Is thunderstorm asthma becoming more common?

When spring arrives, so do warnings about thunderstorm asthma. But a decade ago, most of us hadn't heard of it.

So where did thunderstorm asthma come from? Is it a new phenomenon?

In 2016, the world's most catastrophic thunderstorm asthma event took Melbourne by surprise. An increase in warnings and monitoring is partly a response to this.

But there are also signs climate change may be exacerbating the likelihood of thunderstorm asthma, with more extreme weather, extended pollen seasons and a rise in Australians reporting hay fever.

The first time many Australians heard of thunderstorm asthma was in November 2016, when a major event rocked Melbourne.

During a late night storm, an estimated 10,000 people were rushed to hospitals with severe asthma attacks. With thousands of calls on emergency lines, ambulances and emergency departments were unprepared to handle the rapid increase in people needing urgent medical care. Tragically, ten of those people died.

This was the most catastrophic thunderstorm asthma event in recorded history and the first time deaths have ever occurred anywhere in the world.

In response, the Victorian Department of Health implemented initiatives, including public awareness campaigns and improvements to health and emergency services, to be ready for future thunderstorm asthma events.

A network of pollen monitoring stations was also set up across the state to gather data that helps to predict future events.

While this event was unexpected, it wasn't the first time we'd had thunderstorm asthma in Australia - we've actually known about it for decades.

Melbourne reported its first instance of thunderstorm asthma back in 1984, only a year after this phenomenon was first discovered in Birmingham in the United Kingdom.

Thunderstorm asthma has since been reported in other parts of Australia, including Canberra and New South Wales. But it is still most common in Melbourne. Compared to any other city (or country) the gap is significant: over a quarter of all known events worldwide have occurred in Melbourne.

Melbourne's location makes it a hotspot for these kinds of events. Winds coming from the north of Melbourne tend to be dry and hot as they come from deserts in the centre of Australia, while winds from the south are cooler as they come from the ocean.

When hot and cool air mix above Melbourne, it creates the perfect conditions for thunderstorms to form.

Northern winds also blow a lot of pollen from farmlands into the city, in particular grass pollen. This is not only the most common cause of seasonal hay fever in Melbourne but also a major trigger of thunderstorm asthma.

There's a particular reason grass pollen is the main culprit behind thunderstorm asthma in Australia. During storms there is a lot of moisture in the air. Grass pollen will absorb this moisture, making it swell up like a water balloon.

If pollen absorbs too much water whilst airborne, it can burst or "rupture," releasing hundreds of microscopic particles into the air that can be swept by powerful winds.

Normally, when you breathe in pollen it gets stuck in your upper airway - for example, your nose and throat. This is what causes typical hay fever symptoms such as sneezing or runny nose.

But the microscopic particles released from ruptured grass pollen are much smaller and don't get stuck as easily in the upper airway. Instead, they can travel deep into your airways until they reach your lungs. This may trigger more severe symptoms, such as wheezing or difficulty breathing, even in people with no prior history of asthma.

You might think asthma is the biggest risk factor for thunderstorm asthma. In fact, the biggest risk factor is hay fever.

Up to 99% of patients who went to the emergency department during the Melbourne 2016 event had hay fever, while a majority (60%) had no prior diagnosis of asthma.

Every single person hospitalised was allergic to at least one type of grass pollen. All had a sensitivity to ryegrass.

Thunderstorm asthma events are rare, with just 26 events officially recorded worldwide.

However there is evidence these events could become more frequent and severe in coming years, due to climate change. Higher temperatures and pollution could be making plants produce more pollen and pollen seasons last much longer.

Extreme weather events, including thunderstorms, are also expected to become more common and severe.

In addition, there are signs rates that hay fever may be increasing. The number of Australians reporting allergy symptoms have risen from 15% in 2008 to 24% in 2022. Similar trends in other countries has been linked to climate change.

Read more:

Why do I have hay fever? I didn't have it as a child

Here are three ways you can reduce your risk of thunderstorm asthma:

stock up on allergy medication and set up an asthma action plan with your GP

check daily pollen forecasts for the estimated pollen level and risk of a thunderstorm asthma event in your local area

on days with high pollen or a high risk of thunderstorm asthma, spend less time outside or wear a surgical face mask to reduce your symptoms.