Thunderstorm Asthma: What Happened in Melbourne on 21 November 2016
Thunderstorm asthma is a rare phenomenon in which a particular kind of storm, arriving on a day of very high grass pollen, triggers asthma symptoms in large numbers of people at the same time. Most recorded events have been small. The event that struck Melbourne, Australia, on 21 November 2016 was not: it remains the largest and most serious epidemic thunderstorm asthma event documented anywhere in the world.
Key takeaways
- On 21 November 2016, Melbourne experienced the largest and most serious epidemic thunderstorm asthma event documented anywhere in the world (The Lancet Planetary Health, 2018).
- In roughly 30 hours, emergency departments in Melbourne and Geelong recorded 3,365 more respiratory presentations than expected — a 672 per cent increase (Victorian Chief Health Officer assessment).
- Only 28 per cent of surveyed emergency patients had current doctor-diagnosed asthma, but almost all had seasonal hay fever, and sensitisation to ryegrass pollen was found in 87 to 100 per cent of those tested.
- The leading explanation: storm moisture ruptures ryegrass grains, releasing hundreds of tiny starch granules under 3 microns — small enough to be inhaled deep into the lower airways.
- Victoria now issues an epidemic thunderstorm asthma risk forecast during grass-pollen season, combining pollen monitoring with storm prediction.
What thunderstorm asthma is — and what it is not
Ordinary grass pollen grains are relatively large — generally too large to penetrate deep into the lungs, which is why grass pollen allergy usually shows up as hay fever: itchy eyes, sneezing, a blocked nose. The leading explanation for thunderstorm asthma, first set out in research on ryegrass pollen in the early 1990s, is that storm moisture changes that. When ryegrass grains absorb water they can rupture from osmotic shock, releasing hundreds of tiny starch granules — each small enough, at under 3 microns, to be inhaled deep into the lower airways. A storm outflow then sweeps those particles across a city at ground level, concentrating them where people are breathing.
A rare combination, not a general rule
Two cautions belong with that explanation, and official reviews state both. First, the event requires a rare combination: extreme grass pollen, the right storm structure, and a large sensitised population in its path. Second, while the rupture mechanism is the generally accepted account, investigators have been careful to note that the precise sequence in any given event is not completely established. Thunderstorm asthma is not "storms make pollen worse" in general — most thunderstorms, on most high-pollen days, produce nothing like it.
The evening of 21 November 2016
Melbourne was at the peak of its spring grass-pollen season, surrounded by ryegrass pasture. According to the peer-reviewed investigation published in The Lancet Planetary Health in 2018, grass pollen concentrations that day were extremely high, above 100 grains per cubic metre. Late in the afternoon it was hot; at around 6 p.m. a gust front crossed Melbourne and Geelong, temperatures fell about 10°C, and humidity rose above 70 per cent.
The roughly 30 hours that followed
What followed, over roughly the next 30 hours, is quantified in the Victorian Chief Health Officer's assessment and summarised in the Australian Journal of General Practice (2022). Emergency departments in Melbourne and Geelong recorded 3,365 more respiratory presentations than expected — a 672 per cent increase. Asthma-related hospital admissions rose by 476 above expectation, a 992 per cent increase. Ambulance services received 1,626 more emergency calls than forecast. Thirty-five people were admitted to intensive care. Ten people died; their deaths were investigated by the Victorian Coroner. We do not name them here: they were private citizens, and the official reports treat this event as a public-health case study, which is how it should be read.
| Measure, Melbourne and Geelong (roughly 30 hours) | Recorded figure |
|---|---|
| Excess emergency department respiratory presentations | 3,365 more than expected (+672%) |
| Excess asthma-related hospital admissions | 476 above expectation (+992%) |
| Excess ambulance emergency calls | 1,626 more than forecast |
| Intensive care admissions | 35 people |
| Deaths | 10 people |
Who was affected: the finding that changed the advice
The patient data overturned an assumption. Of the emergency patients surveyed for The Lancet Planetary Health study, only 28 per cent had current doctor-diagnosed asthma — for many, this was their first asthma attack. But almost all had seasonal hay fever, and among those tested, sensitisation to ryegrass pollen was found in 87 to 100 per cent. In other words, the at-risk group was much wider than "people with asthma": it was people with grass-pollen hay fever, most of whom had no reason to think of themselves as at risk of an asthma emergency.
