Eight major Italian cities were on "red alert" on Wednesday, as Italy continues to "bake" in the second summer heat wave. 

If a city has been put on "red alert" by the Ministry of Health, this means that the heat is so intense that it poses a threat to the entire population, not just groups such as the elderly, the clinically vulnerable and small children. 

The ANSA news agency wrote that eight cities were also on "red alert" on Tuesday, including Bologna and Viterbo. The other six are Florence, Frosinone, Latina, Perugia, Rieti and Rome. 

Number will increase to 10 on Thursday, with Campobasso and Pescara joining these cities. The current heat wave is expected to last throughout the week, with temperatures above 40 degrees Celsius. 

A study coordinated by the Barcelona Institute for Global Health, and published in the journal Nature Medicine this week, estimated that more than 18,000 people died in Italy due to the intense heat the nation endured last summer. 

Scientists say the climate crisis caused by human greenhouse gas emissions is making extreme weather events such as heat waves, droughts, severe storms and floods more frequent and intense.