What is the best clothing to keep you cool in a heatwave?

As the world continues to grapple with extreme heatwaves, which are becoming ever more regular thanks to climate change, the clothing we wear is a vital component in how we stay cool. Researchers have found that by wearing appropriate clothes, it is possible to turn the air-conditioning up by 2C (3.6F) – which over the long term would save considerable energy, both saving money and cutting greenhouse gas emissions.

So what should you wear to stay cool?

Colour and design

When it comes to colour, most people wear white in the summer – because white reflects the sun's rays, rather than absorbing the light like black does.

However, this theory gets a little more complex when we start talking about the thickness and fit of clothing, because heat doesn't just come from the sun – it comes from our bodies too. When that heat from our bodies hits the white clothing, it is reflected back at us.

In 1980, a study of why Bedouins – an indigenous, semi-nomadic people who inhabit desert regions in the Arabian peninsula, Middle East and North Africa – wear black robes in the desert, found that heat exposure was the same whether tribal members wore black or white robes.

How is that possible?

Black coloured fabrics are a better radiator of heat – meaning they absorb heat emanating from the body – so this can also play a role in cooling your body down. The Bedouin's secret is wearing loose-fitting black clothing, especially if it's windy. The loose black clothes heat up the space between the fabric and the skin, promoting an upward air current – like a chimney – and providing cooling relief.

"The amount of heat gained by a Bedouin exposed to the hot desert is the same whether he wears a black or a white robe," the study notes. "The additional heat absorbed by the black robe was lost before it reached the skin," it says.

So the fit of the clothing is actually more important than the colour. However, if you are going to be wearing tight-fitting clothing, then stick with white. Fabrics with texture – such as seersucker or pique, a fabric often used in sports polo shirts – also help to lift clothing off your skin, rather than staying snug and tight-fitting.

In terms of clothes, it's better to have a material that allows water vapour to pass through so that it doesn't block the sweat evaporation – Rhett Allain

Material matters

"Your choice of fabric is crucial," says stylist and fashion writer Heather Newberger. "If you're wearing an oversized denim jumpsuit - you're going to feel way hotter than your friend in a more constricted outfit made of gauze or chiffon."

When it comes to fit, lightweight woven fabrics such as cotton and silk are usually better at hanging loosely than knits. This is especially important when it's humid – in dry heat, wicking alone may be enough because the sweat will be absorbed from your body and evaporate in the heat. When it's humid and hot, the air around you is already saturated with water vapour, meaning the sweat your clothes just soaked up doesn't have anywhere to go.

"In terms of clothes, it's better to have a material that allows water vapour to pass through so that it doesn't block the sweat evaporation." says Rhett Allain, associate professor of physics at Southeastern Louisiana University. "Some of the newer sports-based materials do this.  Cotton does not do so great with this."

All textiles trap infrared radiation given off by the body to some degree, which helps to keep us warm in cold weather – but isn't ideal on a hot day and so wearing breathable clothes is important. Uncoated cotton, linen, nylon and polyester are all classed as breathable fabrics to some degree – meaning they allow sweat and heat to escape through the material. They're different to wicking fabrics, though, which actively pull water from your body.

Cotton and polyester absorb and reflect the majority of the infrared that hits them – nearly 99% – meaning they often appaer white in infrared images. But these materials also allow 30-40% of visible light through. This combination can cause the body to warm up faster than it otherwise would, according to researchers at the Massachusetts Institute of Technology (MIT). This occurs because the incoming visible light can generate heat that can then not escape as infrared radiation generated by the body.

But the body's other cooling mechanism – sweating – also plays a role. Cotton absorbs moisture but it doesn't dry quickly, so if you're sweating a lot your clothes will stay wet, making them less comfortable. Linen is widely worn as it has excellent breathability due to its large fibres, but like cotton it is slow to dry. Merino wool has been a popular choice for outdoor enthusiasts as it's breathable and wicks moisture without retaining odour.

Nylon and polyester are used in most activewear as they wick moisture and dry quickly – but they retain odour. Research has also shown that nylon has a higher moisture absorption and better wicking capabilities than polyester, but is slower to dry. Synthetic fibres like nylon and polyester can feel uncomfortable when they get wet though, and one study suggested wearing clothing made of bamboo, which is a low conductor of heat, and doesn't compromise on comfort.

If you really want to keep cool, then strip off completely – as long as this is appropriate, of course, says George Havenith, a professor of environmental physiology and ergonomics at Loughborough University in Leicestershire, UK. Clothes protect your skin from burning, but being naked is better for keeping cool. The less clothing you wear, the more opportunity there is for an evaporative heat exchange between your skin and the air. Although of course, protecting your skin from UV rays is a priority.

But there may be alternatives – new materials and fabrics that will help to cool the body. 

New science

Sports companies such as Nike and Adidas invest millions in designing smart fabrics, and scientists have also been pouring resources into researching which fabrics are better at keeping the body cool. Scientists at MIT found the key balance for allowing heat to radiate away from the body more effectively is a material that's opaque to visible light – reflects and doesn't absorb sunlight – but transparent in the infrared – allowing heat to leave the body rather than trapping between the material and the skin. They found that by making nylon and polyester fibres thinner – around one micrometre in diameter – and weaving them into a yarn 30 micrometres thick could help the wearer maintain a more comfortable temperature.

Scientists at the University of Maryland in the US also developed synthetic fibres with coatings that actually change their structure in response to outside conditions – releasing more heat as temperature rises. The adaptive yarn expands and collapses, changing the space between the fibres. Wider spacing allows the textile to breathe, allowing heat to radiate so the wearer can cool down.

Another group experimented with garments laced with strips that flatten and bend to help cool the body down by more than 2C (3.6F). In hot weather, the strips bend towards the air, dissipating heat from the body. Other teams have been exploring "phase-change" materials that incorporate capsules or fibres of material that melt as they warm up, helping to absorb excess heat.

Getting wet

Perhaps, the best way to stay cool in the heat when it comes to clothing is actually just wearing wet clothes, Allain points out. Water needs heat energy to evaporate, and as it makes this transition from a liquid to a gas, it uses the heat coming from your body, cooling your skin and lowering your body temperature.

So it turns out that picking an outfit to stay cool is a little more complex than simply throwing on a white T-shirt. But the right fabric, appropriate fit and occasionally a splash of water when possible should help you keep your temperature down when the mercury heats up – and save on air-conditioning too.

-bbc