If you’ve ever walked barefoot across a sunbaked parking lot, you know firsthand how concrete soaks up and retains the sun’s heat. When temperatures rise, the countless miles of concrete streets, sidewalks, walls and roofs in cities magnify that effect, creating a phenomenon known as urban heat islands. When combined with the heat released by vehicle engines, paved areas can boost the temperature in cities by as much as 22°F, according to the Environmental Protection Agency.

Soaring city temperatures aren’t just unpleasant. They can be lethal. Heat already kills more Americans than hurricanes, lightning, tornadoes, floods, and earthquakes combined. The urban heat island effect is ever more worrisome because more and more people are moving into cities. Over 81% of all Americans already live in urban areas. In developing countries from China to Nigeria, millions are leaving the countryside every year for a shot at a better life in cities . The number of urban dwellers worldwide is rising by as much as 78 million people annually, according to the United Nations Population Division. That’s the equivalent of adding nine New York Cities to the planet every single year.

Concrete production worsens the problem, too. The cement industry produces somewhere between 5-10% of all carbon-dioxide emissions worldwide, making it the third-largest source of global-warming, behind only coal-fueled power plants and combustion-engine vehicles.