In the hottest parts of the world, high temperatures and humidity will, for longer stretches, surpass a threshold that even young and healthy people could struggle to survive as the planet warms, study says.

Heat waves can already be deadly for the most vulnerable people — but in a warming world, temperatures and humidity will, for growing stretches of every year, surpass a threshold that even young and healthy people could struggle to survive, according to new research published Monday.

Lahore, Pakistan, already an epicenter of human ills linked to climate change, could surpass that survivability threshold for two or three weeks out of the year by the middle of the century, for example, the study in the Proceedings of the National Academy of Sciences found. Under the most dramatic global warming scenarios, it could last for months.

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