After a sweltering summer afternoon in Washington D.C., a cool evening can provide a much-needed break from the heat. But during heat waves, not only does it get hot during the day, but it stays hot at night.

“So people don’t get a break from it. … The heat builds up and the humidity builds up day after day,” says Steve Walz, who recently retired from his position as director of environmental programs at the Metropolitan Washington Council of Governments.

He says prolonged exposure to extreme heat is not only uncomfortable. It can be deadly – especially for the elderly and people with heart or lung problems.

Some people stay safe by closing the windows and running the AC. But many low-income people do not have air conditioning at home.

“They don’t get a break from the high heat at night, and so it just builds the stress and it increases the health risk to the population,” Walz says.

He says low-income neighborhoods also tend to be hotter because they often have fewer trees and more stretches of concrete and black asphalt, which absorb heat.

So as the climate warms, D.C. and other at-risk cities may need to plant trees in vulnerable neighborhoods and provide cooling centers so that people can get some relief.

Reporting credit: Sarah Kennedy/ChavoBart Digital Media