The gravest outcomes were among people with diagnosed asthma
The patients admitted to intensive care, and those who died, all had diagnosed asthma — a distinction the official reports draw carefully, and one reason health authorities afterwards focused on both groups: hay-fever sufferers who had never had asthma symptoms, and asthma patients whose condition was not fully controlled.
What officials changed afterwards
In December 2016 the Victorian Government asked the Inspector-General for Emergency Management to review the response. The review's final report (April 2017) made 16 recommendations, all accepted in principle; a May 2018 progress report found 11 complete and the rest progressing or closed.
The practical legacy: a thunderstorm asthma forecast
The practical legacy is a forecasting system: Victoria now issues an epidemic thunderstorm asthma risk forecast during grass-pollen season, combining pollen monitoring with storm prediction, alongside public education aimed at people with hay fever — not only people with asthma. Melbourne had experienced smaller thunderstorm asthma events before, in 2003 and 2010 among others; 2016 was different in scale, not in kind, and the system built afterwards is designed for exactly that recurrence.
What thunderstorm asthma means for readers in the United States
Nothing on the Melbourne scale has been documented in the US, and the specific ingredients — vast ryegrass pasture upwind of a major city in peak season — are distinctly Victorian. But the underlying ingredients are familiar: high grass-pollen days occur across Dallas, Tulsa, and Memphis, and summer storms routinely sweep pollen-laden air across Phoenix and the plains. The transferable lesson is the one Victorian authorities now repeat every spring: on extreme pollen days, weather is not background — a front can change what is in the air within an hour. That is why daily, local numbers matter more than seasonal averages, why Pollen Atlas tracks grass pollen as its own index (see Grass Pollen Season and What Is a Pollen Count?), and why anyone whose hay fever is severe — or who has ever had asthma symptoms — is better off discussing their situation with a clinician before peak season than during it. You can find an allergist through the Pollen Atlas directory. This article is general information only and is not medical advice.
Frequently Asked Questions
What is thunderstorm asthma?
Thunderstorm asthma is a rare phenomenon in which a particular kind of storm, arriving on a day of very high grass pollen, triggers asthma symptoms in large numbers of people at the same time. The leading explanation is that storm moisture ruptures grass pollen grains into tiny particles small enough to be inhaled deep into the lower airways.
What happened in Melbourne on 21 November 2016?
Melbourne was at the peak of its spring grass-pollen season, with grass pollen concentrations above 100 grains per cubic metre, when a gust front crossed the city at around 6 p.m. Temperatures fell about 10°C and humidity rose above 70 per cent, and over roughly the next 30 hours emergency departments recorded 3,365 more respiratory presentations than expected.
Who is at risk of thunderstorm asthma?
The Melbourne event showed the at-risk group is wider than people with asthma: almost all of the emergency patients surveyed had seasonal hay fever, and ryegrass sensitisation was found in 87 to 100 per cent of those tested, while only 28 per cent had current doctor-diagnosed asthma. Health authorities afterwards aimed warnings at people with hay fever as well as people with asthma.
Can thunderstorm asthma happen in the United States?
Nothing on the Melbourne scale has been documented in the US, and the specific ingredients are distinctly Victorian. But high grass-pollen days and pollen-laden summer storms are familiar across much of the United States, which is why the transferable lesson is to watch pollen counts and weather together on extreme pollen days.
Conclusion: why thunderstorm asthma is now part of every forecast conversation
Thunderstorm asthma entered the global vocabulary on 21 November 2016, when extreme ryegrass pollen and a gust front converged over Melbourne: 3,365 excess emergency presentations, 476 excess admissions, and ten deaths, documented by the Victorian Chief Health Officer, the Inspector-General's review, and The Lancet Planetary Health. The response it produced — pollen monitoring joined to storm forecasting, and warnings aimed at hay-fever sufferers as well as asthma patients — is the model now. Watch both halves of that equation where you live: check the daily city rankings alongside the weather, and keep an eye on your own city's free pollen counts and forecasts at PollenAtlas.com — for your neighbourhood and worldwide — especially on the season's biggest grass-pollen days.
Sources cited in text: Thien et al., "The Melbourne epidemic thunderstorm asthma event 2016," The Lancet Planetary Health (2018); Victorian Chief Health Officer assessment, summarised in the Australian Journal of General Practice (2022); Inspector-General for Emergency Management, Review of response to the thunderstorm asthma event of 21–22 November 2016, Final Report (2017) and implementation progress report (May 2018). This article is general information only and is not medical advice.