Booming populations and rising temperatures mean that the number of days when city dwellers are exposed to extreme heat and humidity has tripled since the 1980s.
The study analysed 13,000 cities worldwide, using satellite imagery and temperature readings.
Over recent decades, hundreds of millions have moved from rural areas to cities, which now hold more than half the world's population.
Lead author Cascade Tuholske a postdoctoral researcher at Columbia University's Earth Institute, said: "This has broad effects.
“It increases morbidity and mortality. It impacts people's ability to work, and results in lower economic output. It exacerbates pre-existing health conditions."
The researchers combined infrared satellite imagery and readings from thousands of ground instruments to determine maximum daily heat and humidity readings in 13,115 cities from 1983 to 2016.
They defined extreme heat as 30C on the so-called "wet-bulb globe temperature" scale, a measurement that takes into account the multiplier effect of high humidity.
"Wet-bulb temperature" refers to temperatures taken with a thermometer covered in a wet cloth, which are normally slightly cooler than "dry-bulb" temperatures.
Human beings can survive very high temperatures (well over 50C) when humidity is low, but they cannot survive temperatures of even 35C in high humidity for long periods, because there is no way to cool down by sweating.
To come up with a measure of person-days spent in such conditions, the researchers matched up the weather data with statistics on the cities' populations over the same time period. The population data was provided in part by Columbia's Center for International Earth Science Information Network, where Tuholske is based.
The analysis revealed that the number of person-days in which city dwellers were exposed went from 40 billion per year in 1983 to 119 billion in 2016 - a threefold increase.
By 2016, 1.7 billion people were being subjected to such conditions on multiple days.
Sheer urban population growth accounted for two-thirds of the exposure spike, while actual warming contributed a third.
The worst-hit city in terms of person-days was Dhaka, the fast-growing capital of Bangladesh; it saw an increase of 575 million person-days of extreme heat over the study period.
Other big cities showing similar population-heavy trends include Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and various cities in Pakistan, India and the Arabian Peninsula.
Tuholske said: "A lot of these cities show the pattern of how human civilisation has evolved over the past 15,000 years.”
She says they tended to be on river deltas.
"The Nile, the Tigris-Euphrates, the Ganges. There is a pattern to the places where we wanted to be. Now, those areas may become uninhabitable. Are people really going to want to live there?"
Watch: How climate change is threatening your cup of coffee