A rare phenomenon called "thunderstorm asthma" is currently plaguing the Australian city of Melbourne.

On Monday evening, a sudden drop in temperature from 35 degrees Celsius along with severe thunderstorms triggered mass pollen allergy asthma attacks, where over 8,000 people in the state of Victoria were rushed to the hospital.

To date, death toll has risen to six and three more are in critical condition, BBC reports.

Within a four-hour period that day, Ambulance Victoria had received over 1,900 calls and extra 60 ambulances were deployed, together with police and several groups of firefighters, Business Insider reports.

What is thunderstorm asthma?

According to the American Academy of Allergy Asthma and Immunology (AAAAI), asthma is a condition characterized by inflamed bronchial tubes or the airways in the lungs, typically triggered by an allergic reaction. In Australia, the asthma attack was triggered by hay-fever, the allergy to pollen. Heavy rain can make hay-fever symptoms - like asthma - worse, said the Australasian Society of Clinical Immunology and Allergy.

When rain falls, pollen grains absorb moisture and erupt, spreading small allergenic particles that could irritate the lungs when inhaled. According to Business Insider, these particles could irritate and inflame bronchial tubes, which are filled with mucus and cause difficulty of breathing.

"When you have a perfect storm coming together [of] a very high pollen day, high humidity, and a thunderstorm, the grains of rye grass absorb water with the humidity and they break up into thousands of pieces," Robin Ould, chief executive of the Asthma Foundation of Australia, told CNN.

"Normally with rye grass the pollen would be trapped by nose hairs. When it breaks up it goes straight to the lungs."

According to CNN, pollen levels peak in late spring, as it did in Victoria. The pollens combined with strong winds, rain and high temperatures, and this led to incidents of thunderstorm asthma.

An effect of climate change?

An international group of researchers suspects that the event could be caused by climate change. According to Paul Beggs, an environmental health scientist at the Macquarie University in Sydney, climate change could be responsible for this growing pollen potency because of carbon dioxide emissions.

"Plants use carbon dioxide through photosynthesis, and if you change the concentration of carbon dioxide in the atmosphere then their growth conditions are changing," Beggs told The Guardian. Researchers found that increasing amount of carbon dioxide led to changes in plants, including more potent pollen.

"All of these things are happening in the background: climate change, increasing pollen potency, and they all mean a thunderstorm asthma episode is more likely than it was 20 years ago," Beggs added.