It’s summer in the northern hemisphere, and we’re once again in record-breaking territory for temperatures. If you live in a city, it’s even worse. Cities are often 10-15 °C hotter than their rural surroundings, something called the “urban heat island” effect. With temperatures continuing to rise and 68% of all people predicted to live in cities by 2050, this heat island is a growing, deadly problem.

This new fabric that reflects both visible and infrared light, won’t make all that go away. But it may just make it a bit more bearable.

The sun’s heat is different than city heat

As global temperatures and urban populations rise, cities can become scorching ovens during the summer. Extensive use of heat-absorbing materials like asphalt and concrete, dense concentrations of buildings, and limited vegetation cause cities to store and release heat, increasing temperatures. This is not just an environmental problem, it’s become so pressing that in some areas, it’s a severe health crisis.

Reflective fabrics are not exactly a large-scale solution, but they can be of help.

Everyone knows you shouldn’t wear black during a heatwave, but what if we took things one step further? Reflective fabrics reflect sunlight and its heat, effectively reducing the heat absorbed by the body. But in cities, there’s a catch: it’s not just the sun’s heat, it’s also the heat coming from buildings and pavement. And it’s not all the same type of heat.

“Solar is visible light, thermal radiation is infrared, so they have different wavelengths. That means you need to have a material that has two optical properties at the same time. That’s very challenging to do,” said co-first author Chenxi Sui, a Ph.D. candidate at PME. “You need to play with materials science to engineer and tune the material to give you different resonances at different wavelengths.”

Not your average cooling material

Existing cooling fabric typically reflects the sun in a diffuse pattern — otherwise it would be blinding for the people around. But this only works for the sun’s rays and does little against urban heat. Furthermore, most of our body isn’t really directly exposed to sunlight. Just hats, shoulder coverings, and some parts of our tops are exposed to a lot of direct sunlight. That’s less than 10% of our overall clothing.

“People normally focus on the performance or the material design of cooling textiles,” said co-first author Ronghui Wu, a postdoctoral researcher at PME. “To make a textile that has the potential to apply to real life, you have to consider the environment.”

The rest of our clothes are being heated by thermal radiation coming from our sides and below, which current fabrics don’t reflect.

The new fabric, which has received a provisional patent, works against both types of heat. In tests carried out in Arizona, one of the hottest places in the world, the material kept 2.3 degrees Celsius (4.1 degrees Fahrenheit) cooler than the broadband emitter fabric used for outdoor endurance sports and 8.9 degrees Celsius (16 degrees Fahrenheit) cooler than the commercialized silk commonly used for summer clothing.

This textile can help provide a passive cooling system that can benefit people in urban heat islands, particularly as temperatures continue to rise. However, the applications go beyond clothing. You could use it on the sides of buildings or cars to reflect heat, which not only makes them cooler, but reduces the cost (and emissions) associated with air conditioning. The material could also be used in transportation to help keep goods like milk or eggs cooler.

Clothing and climate

With the current state of the climate emergency, we need all the help we can get.

Cities around the world — in Mexico, Egypt, India and the US — have experienced record-breaking temperatures near or exceeding 50 degrees Celsius (122 degrees Fahrenheit) this year alone. Urban heat islands exacerbate the problem, with densely populated areas trapping and magnifying heat. The situation is dire as urban populations continue to grow and we are not reducing our emissions at a fast enough pace, endangering the health and safety of millions.

The study “Spectrally engineered textile for radiative cooling against urban heat islands”, was published Science (2024). DOI: 10.1126/science.adl0653.