Whitest white paint could help fight climate change

The whitest white paint reflects up to 98.1% of sunlight and can be used to cool buildings the world over.

This is an Inside Science story. Typical commercial white paints reflect only 80% to 90% of sunlight and fail to help buildings stay cool during the day. For the past six years, mechanical engineer Xiulin Ruan at Purdue University in West Lafayette, Indiana, and his colleagues sought to create whiter paints to cool buildings "similar to an air conditioner, but without the need of electricity," he said. They explored more than 100 materials, narrowing them down to 10 and testing roughly 50 different formulations of each material. The one that performed best is based on barium sulfate, a compound used to whiten photo paper and cosmetics. Its molecular structure can make it highly reflective to solar wavelengths of light. When it does absorb energy from heat or other sources, it typically emanates it out at infrared wavelengths that zip unobstructed through air into outer space instead of heating its surroundings. The scientists made sure the paint had a high concentration of barium sulfate particles of many different sizes. The wavelength of light each particle scatters depends on its size, so a wide range of particle sizes helps the paint scatter more light from the sun.