How hot can it be for humans live in heat?

How hot is it before humans can't live in a specific location?

Turns out, that is a hell of a difficult question to answer. I would assume the 126 recorded in Iran is near the limits of what the human body can withstand for a while. But, again, it depends.

Here is the thing, the human body itself cannot stand temperatures anywhere near that high (before brain damage, the human body internally can only stand 108 degrees for a few minutes).  But humans, being warm blooded, have adaptations to fight high external heat. Naturally (i.e. without air con inside), humans can live as long as they can cool their bodies with evaporation to a reasonable level.

So for the same reason Palm Springs at 100 is okay (very little humidity), but 100 degrees with humidity is very dangerous. In fact, that temperature of 100 degrees - without cool breezes and with 50% humidity killed 2,000 cows in Kansas last week.

Dead cows last week from heat in Kansas

New Measurements

Just as we use a "heat index" to give us a "feels like" temperature, scientist have defined a "wet bulb" temperature index. Say 126 with over 50% humidity might kill a person in 4 hours of exposure. This becomes massively important in the two large population areas that regularly flirt with these temperatures. Pakistan and India both record near deathly wet blub temperatures a few times last year. This promises to happen more and more with Climate Change.

And heat bubbles in the Middle East are hitting records heat as well. Combine that with a scarcity of water in the Middle East

And we (America and the "West") can't really ignore this issue. Imagine what happens if swathes of the Middle East become uninhabitable. The poor will migrate to cooler and richer countries. Australia is already wrestling with economic migrants due to weather (Australia is closer to India and Bangladesh that the "west"). Europe is already feeling mild effects of heat refugees from Africa and the Middle East that will only get worse.

The South East Asai region could easily fall into wars over water access. That is part of the skirmishes between India and China, control of the headwaters and glaciers of the Himalaya Mountains. And since China, India and Pakistan all have nuclear weapons, the area is ripe for a mistake to result in nuclear war. Iran will soon join that group.

Most of the "West" has air conditioning or is trying to figure out how to install it quickly as the wrold heats up (Europe in particular has to install a shit load quickly). What we think of as "Western" Asian countries already have installed it (Singapore, Malaysia, Australia, Hong Kong, Thailand). 

But, it exacerbates the problem as well. It takes energy to run air conditioning. Generation of that energy generally contributes to Climate Change.

This is one of the "there is no easy solution" problems. The world is past the tipping point here and it will get worse no matter what we do. So the world needs to define how to address this. The world can't, so we (American, Canada, Europe) need to define how we will address this going forward.

Britain sending refugees to Africa, and the United States forcing refugees to camp out in Mexico with deadly heat are only stopgaps. These solutions cannot continue with a 3 fold increase in migration without some planning. Refugees from deadly situations will not / can not go home. What do we do?