Scientists have identified the temperature and humidity level above which the human body cannot survive.
The traditionally thought-of 'web bulb temperature' above which humans cannot survive has only been breached around 12 times.
Wet bulb temperature
Even someone who is young and healthy will die after enduring 35C of heat coupled with 100 per cent humidity in what is called the wet bulb temperature.
This is the point at which sweat cannot evaporate off the skin.
Scientists who worked on new research found the "critical environmental limit" where people's bodies were not able to stop their core temperature from climbing was actually at 30.6C, well below the previously theorised 35C.
The research showed that "wet-bulb temperature threshold cannot be applied to human adaptability across all climatic conditions and where appropriate (high humidity), that threshold is well below 35C."
Sweat is the body's main method or reduce its core temperature.
If the body cannot cool down this will lead to heatstroke, organ failure and death.
Although this temperature has been reached 12 times, none of the instances lasted for more than two hours.
The frequency of these events has doubled in the last 40 years.
Their frequency is expected to increase even more if the world warms 2.5C above pre-industrial levels.
Current projections put the world on track to be 2.7C warmer on average by 2100.
Extreme heat kills
Although this wet bulb temperature will kill people, the heat doesn't need to be near that level to be a killer.
Last summer, more than 61,000 people are estimated to have died due to the heat in Europe alone.
In central and southern Italy last month, the death rate was 7 per cent higher than usual. Italy endured the Cerebrus heatwave and wildfires during July.
Other countries in Europe including Greece, Spain and Portugal saw workers refuse to work or not be allowed to work during the hottest parts of the day due to the extreme temperatures.
In Italy alone at least five people died after working in extreme heat.
Small children and the elderly who are generally less able to regulate their body temperature are at greater risk.
Almost 90 per cent of the heat-related deaths in Europe last year were in people over the age of 65.
As ocean temperatures risk, wet bulb temperatures are also likely to increase.
Last month, the world's oceans hit an all-time high, beating the previous record in 2016.