Global warming could create peaks of humid heat in the Persian Gulf beyond human tolerance by century's end, according to a study published in the journal Nature Climate Change.
If greenhouse gas emissions continue unabated, heatwaves occurring on average once every 10 or 20 years would, by 2100, exceed the capacity of a young, healthy person to maintain a normal body temperature, the study found.
Even annual summer heatwaves would become dangerous, especially for the elderly and very young, the researchers said on Monday.
"The human body regulates its core temperature to around 37 degrees Celsius, and its skin to a cooler 35C," explained co-author Jeremy Pal, a professor at Loyola Marymount University in Los Angeles.
That difference allows us to shed excess heat, especially when the air is muggy.
But if a double whammy of high heat and humidity reaches a certain level, "the body is no longer able to cool itself and begins to overheat," Pal told journalists in a phone conference.
That threshold - measured with an index scientists call "wet-bulb" temperature - would be crossed, for example, if a humidity level of 50 percent were combined with a temperature of 46C.
A slightly higher temperature with a lower humidity level, or vice versa, would have the same effect.
Without air conditioning, humans could last about six hours under such conditions before their bodies began to shut down, the authors said.
They pointed out that some outdoor activities - such as the Muslim pilgrimage known as the Hajj - could literally become life-threatening due to the climactic conditions.
While the temperatures in Mecca and nearby Jeddah are not predicted to be the highest in the region, they would still pose a risk.
To date, no place on Earth has crossed the red line of a deadly "wet-bulb temperature".