A freak illness known as thunderstorm asthma has now killed at least six people in Australia.
Thousands were hospitalized in Melbourne and other parts of Victoria last Monday with breathing problems due to a rare combination of weather and pollen.
One week later, 12 people are still receiving hospital treatment, including three who are in a critical condition, according to the Victoria Department of Health and Human Services.
Thunderstorm asthma occurs when a storm hits during a period of unusually high pollen and high humidity, causing the grains to break up and disperse, entering people's lungs and making it hard for them to breathe.
In a survey by the University of Melbourne, 74% of respondents said they experienced an asthma attack during the storm last week.
Though grass pollen is the most common known cause of thunderstorm asthma, attacks can also be triggered by excessive levels of tree pollen and fungal spores in the atmosphere.
"This will vary by geography," said Aziz Sheikh, Professor of Primary Care Research and Development at the University of Edinburgh, adding that pollen from olive trees, for example, was reported in a previous thunderstorm asthma event in Italy in 2010.
Levels of fungal spores in the atmosphere typically peak during harvest, which can also be drawn up and broken down during large thunderstorms due to the rise in atmospheric pressure, according to Sheikh.
An official review is currently underway into how Victoria's state emergency services and health system responded to the thunderstorm asthma emergency.
An extra 60 ambulances had to be deployed as more than 1,900 calls flooded emergency lines in four hours, or one call every four to five seconds.
What can be done?
While thunderstorm asthma has occurred all over the world in different conditions, there are persistent factors, according to Reena Ghildyal, an expert in biomedical sciences at the University of Canberra.
"There are many common threads in all reports of thunderstorm-related asthma -- a high concentration of potentially allergenic material such as that in late spring in Melbourne (pollen grains or fungi), a thunderstorm that sweeps up the allergens, which burst when wet and release very small particles (such as starch granules or fungal spores)," she wrote last week.
People with pre-existing asthma are particularly at risk of complications from thunderstorm asthma, and should therefore take necessary precautions.
"Keep updated on local pollen counts and weather forecasts, especially in spring; keep your asthma medication up to date; enjoy the spectacle of the thunderstorm from inside your house; and call (emergency services) if your asthma worsens or you feel any breathing difficulty," she said.