It's very hot in the US. Today at 2pm the temperature where I live will be around 29 degrees celcius. I googled ukraine's weather right now and it's 14C over there.
That means that when you sweat, it doesn't evaporate. And it doesnt make you cooler. And the sweat then warms up to the air/body temperature, and further blankets you in hot... And you sweat more.
Some scarier numbers, mammals die without shelter in around 8 hours if the temp is 35°C (95 °F) in saturated (100% humidity) air.
Although, it's a dry heat - I pity our east-coast and southern friends... the humidity makes it feel a whole lot worse!