How much heat can the human body possibly take?

How far can you push the human body before it fails? The New Scientist posed that question a few years ago, listing out a few “ultimate limits” for the human body: how fast can we run, how long can we concentrate, how long can we survive without sleep and so on. The one question they did not ask, and that’s increasingly bothering us, is: how much heat can we tolerate before we perish?

It’s just May. And we are already battling an intense heat wave. Last year, NASA experts predicted that in 2015, planet Earth will face an unprecedented heat wave. The three decades from 1983 to 2012 have been the warmest in the last 1,400 years. And summer of 2015 will be excruciatingly hot in several parts of the world. Clearly, India is one of those.

What’s excruciatingly hot for us? Remember 2010, when temperature hit a 50-year-high with Jalgaon in Maharashtra crossing 49 degrees? No. India has seen more: the highest so far has been 50.6 degrees at Alwar, Rajasthan, in May 1956.

Apparently, that’s not enough to make us one of the most spectacular hotspots in the world. At least, we haven’t seen temperature going up to 71 degrees, as it did at the salt deserts of Dasht-e Lut, Iran, in 2005. Or 69.3 degrees as in Queensland, Australia, in 2003. Or even neighbour Pakistan’s 2010 record, when temperature soared to 53.7 degrees at Mohenjo-daro — one of the warmest temperatures ever recorded in the world.

So how much heat can you tolerate? As much as your genes allow. Heat tolerance is coded in your genes. The mechanism goes back to human evolution over 1.7 million years, when humans started coming out of forested areas into open grassy lands. To survive the heat and to keep cool, our ancestors started losing body hair. A genetic adaptation for survival — just as bipedalism, a prominent nose, a large brain, and the ability to speak are. There’s more: people who are tall, slim and slender tolerate heat better. They have more skin to perspire. Woe be unto those who are fat: they have less skin surface for their weight.

Generally, the range between 18 degrees and 24 degrees is best for the body, says the World Health Organisation (WHO). But when temperature nears 40 degrees, the body finds it difficult to cool down — with or without humidity — leading to heat cramps, exhaustion, breathing difficulty and increased heart rate. If mercury soars well above 40 degrees, the heat can cause permanent damage to vital organs. As we all know, heat can be a killer.

Well, there’s a caveat from WHO: Your heat tolerance is linked to what you are used to. So Dilliwalas would fare better, than say Londoners, if the mercury goes beyond reasonable limits. Simply because we are used to it.

Some consolation that.