Victoria's Department of Health has forecast a "high risk" for epidemic thunderstorm asthma across the Northern Country, North Central, North East and East Gippsland districts on Saturday.
A cooler change is expected to cross the state from the west, creating the potential for severe thunderstorms with damaging winds, possible large hail and heavy rain in central and eastern areas in the afternoon and evening.
The winds, combined with high to extreme pollen levels, increase the risk a large number of people may develop asthma symptoms over a short period of time.
A thunderstorm asthma event in Melbourne in 2016 killed at least nine people and put thousands more in hospital.
Health authorities are recommending asthma sufferers in high-risk areas stay inside during the storms, take preventative medication, and carry their reliever.
"Health and emergency services are monitoring the situation closely and are ready to respond," the health department said in a statement.
"Our hospitals are experiencing significant demand due to COVID-19, so it's important you stay well and avoid unnecessary use of emergency health services."
Meanwhile, a total fire ban has also been declared for Sunday in the Mallee and Wimmera regions to the state's west.
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