Most people know that a thunderstorm can interrupt a picnic. Fewer realise that in certain conditions, a summer thunderstorm can trigger sudden, severe breathing difficulties in hay fever sufferers - including people who have never had asthma. This phenomenon, called thunderstorm asthma, is documented by UK health authorities including GOV.UK and Allergy UK, and it is particularly relevant during the May to July peak of UK grass pollen season. Here is what you need to know, and why sudden severe breathing difficulty should always be treated as a medical emergency.
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Thunderstorm asthma occurs when specific atmospheric conditions during a storm cause pollen grains to rupture into tiny fragments that can penetrate deep into the lungs. Under normal conditions, grass pollen grains are 20 to 40 microns in diameter - large enough that they are mostly filtered by the nose and throat.
When a storm front moves through a high-pollen environment, the turbulent airflow at the storm's leading edge draws pollen grains high into the atmosphere. There, the humidity at altitude causes the grains to burst open and release their inner starch granules, which are just 0.5 to 2.5 microns across - small enough to reach the deepest parts of the lungs. When the storm then pushes this fragment-laden air back down to ground level, anyone outdoors or near open windows can inhale a concentrated mass of these microscopic particles, which can trigger sudden bronchospasm.
The Melbourne 2016 Event: Why This Matters
The most documented thunderstorm asthma event occurred in Melbourne, Australia, in November 2016. In a single night, several people died and more than 8,500 were treated in emergency departments - many of whom had no prior history of asthma. The event overwhelmed the region's hospital system.
The UK has experienced similar, though less severe, episodes. During high grass pollen seasons, thunderstorm events across southern England and the Midlands have been linked to sharp spikes in emergency asthma admissions. GOV.UK and Allergy UK both maintain guidance on thunderstorm asthma, and the NHS now includes it in hay fever resources. The risk is highest in the UK from May through July, during peak grass pollen season, when large thunderstorms are also most common.
Who Is Most at Risk in the UK?
The highest-risk group is people who have both hay fever and asthma - estimated at 3 to 5 million UK adults. However, thunderstorm asthma events have also affected people with grass pollen allergy but no asthma diagnosis.
- Your risk is elevated if you are allergic to grass pollen, even without diagnosed asthma.
- Those with poorly controlled asthma are at greatest risk of a severe outcome.
- People with poorly controlled hay fever during storm conditions are also at elevated risk.
- Adults aged 20 to 40 with grass pollen allergy are disproportionately represented in hospital admissions - possibly because they are more likely to be outdoors during a storm and less likely to carry a reliever inhaler.
The risk is concentrated in the first 30 minutes after a storm front arrives, during the period of intense pollen fragment dispersal.
Recognising the Symptoms
Thunderstorm asthma can develop very rapidly, within minutes of exposure. Symptoms include:
- Sudden wheezing, a tight chest, and shortness of breath.
- Severe hay fever symptoms (sneezing, runny nose, itchy eyes) from very high pollen exposure.
- In severe cases, difficulty speaking, blue lips, and an inability to complete sentences.
The key warning sign is that symptoms develop suddenly during or immediately after a storm, rather than building gradually over days. If you experience sudden severe breathing difficulty during a thunderstorm in pollen season, treat it as a medical emergency - call 999 or get to an A&E department immediately.
How to Protect Yourself During UK Storm Season
The most effective protection is staying indoors with windows and doors closed when a thunderstorm is approaching during high pollen periods.
- Check the Met Office pollen forecast each morning during May to July. If pollen is rated high or very high and a thunderstorm is forecast, take particular care.
- Go inside at least 30 minutes before a storm arrives - the risk peaks in the 30-minute window as the front moves in.
- If you have asthma alongside hay fever, carry your reliever inhaler at all times during pollen season, not just when you feel unwell, and follow your personal asthma action plan.
- Close all windows and doors when a storm is incoming, even in hot weather.
- Running a MERV-13 air purifier with windows closed during a storm period reduces indoor pollen fragment concentrations.
If you are not sure how to manage your asthma or hay fever during storm season, speak to your GP, asthma nurse, or pharmacist.
Forecasting and Storm Risk Monitoring
DEFRA's UK-AIR system tracks regional pollen and air quality across the UK in near real-time, and this site's pollen map draws on DEFRA data to show live regional risk levels. During summer storm events, the combination of very high pollen readings followed by an incoming thunderstorm is the key warning signal.
The Met Office has begun integrating air quality and pollen data more closely with storm forecasts precisely because of thunderstorm asthma risk. You can combine the Met Office pollen forecast with the 5-day weather forecast during grass pollen season to anticipate high-risk days: warm, dry, high-pollen days followed by an afternoon or evening thunderstorm are the highest-risk pattern.
Indoor Air Quality During and After Storms
The period immediately after a thunderstorm is also risky - the storm has stirred pollen into the air, and residual fine fragments can remain suspended for hours. Keep windows closed for at least 1 to 2 hours after a storm passes during peak pollen season.
Reduce indoor pollen: see our guide to the which air purifier helps most with hay fever — model picks, room-size guidance and where to place a unit for the biggest overnight reduction.