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Australia is experiencing one of its strongest ever heatwaves.

In fact, it is getting so hot that meteorologists have been forced to increase their temperature scale to 54C and add a new colour code.

The unprecedented temperatures have caused outbreaks of bushfires putting lives at risk.

But such high temperatures also pose another risk - to the body itself.

Extreme temperatures result in stress to the body.

The body works best within a narrow range of body temperature - 36C to 37.5C - and gets rid of heat mainly by sweating, although breathing and an increased heart rate can also expel heat.

The hotter and more humid it gets the more the body has to sweat, increasing the risk of dehydration.

In extreme heat the body starts to struggle to cool itself down, which can then lead to heat cramps, heat exhaustion or even heatstroke - also known as sunstroke.

The latter is a medical emergency which can result in permanent damage to vital organs and even death if it is not treated.

So how hot is too hot?

The World Health Organization says the optimum air temperature for the body is between 18C and 24C. Any hotter and the risks rise.

Some of the risk is linked to what a body is used to. Unsurprisingly people in hotter countries tend to cope better when the mercury starts to rise.

Research has shown that when the temperature gets to 35C, accompanied by high humidity, health is put at danger. Once 40C is reached, it can be dangerous even with low humidity levels.

Suffice to say at 50C, the risk is even higher.

Much also depends if an individual falls into one of the vulnerable categories.

Older people, babies and young children and pregnant women are all more susceptible to extremes in heat as are those with chronic conditions such as heart or breathing problems.

Certain types of medication or infections can have an effect too.

During heatwaves the public are advised to take precautions, such as drinking lots of water, avoiding strenuous exercise and dressing sensibly in lightweight and light-coloured clothes.

But in extreme heat it is essential to find a cool environment in which to artificially reduce the body's temperature.

Professor Virginia Murray, of the Health Protection Agency, who has studied the effect of heat on the body, says: "What is really frightening is when the body is not able to cool itself down.

"That can happen when it is really hot during the day and night. The body does not get a chance to get rid of the heat.

"In those circumstances the most important thing is being able to go somewhere to cool down. People need to find a cool area of a building or somewhere with air conditioning."

When people are not able to do that, history shows heat can be a killer. The European heatwave of 2003 - the hottest summer since the 1500s - was estimated to have caused the deaths of more than 70,000 people across the continent.

Meanwhile, up to 10,000 deaths were said to have been caused by the hot summer of 1988 in the US.

In fact, of all the natural disasters that can strike extreme temperatures are the most lethal, causing more deaths overall than floods, earthquakes and tornadoes.

But whether heatwaves or cold snaps are the biggest killer depends on where you live. In both the US and Australia, heat carries the highest risk. In the UK, it's the cold.