The number of days during which city residents worldwide are exposed to extreme heat and humidity has tripled since the 1980s, a study published Monday by the Proceedings of the National Academy of Sciences found.

The growing number of days with extreme heat and humidity in urban areas now affects nearly one-quarter of the global population and is the combined result of both rising temperatures and booming urban population growth, the researchers said.

The number of person-days -- or the population multiplied by days -- in which city dwellers were exposed to extreme heat and humidity went from 40 billion a year in 1983 to 119 billion in 2016, the data showed.

By 2016, 1.7 billion people were being subjected to these conditions on multiple days per year, according to the researchers.

The increase in extreme heat and humidity "has broad effects," study co-author Cascade Tuholske said in a press release.

Among other concerns, "it impacts people's ability to work, and results in lower economic output [and] it exacerbates pre-existing health conditions," said Tuholske, a postdoctoral researcher at Columbia University's Earth Institute in New York City.

Over the past 40 years, hundreds of millions have moved from rural areas to cities, which now hold more than half the world's population, according to the researchers.

Temperatures in these urban areas are generally higher than in the countryside because of sparse vegetation and abundant concrete, asphalt and other surfaces that tend to trap and concentrate heat, they said.

A study that focused on cities in the United States published in July found that the effects of extreme heat conditions are felt more in poorer neighborhoods.

For this study, Tuholske and her colleagues 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 86 degrees Fahrenheit on the so-called "wet-bulb globe temperature" scale, a measurement that takes into account the effect of high humidity on the human body.

This wet-bulb temperature is roughly equivalent to 106 degrees Fahrenheit on the "real feel" heat index, the point at which even most healthy people find it hard to function outside for long, and the unhealthy might become very ill or even die.

The researchers matched up weather data with statistics on the cities' populations over the same time period.

Urban population growth accounted for two-thirds of the exposure spike, while actual climate warming contributed one-third, the data showed.

The worst-hit city in terms of person-days was Dhaka, the fast-growing capital of Bangladesh, which saw an increase of 575 million person-days of extreme heat over the study period.

Its ballooning population alone -- 4 million in 1983 to 22 million today -- caused 80% of the increased exposure to extreme heat and humidity for people living there, the researchers said.

Other cities with similar population-heavy trends include Shanghai and Guangzhou in China, as well as Dubai, Hanoi and Khartoum, Sudan, according to the researchers.

Cities that saw close to half or more of their exposure to extreme heat and humidity caused by warming climate alone were Baghdad, Cairo, Kolkata and Mumbai, the researchers said.

Seventeen percent of the cities studied added an entire month of extreme-heat days over the 34-year study period, including about 40 sizable cities in Texas and the Gulf Coast of the United States.

In cities such as Houston, Dallas-Fort Worth, San Antonio and Austin, as well as Pensacola, Fla., the cause has been varying combinations of both increasing population and increasing heat, the data showed.

However, in Las Vegas, as well as Savannah, Ga. and Charleston, S.C., the cause has been mainly population growth, while in Baton Rouge, La. and Gulfport, Miss., the primary culprit was warming temperatures.

Because the period covered by the study covered data through 2016, it did not include the series of record heat waves that affected much of the United States and Canada this summer.

Still, the study "could serve as a starting point for identifying ways to to address local heat issues," such as planting trees, said Kristina Dahl, a climate researcher at the Union of Concerned Scientists.

"This study shows that it will take considerable, conscientious investments to ensure that cities remain livable in the face of a warming climate," she said.

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