Three people remain in critical condition following Melbourne's asthma thunderstorm, which has been linked to at least four deaths so far.  Melbourne hospitals treated more than 8500 patients across Monday night and Tuesday, the health department said on Thursday.  "The thunderstorm asthma event on Monday was a health emergency of unprecedented scale," a spokesman said in a statement.  Victorian Health Minister Jill Hennessy says senior doctors are amazed there weren't many more deaths given the number of people who suffered cardiac arrest and breathing difficulties during the storm which overwhelmed emergency services and hospitals.  Nine patients are still in intensive care, the minister said on Thursday.  Three are in critical condition.  "I'm pleased we haven't had more deaths than what we have had ... but those deaths are four deaths too many," Ms Hennessy told reporters.  At least four people died during the rare weather and the cause of those deaths will be investigated by the Inspector General for Emergency Management.  His findings will form part of an overarching review that will examine how Ambulance Victoria and emergency services responded, and how the community was notified of the unfolding crisis.  Ambulance Victoria will conduct its own internal review.  At one point, there were 140 "code one" cases occurring at the same time on Monday evening.  Noble Park father-of-two Clarence Leo, 35-year-old Apollo Papadopoulos, law student Hope Carnevali, 20, and Year 12 student Omar Moujalled all died during the mass asthma outbreak.  A team of experts is now desperately trying to determine whether severe asthma storms can be predicted in the future.  They occur when grass pollen, dust and other allergens are blown ahead of a storm front and then rupture into tiny particles that can penetrate deep into people's airways.  Government agencies, pollen scientists and the Bureau of Meteorology met on Thursday to discuss whether future storms could be accurately forecast.  The man who runs Melbourne's main pollen-counting station thinks it's possible given useful data exists from previous storms.  "We get those conditions of high grass pollen and thunderstorms quite regularly in Melbourne at this time of year, but it's not every time that we get thunderstorm asthma," Melbourne University pollen expert Ed Newbigin told AAP.  "So there is another factor involved that we don't know."  Associate Professor Newbigin is convinced the state's various monitoring bodies can identify the missing ingredient.  But how reliable a forecasting model would be, and whether it could predict when a storm would be as extreme as Monday's, is unknown.
A review into the emergency services response to Melbourne's asthma thunderstorm will include a probe into at least four deaths on the same night.