The Ministry of Health issued the level three, or red alert, weather warnings as the heatwave gripping much of Europe peaks in Italy.
The maximum heat warnings cover much of the north and centre of Italy, while many southern areas are expected to feel relatively cool.
Red alerts apply to the cities of Rome, Florence, Perugia, Turin, Bologna, Bolzano, Brescia, Frosinone, Genoa, Pescara, Rieti, Trieste and Verona.
Level two amber alerts are also in place for much of the country.
Heat map: Italian Ministry of Health
This indicates “high temperatures that can have negative effects on the health of the population,” the Health Ministry writes, adding that the heat is particularly dangerous for over-75s and children under three, as well as people in poor health.
The red warning applies when “high-risk conditions persist for three or more consecutive days.”
Five green spots stand out on the map: the parts of Italy not affected by extreme temperatures today are Palermo, Reggio Calabria, Messina, Bari and Cagliari.
The stifling conditions will also worsen air pollution, the ministry said, meaning that those with respiratory problems or allergies are also liable to suffer.
But even those in good physical health are at risk of dehydration, sunstroke, sunburn and exhaustion.
The health ministry is urging people to take precautions and to check on people living alone.
Tourists keeping cool. Photo: AFP
It has also urged people not to call the emergency services unless essential, to avoid overstretching resources needed for an expected increase in the number of emergency cases.
The Italian Red Cross has set up a 24-hour hotline that people can call for free for advice and assistance: 800 065 510.
During Italy's heatwave of 2015, so many people died that it lowered the country's average life expectancy that year. And in 2003, Europe's worst heatwave in decades killed 18,000 people in Italy alone.