The whitest ever paint which could help tackle global warming by cooling down buildings without the need for air conditioning, has been created by university engineers.
The paint, which could be on the market in two years' time, reflects 98.1% of sunlight and radiates infrared heat away from the surface.
Typical commercial white paint gets warmer rather than cooler.
White paints that are already used for painting buildings reflect between 80% to 90% of sunlight but absorbs UV light meaning it cannot cool surfaces below ambient temperatures.
The newly developed paint was shown to cool surfaces by 4.5C below the ambient temperature, even in strong sunlight.
Coating buildings in the paint could cool the building enough to not need air conditioning, researchers from Purdue University in the US said.
"If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts.
"That’s more powerful than the central air conditioners used by most houses,” said Purdue University Professor of Mechanical Engineering Xiulin Ruan.
Two features give the paint its extreme whiteness.
Firstly, it has a very high concentration of the chemical compound barium sulphate, which is also used to make white photographic paper and cosmetics.
Secondly, the barium sulphate particles are all different sizes.
How much each particle scatters light depends on its size, so a wider range allows the paint to scatter more of the light spectrum from the Sun.
"We looked at various commercial products, basically anything that’s white," said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology, who worked on this project.
"We found that using barium sulphate, you can theoretically make things really, really reflective, which means that they’re really, really white."
He added: "Although a higher particle concentration is better for making something white, you can’t increase the concentration too much.
"The higher the concentration, the easier it is for the paint to break or peel off."
The paint's solar reflectance is so effective, it even worked in the middle of winter, the researchers found.
The white paint is the result of six years of research to develop radiative cooling paint as a feasible alternative to traditional air conditioners.
Prof Ruan’s lab had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material.
Their previous whitest paint used chalk - calcium carbonate – and reflected 95.5% sunlight.