As large swaths of Europe and Asia suffer record heat waves this summer, discussions have focused on an obvious way to keep people cool: more air conditioners. Popular in high-income countries such as the United States, Japan and Australia, these units are less common in homes in Europe and low-income countries. But air conditioners are expensive to operate, consume large amounts of energy and generate heat, so installing them in every home that needs one is not the answer, scientists say. Instead, they say that, in recent years, there have been advances in materials that passively cool buildings, offering a solution without these drawbacks.

Building cooling accounts for nearly 10% of global electricity use, according to a study published earlier this month in Nature Reviews Clean Tech¹. That number is expected to rise as heat waves become more frequent, intense and long-lasting. By 2050, the electricity needed to cool buildings worldwide is expected to have increased by 210% compared to 2024 levels, and greenhouse gas emissions from air conditioning are likely to triple over the same period.

Matthaios Santamouris, an architectural researcher at the University of New South Wales in Sydney, Australia, who co-authored the review, says air conditioning can save the lives of people who are at high risk of dying from extreme heat, including those over 65, but the technology has several drawbacks. During temperature spikes, when there is an increase in air conditioning use, sudden demand for electricity puts pressure on the grid, causing blackouts that endanger people vulnerable to heat stress.

Air conditioning also raises temperatures in cities, says Negin Nazarian, an urban climatologist at the University of New South Wales. Split systems, in which an air conditioning unit cools a single room, work by extracting heat from the building and “pouring it right next door” into the surrounding environment, Nazarian adds. This creates a “vicious cycle,” he says, in which outdoor temperatures rise, causing increased use of air conditioning to cool indoor spaces.

The cost of using air conditioners also makes them more difficult to access for low-income populations, who are particularly vulnerable to damage from exposure to extreme heat. For example, up to 100 million families in India, Mexico, Indonesia and Brazil will still not be able to afford air conditioning in 2040, despite having access to electricity.².

An alternative solution

Santamouris says passive building cooling, in which materials are designed to control temperatures, uses little or no energy. Automatic blinds and window slats that follow the position of the sun are already used in commercial buildings, offices and universities. These control the amount of heat entering a building, reducing dependence on air conditioning.

One of the most exciting advances is the development of materials that can achieve passive radiative cooling, says Nicole Miranda, a sustainable cooling specialist at the University of Oxford, UK. During the day, heat from the Sun radiates in the form of infrared waves, which are absorbed by exposed surfaces. When surfaces become hotter than the environment around them, they release infrared waves. Most wavelengths are absorbed by the atmosphere, except for ‘atmospheric window’ waves (those with wavelengths between 8 and 13 micrometers), which escape directly into space. Passive radiative cooling technologies take advantage of this. For example, special paints, glasses, gels, and other building materials are made to reflect these wavelengths and cool surfaces.

Scientists have developed materials that consist of several layers, or with bubbles or air pockets, to reflect heat. For example, paints that incorporate micro- or nanosized spheres of silicon oxide and aluminum oxide can improve light scattering and increase the amount of heat released into the atmosphere.³. One coating reduced the surface temperature to which it was applied by 5.26°C, reflecting 97% of the solar energy to which it was exposed.⁴.