How hot is TOO hot? As extreme heatwaves sweep across Europe, experts reveal the upper temperature limit for human safety – and it’s much lower than we thought

It may be sultry in Britain, but the rest of Europe has been roasting in record-breaking heat for the past few days.

And it’s not just on the continent where millions of people have been sweltering in temperatures well above ‘normal’ – the mercury also crossed 50C in parts of the US and China earlier this week.

It has raised concerns not only about the elderly, young or those with health problems, but also about the impact this record-breaking heat will have on young, healthy adults as global warming continues to wreak havoc.

So how hot is too hot?

Well, experts have revealed the upper limit for human safety – and it’s actually much lower than we thought.

Analysis: It is calculated from the combination of heat and humidity – known as the ‘wet bulb temperature’. This graph shows the point where temperature and relative humidity together become dangerous to the human body (the red area)

The main point to note is that it’s not just about what the thermometer says.

Instead, it’s the combination of heat and humidity – known as the “wet bulb temperature.”

This is measured by attaching a wet cloth to the bulb of a thermometer.

It is a direct indicator of how well sweating cools the body.

It was previously thought that a wet bulb temperature of 95°F (35°C) — equivalent to a temperature of 95°F at 100 percent humidity, or 115°F at 50 percent humidity — was the upper limit of safety for humans.

At this point, the human body would no longer be able to cool itself by evaporating sweat from the surface of the body to ensure a stable core body temperature.

However, new research now suggests that the upper limit actually is 87°F (31°C) at 100 percent humidity or 100°F (38°C) at 60 percent humidity.

The study, conducted by experts at Penn State University, saw healthy men and women under heat stress in a controlled climate chamber.

Each person was given a small telemetry pill to swallow so that their core temperature could be monitored as they moved slowly through everyday activities such as eating, cooking and showering.

As they did, the researchers increased the heat or humidity in the room to see at what point each participant’s core temperature started to rise toward what’s known as the “critical environmental limit.”

This is where there is suddenly an increased risk of heat-related illness.

Europe is undergoing what could be its hottest week on record. It depicts people cooling themselves off at Rome’s Trevi Fountain as a heat wave sweeps through Italy

This is because the heart has to work harder to pump blood to the skin to dissipate heat, while sweating reduces the fluid balance in your body.

If the body continues to overheat, it can even lead to heat stroke.

This is a serious, life-threatening condition that requires immediate medical treatment to cool the body.

It can damage organs such as your lungs, kidneys, and liver. If left untreated, it can be fatal.

Heat stroke can occur during heat waves or long periods of very hot weather.

Some people — such as children, the elderly, and those with long-term health problems (such as diabetes or heart problems) — are more at risk for heat exhaustion or heat stroke.

As a result, they have to be extra careful in hot weather.

Following their latest research, experts now want to test older men and women to see what their upper safety limit is.

Because of their increased risk of heart disease and respiratory problems, it’s probably much lower.

Such an understanding is important, because as the average global temperature continues to rise and heat waves become stronger and more intense, the impact this will have on human health is not yet fully understood.