people affected by the condition that causes distressed breathing.
In 2016, nine people died when a thunderstorm asthma event hit Melbourne, causing emergency services to be overloaded. It was the largest single thunderstorm asthma event recorded in living memory.
National Asthma Council director Professor Peter Wark said it was "a perfect storm".
"It was the right conditions, lots of wet weather, lots of grass, and then the thunderstorm happened at the right time. It struck when people were leaving work and going home, so people were outside when they were going to get exposed."
Hot, dry northerly winds moved over Melbourne's metropolitan area around 5pm as commuters were travelling home on November 21, 2016.
The combination of the moisture from the storm and the rye grass in Melbourne's north and west resulted in the pollen breaking into tiny pieces, which can easily penetrate a persons airways and trigger an asthmatic reaction.
Experts learned that even people without an asthma diagnosis are at risk.
Vulnerable people include those with springtime hay fever, people who are allergic to grass pollen, and asthmatics whose symptoms get worse in springtime.
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"Occasionally, if you get a bad enough event, people who weren't aware they had asthma, but who are allergic to grass, can also for the first time develop asthma, that's where the danger comes," Professor Wark said.
The prevalence of asthma in Melbourne was due to several complex factors. Geographically, Melbourne is surrounded by planes of rye grass. A growing population and pollution may also be contributing factors.
"It's also a city that has a lot of recent migration, many first generation Australians coming in," Professor Wark said.
"Asthma is something people can acquire within that first or second generation of coming here. We don't understand the reasons for that. It clearly has something to do with our environment, it may well have something to do with our diet, with pollution levels."
Steps can be taken to prevent an asthmatic reaction during thunderstorm asthma season. Asthmatics should use preventer medication regularly and carry an asthma puffer at all times.
Those who suffer hay fever in spring should see a doctor to determine if you have asthma and develop an action plan.
"Good day-to-day control of your asthma is the best insurance against these events in the long run," Professor Wark said.
Vulnerable people should pay attention to pollen counts and stay indoors during thunderstorm warnings.
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