Explained: Why your body feels beat by the heat

If you're feeling beat by the heat right now, don't worry: it's perfectly physiological. Science reporter Jamie Morton explains.

What happens to my body when it gets hot?

As amazing as the human body is, like anything else, it can overheat.

When temperatures rise, as they have markedly since the weekend, we shed heat through the evaporation of sweat from our skin, which should occur smoothly when there's little moisture in the air.

Because our sweat is essentially filtered blood - it's the plasma of our blood that leaves through our sweat ducts - our bodies lose fluid that could have been used elsewhere, which leaves us feeling sapped of strength and energy.

That's why it's important to stay cool – and hydrated.

If we've had enough time to adapt to the warmth, this shouldn't be such a bad thing.

But when we're suddenly plunged into scorching temperatures, the shift can have wide-ranging effects on our body's normal biological cycle, or biorhythm, with implications for our mental, physical and emotional state.

Biorhythms rapidly disturbed by higher body temperatures can alter sleep quality and stress levels, and the demand of higher sweat rates and oxygen to shed heat energy results in uncomfortable clamminess and fluid loss.

Other risk factors make some people more vulnerable: such as being very young or very old, obese or suffering from heart disease or poor circulation.

Thankfully, this week's heatwave has come with relatively little humidity, which poses its own difficulties in efficiently balancing fluids and sweating.

Three-year-old Khalani Mase, of Otahuhu, cools off in the water at Auckland's Browns Bay this week. Photo / Dean Purcell

How hot is too hot?

Our ability to regulate body heat depends on the temperature and humidity of the surrounding air.

And yes, there is an upper survival limit, beyond which we can no longer effectively cool ourselves.

Researchers have suggested this happens when what's called the "wet bulb" temperature passes 35C.

Rather than the temperatures we see on evening news forecasts, the wet-bulb temperature is equivalent to what's felt when wet skin is exposed to moving air.

It includes temperature and atmospheric humidity and is measured by covering a standard thermometer bulb with a wet cloth and fully ventilating it, hence the name.

One study calculated that humans and most mammals, which have internal body temperatures near 37C, could experience a potentially lethal level of heat stress at "wet-bulb" temperature above 35C, sustained for six hours or more.

In already hot parts of the world, there's concern that climate change could be pushing conditions to these extremes and making areas unliveable.

Despite its relatively cool, temperate climate, New Zealand isn't immune to extreme heat and the health risk that comes with it.

By the end of the century, assuming greenhouse gas emissions continue to climb close to current levels, many parts of our country will record more than 80 days a year above 25C.

Most places typically only have between 20 and 40 days above that now.

Unfortunately, there's a dearth of research into the public health risk of extreme heat among Kiwis.

One of the few such studies, carried out more than a decade ago, found about 14 elderly people in Auckland and Christchurch die annually when the mercury climbs above 20C.

If global temperatures climbed just 1, 2 or 3C above current levels, that same death rate could rise to 28, 51 and 88 respectively.

With about one in four New Zealanders projected to be 65 and over by 2043 - that's including many people in their 40s today - the problem will be amplified.

Although 25C has long been used as a benchmark of extreme heat in New Zealand – the measure purportedly stemming from the claim that beef and dairy cattle start experiencing heat stress at that point – Victoria University climate scientist Dr Luke Harrington has contended that "extreme" is a relative concept.

Indeed, there's ample evidence from overseas that people are adapted to the temperatures they're familiar with – and that's no different here.

"What we can cope with comfortably varies on the basis of what you're exposed to – so a person who lives in Poland is going to be quite different to someone who lives in Singapore," Harrington said.

But he said there was a big risk in deviations from normal local temperatures – hence a new joint pilot, in which MetService is issuing "heat alerts" this summer - has set different thresholds for different places.

They ranged from 24C in Hokitika to 31C in Gisborne, and also accounted for effects like high humidity.

How can I cool down?

Obviously, drinking lots of water - especially during exercise - helps us replenish that lost fluid.

And being somewhere with good air conditioning could help escape the clamminess.

For too many Kiwis, that might be a luxury: a recent Stats NZ survey of 6700 homes found 36 per cent were sitting at 25C or more over summer - and sometimes even above 30C - compared with a comfortable indoor range of 20C to 25C.

In homes without air conditioning, getting to sleep could prove a struggle.

We slumber best in ambient temperatures of between 13C and 23C, but first need to cool our core body temperatures enough to doze off.

But we could trick our bodies into sleepiness with simple hacks like opening a window, turning on a fan or taking a cooler shower about 30 minutes before bed.