Six people have died and five are being kept on life support in Australia due to a rare condition caused by environmental factors during a thunderstorm that hit Australia’s second-largest city of Melbourne.

The sixth victim of the rare condition, known as “thunderstorm asthma,” died in hospital on Saturday while five patients remained in intensive care units, three of whom were in critical conditions, the Australian Health Department said in a statement.

Another 12 patients were in hospital with less serious respiratory and related conditions.

A wild thunderstorm hit Melbourne on Monday night, causing widespread asthma attacks across the city.

The storm overwhelmed emergency services and hospitals in the city of 4.5 million people, with 8,500 receiving hospital treatment over “thunderstorm asthma.”

It is believed that during a thunderstorm, pollen grains can absorb moisture and then burst into much smaller fragments, with the fragments being easily dispersed by wind over the city. The tiny pollen particles can penetrate deep into lungs, causing asthma attacks in some people.

Many of those affected during a “thunderstorm asthma” outbreak may have never experienced an asthma attack before.

Around a third of the patients who suffered asthma attacks on Monday reported never having had asthma before.

The world’s first recorded “thunderstorm asthma” outbreak occurred in Melbourne in 1987, when hospitals reported a five-fold increase in asthma cases.

Similar events have happened in the United States, Canada, Britain, and Italy.

The last thunderstorm asthma attacks in Melbourne had been reported in November 2010.