Melbourne has suffered what appears to be the most lethal episode of thunderstorm asthma on record. With at least four deaths and more patients left in intensive care, the storm has served as a grim reminder that the seasonal weather occurrence can have tragic knock-on effects for human health. “This is an extraordinary event,” said Prof Anthony Seaton, who has worked on thunderstorm asthma at the University of Aberdeen. “I don’t know of any event as severe as this.”

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Thunderstorm asthma has been studied for more than 30 years. Since the early 1980s, rapid spikes in sudden asthma attacks have been linked to thunderstorms in the UK, Australia, Italy and the US, but they no doubt occur in other countries. “The evidence for the connection is pretty robust,” said ProfAziz Sheikh, co-director of the Asthma UK Centre for Applied Research, a collaboration between 14 universities.



The largest known outbreak of thunderstorm asthma previously coincided with a heavy thunderstorm in London on 24 June 1994. In a 30-hour period which started at 6pm that evening, 640 patients with asthma or other breathing problems overwhelmed hospital emergency departments. Under normal conditions, the staff would have expected only 60 or so asthma patients. More than 100 were admitted, and five ended up in intensive care. In Melbourne, two consecutive thunderstorms saw a five to ten-fold rise in asthma cases in 1992, a report from the city’s Monash medical centre found.



But while scientists broadly agree that thunderstorms can unleash dramatic asthma outbreaks, the precise causes of the epidemics are less clear. One leading theory is that during thunderstorms, certain types of pollen grains are drawn up to the humid cloud base where they rupture and shed much finer highly allergenic granules into the atmosphere. When broken open, each pollen grain can release more than 500 allergy-causing granules. These fall back to the ground in cold downdrafts and, because they are much smaller than pollen particles, can be inhaled deep into the lungs. There, the particles may bring on severe asthma attacks in people who suffer from pollen allergies.



As a working theory it makes sense. Thunderstorm asthma coincides with high pollen counts and affects people who have hay fever. Those with pollen allergies who stay indoors and shut the windows during thunderstorms do not seem to suffer. But scientists have so far failed to prove the theory. Prof Jeroen Buters at Technical University Munich’s Center of Allergy and Environment monitored pollen allergens in the air for 24 hours a day over six years. He found no spike in the fine particles spewed out of rupturing pollen particles. But the measurements are hard to do. Pollen rupturing may occur only very briefly during a storm and could simply be missed in experiments. Another headache is that the filters used to monitor the air capture diesel particles too, which absorb pollen allergens on contact. “It’s a sexy theory,” said Buters. “But we have not found these small particles in ambient air in real life.”



The fact that most thunderstorms, and even those in the high-pollen season, do not unleash asthma epidemics makes it clear that scientists are missing something. In Melbourne, pollution is unlikely to play a role in the latest deaths. But thunderstorms suck up everything from pollen and fungal spores to bacteria and dust before they dump it back down in cold streams of air.

In a report published this year, doctors in Napoli warned that climate change is predicted to ramp up the frequency of storms, which could lead to more cases of thunderstorm asthma. “There’s an argument for public health advice,” said Seaton, “People with asthma, with allergy to pollen, should take precautions if a thunderstorm occurs.”