Thunderstorm asthma is not known to occur in Perth. During Victoria’s annual grass pollen season (October through December) there is a seasonal increase in asthma and hay fever symptoms. There is also a small chance of a thunderstorm asthma event. Thunderstorm asthma is thought to be triggered by a unique combination of high grass pollen levels and a certain type of thunderstorm, causing many people to develop asthma symptoms over a short period of time. The sudden onset and large increase in asthma symptoms following a storm is described as thunderstorm asthma.