While India, Pakistan and the Gulf already have briefly touched dangerous humid heat in recent years, the study found it will afflict major cities from Lagos, Nigeria, to Chicago, Illinois, United States (US) if the world keeps heating up.

Towards the higher end of warming scenarios, potentially lethal combinations of heat and humidity could spread further including into areas such as the US Midwest, the authors of the report said.

"It's very disturbing," study Co-author Matthew Huber of Purdue University in the US state of Indiana told Reuters. "It's going to send a lot of people to emergency medical care."

The study found that around 750 million people could experience one week per year of potentially deadly humid heat if temperatures rise 2 degrees Celsius above preindustrial levels.

At 3C of warming, more than 1.5 billion people would face such a threat, according to the paper published in the journal Proceedings of the National Academy of Sciences.

The world is on track for 2.8C of warming by the year 2100 under current policies, according to the 2022 United Nations Emissions Gap report.

--Reuters--