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Could an epidemic thunderstorm asthma event happen again in Melbourne?

Ben Domensino, Tuesday November 21, 2017 - 16:13 AEDT

Today marks the one year anniversary of a tragic thunderstorm asthma event in Melbourne that resulted in nine deaths and directly impacted the health of thousands of Victorians.

One year on, there is still a lot that we don't know about thunderstorms asthma, mainly because it doesn't happen very often in Australia or around the world.

In Australia, epidemic thunderstorm asthma events are known to have occurred in Wagga Wagga, Tamworth and Canberra, although the most frequent and severe cases have taken place in Melbourne.

Prior to 2016, there were five documented cases of epidemic thunderstorm asthma in Melbourne since 1980, all of which have occurred in November, at the peak of central Victoria's grass pollen season.

Last year's event in Melbourne is thought to have been unprecedented in scale and severity when compared to all other documented cases, both here in Australia and the rest of the world.

Research suggests that four ingredients are needed for a thunderstorm asthma event to occur and be widespread enough in its impacts to be defined as an epidemic. They are:
- A high concentration of allergenic material, such as pollen
- A thunderstorm capable of sweeping up this allergenic material and blowing it over a populated area
- The formation of allergenic particles that are small enough to be breathed in
- Exposure of people who are sensitive to the airborne allergen

The 2016 epidemic thunderstorm asthma event is thought to have affected so many people because it was simply the right mix of ingredients.

Rainfall during the leadup to November in 2016 was prolific. Victoria experienced its 10th wettest spring on record, including its second wettest September in more than a century and a wetter-than-average October. This early-season rain boosted vegetation growth across central and western Victoria, which then dried out as rainfall tapered off in November.

On Monday 21st of November 2016, a broad band of thunderstorms swept across western Victoria and reached Melbourne early in the evening. While the storms weakened before reaching the city, a burst of dry air rushing out ahead of the storms - called a gust front - swept across Melbourne carrying airborne pollen fragments that were small enough for people to breathe in.

Another important feature is that Melbourne's population is situated to the south of a large area of grassland that has been converted from bush habitat to pasture.

The time of day also meant that lots of people were outside, which is thought to have further exacerbated the event.

While there is still a lot that we don't know about thunderstorm asthma, there's one question that's worth asking: Will these events happen in the future?

It's not yet possible to know when, where and how frequently epidemic thunderstorm asthma events will happen in the future. However, we can look at how some of the ingredients required to produce these events may change in the years ahead.

Research has shown that if maximum temperatures increase in the future, this may cause intense thunderstorms to become more frequent in Melbourne by the end of this century. Warmer air near the ground may cause more energy for storm development.

In addition to temperatures, rising levels of carbon dioxide in the atmosphere are likely to enhance pollen production in grass species around the world. Some studies show that the production of grass pollen could rise by 200 per cent if the concentration of carbon dioxide in our atmosphere is doubled.

More research needs to be done before we can figure out exactly how the complex processes involved in epidemic thunderstorm asthma are going to change in the future. However, increasing the temperature and concentration of carbon dioxide in our atmosphere may make these events more likely in the years ahead.

- Weatherzone

© Weatherzone 2017

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