Seasonal allergy sufferers are being hit with more pollen over a longer season due to rising temperatures, but global warming is also triggering alarming extreme allergy events, say experts.

People could see the thunderstorm, but they couldn't see what was going on inside it. Trillions of pollen particles, sucked up into the clouds as the storm formed, were now being splintered by rain, lightning and humidity into ever-smaller fragments – then cast back down to Earth for people to breathe them in.

It was around 18:00 on 21 November 2016 when the air in Melbourne, Australia, turned deadly. Emergency service phone lines lit up, people struggling to breathe began flooding into hospitals, and there was so much demand for ambulances that the vehicles were unable to reach patients stuck at home. Emergency rooms saw eight times as many people turning up with breathing problems as they would normally expect. Nearly 10 times as many people with asthma were admitted to hospital.

In total, 10 people died, including a 20-year-old law student who passed away on her lawn, waiting for an ambulance while her family tried to resuscitate her. One survivor described how he had been breathing normally and then, within 30 minutes, found himself gasping for air. "It was insane," he told reporters from his hospital bed.

Paul Beggs, an environmental health scientist and professor at Macquarie University in Sydney, Australia, remembers the incident well. "It was an absolutely massive event. Unprecedented. Catastrophic," he says. "The people in Melbourne, the doctors and the nurses and the people in pharmacies – they all didn't know what was happening."

It soon became clear that this was a massive case of "thunderstorm asthma", which occurs when certain types of storms break up pollen particles in the air, releasing proteins and showering them on unsuspecting people below. The widely dispersed proteins can trigger allergic reactions in some people – even among those who weren't previously asthmatic.

Thunderstorm asthma events like the one that hit Melbourne are one extreme example of how pollen from plants and the allergies it causes are being dramatically altered by climate change. As temperatures rise, many regions – especially the US, Europe and Australia – are seeing seasonal allergies affect an increasing proportion of people, over a longer season and with worse symptoms, say scientists.

This year, in the US, pollen levels are predicted to be higher than the historical average across 39 states this season. And that is only likely to get worse in the years to come, experts warn.

Pollen itself is an essential and ever-present part of our world. These microscopic particles pass between plants, enabling them to reproduce. While some plants spread their pollen with the help of insects, others rely on the wind, sending huge volumes of this powdery substance airborne. Many trees, grasses and weed species rely upon wind dispersal for their pollen. It is these that are especially likely to cause seasonal allergies, or hay fever.

This occurs when your immune system mistakenly identifies the pollen as a harmful substance, triggering a response normally reserved for pathogenic bacteria or viruses. Common symptoms can include a runny nose, itchy eyes and sneezing. In some cases, seasonal allergies can trigger breathing difficulties when inflammation in the airways leads to swelling, making it hard to get enough air into the lungs.

While Melbourne has been the unlucky epicentre of thunderstorm asthma, with seven thunderstorm asthma major events recorded since 1984, similar incidents have occurred around the world, from Birmingham in the UK to Atlanta in the US. Although these are still rare events, climate change may be increasing the likelihood of thunderstorm asthma incidents, in part because it is extending pollen seasons, but also because it is increasing the frequency of extreme weather events, including storms.

While it isn't possible to determine exactly how much climate change influenced the 2016 thunderstorm asthma incident in Melbourne, Beggs is "reasonably certain" it had some impact.

"We know that climate change is leading to greater amounts of pollen in the atmosphere," he says. "It's changing the seasonality of the pollen. It's changing the types of pollen that we're exposed to." Beggs, who has researched thunderstorm asthma extensively, published a paper in 2024

© BBC