What is thunderstorm asthma?

A sudden outbreak of asthma symptoms during storms in the grass-pollen season. Storm humidity ruptures grass pollen into granules of 0.6 to 2.5 microns that travel deep into the lungs.

How does a storm cause asthma attacks?

An intact grass-pollen grain measures about 30 microns, too big to reach the lower airways, but storm moisture bursts it into roughly 700 microscopic fragments that gust fronts carry straight back to ground level.

What happened in Melbourne in 2016?

On November 21, 2016, a gust front with sky-high grass-pollen counts created the largest recorded event: a 672% surge in respiratory emergency-department visits, a 992% rise in asthma admissions, and 10 deaths.

Individuals allergic to grass pollen—especially those with seasonal hay fever—are at greatest risk, and many of them have never received an asthma diagnosis.

Does thunderstorm asthma happen in the US?

Yes. The US has recorded smaller clusters, such as a documented Atlanta outbreak, and a 2024 analysis connects thunderstorms to bursts of asthma-related emergency visits.

How do I protect myself?

During grass season, move indoors before and during storms, shut windows, set AC to recirculate, and keep asthma and allergy medications current and taken exactly as prescribed.

Thunderstorm Asthma: How Storms Trigger Sudden Allergy Attacks

Thunderstorm asthma isn’t ordinary asthma—it’s a sudden spike in breathing trouble that hits when storms roll through during grass-pollen season. A complete grass pollen grain measures roughly 30 microns, far too large to slip into the lower airways, yet humidity within the storm makes the grain swell and then literally pop, scattering hundreds of starch granules only 0.6 to 2.5 microns across. According to D'Amato and Emmerson, a single ryegrass grain can contain about 700 of these fragments.

Those minuscule granules travel deep into the lungs, while gust fronts sweep near-ground pollen aloft and slam the concentrated mix right back down to street level. Within hours, neighborhoods can see an eruption of asthma attacks in people who were not in distress a moment earlier.

Melbourne, 21 November 2016

A single grass-pollen thunderstorm produced the world's largest recorded epidemic asthma event.

Excess ER presentations672%

Thien et al. 2018, Lancet Planetary Health

Asthma hospital admissions992%

Thien et al. 2018, Lancet Planetary Health

Increase above baseline during the event. Source: Thien et al. 2018.

How a storm triggers an attack

When grass-pollen counts are already high, the storm’s outflow pulls pollen up into moist air. There, the grains soak up water and burst thanks to osmotic shock. The respirable fragments that emerge ride the downdraft and gust front back to ground level in one tight surge, delivering lung-size allergen to individuals who might usually manage with only a runny nose. Bronchospasm can follow almost immediately.

The 2016 Melbourne event

On November 21, 2016, the largest epidemic on record struck Melbourne, Australia. Grass-pollen levels were off the charts when a gust front rolled in and temperatures tumbled about 10 degrees. Within around 30 hours, emergency departments reported a 672% surge in respiratory visits, a 992% jump in asthma admissions, 35 intensive-care admissions, and 10 deaths—figures documented by Thien 2018 in Lancet Planetary Health.

Who is at risk

The highest-risk people are those allergic to grass pollen, particularly individuals with seasonal hay fever; many have never been told they had asthma. In Melbourne, a large portion of the sick had hay fever but no prior asthma diagnosis. The US has seen smaller clusters—an Atlanta outbreak is on record—and a 2024 analysis tied thunderstorms directly to spikes in asthma emergency visits (per ACAAI). If grass pollen gives you hay fever but you have never wheezed before, a powerful storm during pollen season is a moment to take seriously, not ignore.

How to protect yourself

During grass-pollen days, treat an incoming storm like a toxic cloud if you react to grass. Head indoors ahead of the rain and especially before the leading wind gusts, shut windows and doors, and set any air-conditioning to recirculate. Anyone with asthma or a confirmed grass allergy should keep reliever and preventer medications up to date and use them on schedule ahead of high-risk days, exactly as the Victoria Department of Health advises.

How a storm turns pollen dangerous

700

lung-deep granules per ruptured ryegrass grain

Emmerson et al., NIH-PMC

0.6–2.5µm

granule size — small enough to reach the airways

NIH/PMC

deaths in the 2016 Melbourne event

Thien et al. 2018

A whole grass grain is ~30 µm — too big to inhale deeply until a storm bursts it.

Check your local grass pollen forecast

Pollen seasons vary sharply by region. These metros see some of the worst grass pollen pressure — check the current forecast for each, or look up any US city on the pollen count hub:

Oklahoma City, OK pollen count — Grass pollen plus frequent spring storms make this prime thunderstorm-asthma territory.
Atlanta, GA pollen count — Site of a documented US thunderstorm-asthma outbreak, with heavy spring grass.
Dallas, TX pollen count — Warm-season grass and an active storm season overlap each spring.
Wichita, KS pollen count — Plains grassland and supercell season collide here.
Memphis, TN pollen count — Mid-South grass season and frequent thunderstorms raise the risk.
Kansas City, MO pollen count — Grass pollen and Great Plains storms peak together in late spring.

Frequently asked

What is thunderstorm asthma?: A sudden outbreak of asthma symptoms during storms in the grass-pollen season. Storm humidity ruptures grass pollen into granules of 0.6 to 2.5 microns that travel deep into the lungs.
How does a storm cause asthma attacks?: An intact grass-pollen grain measures about 30 microns, too big to reach the lower airways, but storm moisture bursts it into roughly 700 microscopic fragments that gust fronts carry straight back to ground level.
What happened in Melbourne in 2016?: On November 21, 2016, a gust front with sky-high grass-pollen counts created the largest recorded event: a 672% surge in respiratory emergency-department visits, a 992% rise in asthma admissions, and 10 deaths.
Who is most at risk?: Individuals allergic to grass pollen—especially those with seasonal hay fever—are at greatest risk, and many of them have never received an asthma diagnosis.
Does thunderstorm asthma happen in the US?: Yes. The US has recorded smaller clusters, such as a documented Atlanta outbreak, and a 2024 analysis connects thunderstorms to bursts of asthma-related emergency visits.
How do I protect myself?: During grass season, move indoors before and during storms, shut windows, set AC to recirculate, and keep asthma and allergy medications current and taken exactly as prescribed.