This year is on track to be the hottest in recorded history. With rising temperatures and more intense and frequent heatwaves, keeping cool in summer will get harder. Air conditioning can only go so far, especially when extreme heat raises the risk of electricity outages.
Wearing light, loose-fitting clothing is well recognised health advice to beat the heat in sweltering temperatures, especially important given heatwaves kill more people than any other extreme weather event.
But increasingly, researchers and fashion brands are turning to clothes that claim to make people physically cooler. New fabrics that take the heat off the wearer purport to cool the body and reduce energy bills – could they make living through extreme heat more bearable?
Reducing skin temperature by 2.3C
Read the description of any activewear and the terms “sweat-wicking” or “moisture-wicking” are likely to feature. Through capillary action, these clothes move sweat away from the body towards the surface of the fabric, where it evaporates and cools the wearer. This is more effective in dry heat, because evaporation is slower when humidity is high.
In addition to moisture, the body emits heat in the form of infrared radiation. Most fabrics in common use absorb a significant amount of infrared radiation from the body rather than releasing it, says Dr Alex Song, a senior lecturer at the University of Sydney. “The traditional textiles like cotton, wool, polyester … they will absorb thermal radiation and [trap] it,” he says. It’s useful for staying warm in cold weather, but not so much at the height of summer.
Newer textiles aim to maximise heat transfer to the surrounding environment, a process known as radiative cooling. In 2018, Song and his colleagues engineered a fabric made from polyethylene, a common plastic, which they embedded with nanoparticles of zinc oxide, a compound used in sunscreen. In field tests using simulated skin, under full sun the fabric resulted in temperatures 5C to 13C cooler than cotton. “The new textile actually transmits about 90% of the body’s thermal radiation,” Song says. “It feels just like bare skin.”
But fabric that lets body heat pass through is only part of the picture – ideally, cooling materials also need to reflect sunlight.
Common wisdom suggests that wearing white in summer will keep you cooler than black. That’s because black fabrics typically absorb more energy from the sun. “Black cotton fabrics typically absorb 80-90%, or even higher, of incoming solar radiation,” Song says. In comparison, white cotton fabrics absorb somewhere between 30% and 60%, he estimates. “So you will still feel hot under the sun even if you’re wearing white cotton.”
The fabric Song and his collaborators engineered reflected more than 90% of incoming energy from the sun. How? The embedded nanoparticles reflected and scattered solar radiation – a property Song compares to milk.
To the naked eye, milk is obviously white. “Actually, if you zoom in, milk is mostly water, and the white colour is from the suspension of proteins and fat,” he says. “These are very small molecular clusters on the order of hundreds of nanometres,” which collectively reflect light.
A similar fabric his team developed can reduce skin temperature by 2.3C indoors, a difference they suggest corresponds to a 20% saving in the energy needed for air conditioning.
‘Like a really thin layer of tissue’
Cooling textiles made of plastics such as polyethylene and polypropylene aren’t yet in wide use. Song concedes they can be uncomfortable to wear. There’s a reason, he says, that people have used natural fibres such as cotton for thousands of years – “they’re cheap, they’re comfortable, they’re very safe to the human body, they have high cultural acceptance”.
“These new materials … do have good performance but they lack significantly in all the other aspects – cost, comfort, water wicking, a mature industry supply chain, biodegradability.”
He and his team are now turning their efforts to engineering natural fabrics with similar cooling abilities. They’re not the only ones.
Dr Shadi Houshyar, a senior lecturer at RMIT University, and her colleagues have developed a method of coating cotton with a thin layer of nanodiamonds, resulting in temperatures 2C to 3C cooler than regular cotton.
“It’s like a really thin layer of tissue [paper] on the surface of the fabric,” she says. “The fabric feels comfortable.”
Houshyar points out that nanodiamonds – which are made from carbon – are cheap and sustainable to manufacture. The team is in discussions with several major international sportswear brands, and is working to improve the durability of the material.
“The current treatment doesn’t last more than five to six washes – we want to improve that side of it,” she says.
“In terms of comfort in a changing climate, if we can reduce temperatures by even one, two or 3C, we can save a lot of energy. That’s going to be really important for us in the future.”