Scientists at Johns Hopkins University analyzed three informal settlements in Nairobi, including the largest, Kibera, home to nearly a million people.
Along the settlements' narrow alleyways, mud-walled homes and metal roofs, they found stifling temperatures, "between five and nearly 10 degrees Fahrenheit (2.7-5.5 C) higher than those reported at Nairobi's official weather station less than half a mile away," said the study in the journal PLOS ONE.
The study was conducted by 11 researchers over the course of 80 days from late 2015 to early 2016, one of Nairobi's hottest summers since the 1970s.
Researchers posted 50 thermometers on trees and wooden posts, most in shaded areas.
At the Kenya Meteorological Department headquarters, in a grassy, wooded area, average daytime temperature was 78 Fahrenheit (25 C).
In the slums, the average was nearly 82 in Kibera, 85 in Mathare, and 87 in Mukuru.
The higher temperatures found in the study are "certainly consistent with excess deaths," said lead author Anna Scott, a climate scientist in the Department of Earth and Planetary Sciences at Johns Hopkins.
However, researchers were unable to quantify how many people are likely to die from heat waves in these urban areas, since many variables are at play.
Up to 60 percent of Nairobi's residents live in these informal settlements.