In the summer, urban areas can feel sweltering because dark pavement and buildings absorb heat from the sun.

And as the climate warms, cities are getting even hotter. At the same time, urban populations are booming, so more and more people are feeling the heat.

“Our main finding, globally, is that urban extreme heat exposure increased 200% over a 34-year period,” says Cascade Tuholske of Columbia University’s Center for International Earth Science Information Network.

His team analyzed weather data from more than 13,000 cities, from 1983 to 2016.

They found that, globally, more than 1.7 billion city dwellers now experience multiple days each year when heat and humidity reach levels that can make people sick.

The risks are growing fastest in cities that are already hot — places like Kolkata, Dhaka, and Lagos.

“While … the added warming may only be a few degrees compared to the poles where we’re having much bigger extremes, those few degrees when we start accounting for humidity are really important in terms of human health and well-being,” Tuholske says.

So there’s a growing need for cities to protect vulnerable residents as temperatures rise.

Reporting credit: Sarah Kennedy/ChavoBart Digital Media and Molly Matthews Multedo