SYDNEY • The death toll from the "thunderstorm asthma" episode has risen to six, and three others are in critical condition, the authorities said yesterday, as they assessed the fallout from the unprecedented event.

Four victims, ranging in age from 18 to 35, were last week linked to the rare phenomenon, as a thunderstorm in Victoria state coincided with a high pollen count and sent more than 8,500 patients to hospital emergency departments. The event triggered respiratory problems in those with asthma and hay fever .

"There have now been six deaths that might have occurred because of conditions relating to the thunderstorm asthma events on Monday," the state health department spokesman said in a statement.

"Five patients are continuing to receive specialist care in intensive care units in hospitals in Melbourne, with three still in a critical condition."

A further 12 people were being treated for respiratory and other health problems, he noted.

No further details were released about the two latest victims.

Victorian Health Minister Jill Hennessy said last Thursday that demand for ambulances was so acute at one point that "it was like having 150 bombs going off right across a particular part of metropolitan Melbourne".

The government has launched a review into how emergency and health services can better respond to such events and manage them effectively.

The rare phenomenon is known to have occurred in Australia only a few times, said Mr Robin Ould, who heads the Asthma Foundation of Victoria.

"When ryegrass pollen becomes wet through humidity or water, it breaks up into a lot of small pieces, and those small pieces can get past the nasal passage into the lungs. Normally, ryegrass would be trapped in the nasal passage," he told Agence France-Presse.

"When it gets in the lungs, the allergens that are there cause an asthma attack... the small bronchial tubes become inflamed, they fill with mucus, and the muscles around them become tight, and people can't exchange air," he explained.

He said those affected had asthma that was triggered by ryegrass, or suffered from hay fever.

Melbourne, the capital of Victoria, is susceptible to thunderstorm asthma events because ryegrass is predominantly found in parks and farmland around the city, said Mr Ould.

About 10 per cent of Australians have asthma, with 80 per cent of them also experiencing hay fever, according to Asthma Australia.

In asthma victims, the immune system in the airways goes into overdrive, resulting in wheezing, coughing and restricted breathing.

AGENCE FRANCE-PRESSE