An anticyclone – a high-pressure area – named Cerberus (named after the monster from Dante’s Inferno) coming from the south will cause temperatures to rise above 40°C across much of Italy. This comes after a spring and early summer full of storms and floods.
The highest temperature in European history was broken on 11 August 2021, when a temperature of 48.8°C was recorded in Floridia, an Italian town in the Sicilian province of Syracuse. That record may be broken again in the coming days.
Heat exhaustion means you’ve probably been trying to sweat and thus lost minerals. If you drink too much normal water, you can dilute the minerals even more and thus aggravate a condition known as: Hyponatremia (salt insufficiency) https://en.wikipedia.org/wiki/Hyponatremia#True_hyponatremia
The thing with hyponatremia is that it’s very similar to heat stroke in terms of symptoms, but drinking water makes it worse. See: Dehydration, Heat Stroke, or Hyponatremia? The Recognition, Treatment, and Prevention of Hyponatremia Caused by High Exercise Outdoor Activities. https://eric.ed.gov/?id=ED443633
So figuring it out requires knowing some context, what went on before. Or you can hydrate slowly with an ORS like the ones mentioned around here (ending with -lyte) or make your own ORS. Or you can eat some stuff, get some salty nuts, eat some fruits, as a way to stock up when the day looks risky.
I’m not certain, but I think I did get hyponatremia once while hiking in the mountains in summer heat. I drank a lot of spring water in a day, like… 10-20% of my body weight and was still thirsty, and didn’t even really need to urinate, since I was losing so much water through sweat. It was very exhausting. I only felt better at the end of the day when I downed a large bottle of cheap green soda. And later, at home, when I drank A LOT of cold soup straight out of the pot. It just felt necessary, so I’m just figuring it out in hindsight.