What thunderstorm asthma is
Thunderstorm asthma is not ordinary asthma triggered by cold air or exercise. It is a specific event caused by an uncommon combination of very high grass pollen levels and a particular type of thunderstorm. When this combination occurs, pollen grains are swept high into the atmosphere, where moisture causes them to rupture into thousands of tiny particles. These particles are small enough to penetrate deep into the airways, triggering sudden and severe respiratory responses.
The critical point for hikers is this: thunderstorm asthma can affect people with no prior history of asthma or respiratory illness. It is not a condition you know you are vulnerable to until it happens.
Why it matters on trail
In everyday life, thunderstorm asthma events allow most people to move indoors quickly. On trail, that option may not exist. You may be several hours from your car, on an exposed ridge, or in terrain where rapid movement is difficult. If respiratory distress develops on trail, your options narrow quickly.
The onset can be fast. Someone who felt completely fine at the trailhead can move to severe respiratory difficulty within minutes of a storm outflow reaching them. This is not a gradual warning — it is sudden.
The conditions that create the risk
Two factors must combine for thunderstorm asthma to occur:
High grass pollen levels. In southern Australia, grass pollen season runs roughly from October through December, peaking in November. Pollen counts vary by day depending on temperature, wind, and recent rainfall. High pollen days are most common during warm, dry, windy conditions before a cool change.
A specific storm pattern. Not all thunderstorms create thunderstorm asthma risk. The event requires storm outflows — strong winds at ground level ahead of a storm — combined with high atmospheric moisture that ruptures pollen grains at altitude.
When both conditions exist simultaneously, the risk window is real and the BOM issues warnings.
How BOM warnings work
The Bureau of Meteorology issues Thunderstorm Asthma Forecast alerts when conditions create elevated risk. These are separate from standard Severe Thunderstorm Warnings. A Severe Thunderstorm Warning covers hail, wind gusts, and flooding. A Thunderstorm Asthma Forecast is a specific alert covering respiratory risk.
These warnings are available through the BOM website and app, and are typically issued for southern Australian regions during pollen season. They are not issued routinely — they reflect a genuine elevated risk assessment when pollen and storm conditions align.
Check BOM before any hike during spring in Victoria, South Australia, and southern NSW. This takes thirty seconds and is as relevant as checking the fire danger rating.
Who is at risk
People with diagnosed asthma or hay fever face the highest risk, but thunderstorm asthma events have caused serious illness and death in people with no prior respiratory history. The particle size produced during these events is small enough to affect airways that would normally handle typical pollen exposure without difficulty.
On trail, additional risk factors include:
- High breathing rate from sustained exertion
- Extended time outdoors during the storm outflow window
- Being in open terrain with no shelter from wind or precipitation
- No access to medication
If you have asthma or hay fever, always carry your preventer and reliever medication on spring hikes. Do not leave them in the car.
Go, delay, or get off the trail
This is the decision this article is designed to help you make.
Go when pollen counts are low, no Thunderstorm Asthma Forecast has been issued, and storm activity is not forecast for your area and timeframe.
Delay or modify when pollen counts are high but no storm activity is forecast. Consider shorter routes, earlier starts to finish before afternoon storm development, and staying closer to shelter and exit points.
Cancel or get off the trail when a Thunderstorm Asthma Forecast has been issued for your region. This is not a precautionary suggestion — it is the appropriate response to a confirmed elevated risk event. If you are already on trail when a warning is issued and storm conditions are developing, move to exit immediately. Do not wait for symptoms.
If symptoms develop on trail
If you or someone in your group develops sudden shortness of breath, chest tightness, wheezing, or coughing during or after a storm outflow:
- Stop activity immediately
- Use a Ventolin inhaler if available — four puffs via spacer, wait four minutes, repeat if no improvement
- Call 000 if symptoms are severe or not improving
- Move toward shelter and exit but do not increase exertion if breathing is already compromised
Thunderstorm asthma can escalate rapidly. Treat it as a medical emergency, not a manageable discomfort.
Key points to remember
- Thunderstorm asthma affects people with no prior asthma history
- It requires high grass pollen and specific storm conditions to occur
- Pollen season in southern Australia peaks in November
- BOM issues Thunderstorm Asthma Forecasts — check them during spring
- On trail, sudden onset with no nearby shelter makes this hazard more serious than in urban settings
- Cancel or exit if a forecast is issued — do not wait for symptoms
