Every year during grass pollen season (October through December in south-east Australia) there is an increase in asthma and hay fever symptoms. There is also the small chance of an epidemic thunderstorm asthma event.
Epidemic thunderstorm asthma is thought to be triggered by a unique combination of high grass pollen levels and a certain type of thunderstorm, causing a large number of people to develop asthma symptoms over a short period of time. The sudden onset and large increase in asthma symptoms has been described as an epidemic of asthma and hence the term epidemic thunderstorm asthma.
These events result from grass pollen grains being swept up in the wind and carried for long distances. Some can burst open and release tiny particles (allergens) that are thought to be concentrated in the wind gusts that come just before a thunderstorm. These particles are small enough to be breathed deep into the lungs and can trigger asthma symptoms, making it difficult to breathe.
This can become very severe, very quickly and many people may require medical help at the same time. This can placing a lot of pressure on health services.
It is important to note that epidemic thunderstorm asthma events are uncommon and are not responsible for the seasonal increase in asthma symptoms seen every year.