New York, Dec 23 (IANS): The combined effects of rising heat and humidity will affect India's northeast the most in the world close to the end of of the century, a global study says.

Although humidity can greatly magnify the effects of heat, most climate projections tend to leave out this major factor that could worsen things.

The new study, published in the journal Environmental Research Letters, projects that in coming decades the effects of high humidity in many areas will dramatically increase.

While hundreds of millions of people would suffer worldwide, the hardest-hit area in terms of human impact, will probably be densely populated northeastern India, the researchers said.

"The conditions we're talking about basically never occur now -- people in most places have never experienced them," said lead author Ethan Coffel of Columbia University in New York.

"But they're projected to occur close to the end of the century," Coffel said.

Using global climate models, the researchers mapped current and projected future "wet bulb" temperatures, which reflect the combined effects of heat and humidity.

The measurement is made by draping a water-saturated cloth over the bulb of a conventional thermometer.

The study found that by the 2070s, high wet-bulb readings that now occur maybe only once a year could prevail 100 to 250 days of the year in some parts of the tropics.

Lab experiments have shown wet-bulb readings of 32 degrees Celsius are the threshold beyond which many people would have trouble carrying out normal activities outside.

"Lots of people would crumble well before you reach wet-bulb temperatures of 32 C, or anything close," said coauthor Radley Horton, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory.

"They'd run into terrible problems," Horton, said.

The results could be "transformative" for all areas of human endeavor--"economy, agriculture, military, recreation", Horton said.

The study projects that some parts of the southern Mideast and northern India may even sometimes hit 35 wet-bulb degrees Celsius by late century - equal to the human skin temperature, and the theoretical limit at which people will die within hours without artificial cooling.

Other areas which are likely to bear the brunt of the crusing combination of heat and humidity include the southeastern US, the Amazon, western and central Africa, the Arabian peninsula and eastern China.

The research suggests that humidity may prove breaking point for some areas as temperatures rise.

Warming climate is projected to make many now-dry areas dryer, in part by changing precipitation patterns.

But by the same token, as global temperatures rise, the atmosphere can hold more water vapor.

That means chronically humid areas located along coasts or otherwise hooked into humid-weather patterns may only get more so.

And, as many people know, muggy heat is more oppressive than the "dry" kind.

That is because humans and other mammals cool their bodies by sweating; sweat evaporates off the skin into the air, taking the excess heat with it.

It works nicely in the desert. But when the air is already crowded with moisture - think muggiest days of summer in the city - evaporation off the skin slows down, and eventually becomes impossible.

When this cooling process halts, one's core body temperature rises beyond the narrow tolerable range.

The results are lethargy, sickness and, in the worst conditions, death.