https://www.sciencedaily.com/releases/2021/10/211004153741.htm

Over recent decades, hundreds of millions have moved from rural areas to cities, which now hold more than half the world's population. There, temperatures are generally higher than in the countryside, because of sparse vegetation and abundant concrete, asphalt and other impermeable surfaces that tend to trap and concentrate heat -- the so-called urban heat island effect.

"This has broad effects," said the study's lead author, Cascade Tuholske, a postdoctoral researcher at Columbia University's Earth Institute. "It increases morbidity and mortality. It impacts people's ability to work, and results in lower economic output. It exacerbates pre-existing health conditions."