French Turn to Chalk to Combat Record Heatwave, Causing Nationwide Shortage

In late June and early July 2026, France experienced an unprecedented heatwave with temperatures soaring above 40 degrees Celsius. This extreme heat led to a nationwide shortage of crushed chalk, a simple and inexpensive product that became a viral home remedy for cooling. Citizens and schools began applying a white chalk layer on windows to reflect sunlight and reduce indoor temperatures without using electricity.

The chalk, traditionally used for whitening shop windows and agricultural greenhouses, gained popularity after social media spread the technique. Experts, including Purdue University cooling technology researcher Shiyan-Yu Li, confirmed the scientific basis behind the method. Li explained that chalk’s main component, calcium carbonate (CaCO3), reflects most sunlight, including UV and near-infrared radiation, making it highly effective at lowering indoor heat when applied to windows.

French media reported frustrated residents unable to find chalk in stores. Philippe, a buyer quoted by Ouest-France, said, "We knew about the idea for some time and talked about it during the last heatwave, but forgot to buy it. Now it’s too late! It’s out of stock everywhere!" Some schools have started using a chalk-water mixture on classroom windows, though officials cautioned this is not a magic solution. Long-term cooling requires proper roof insulation beyond temporary window coatings.

The surge in chalk use reflects growing concerns over frequent, intense heatwaves linked to global warming, posing serious health risks, especially in dense urban areas. The concept of using white paint or coatings to reduce heat is well-established in southern Europe, such as Greece, where whitewashed buildings reflect solar radiation. Recent scientific advances have developed ultra-white paints that can reflect over 98% of sunlight and reduce daytime temperatures by up to 7.6 degrees Celsius.

This knowledge is now influencing other industries. Jiashou Wang, a University of New South Wales student involved in developing cooling paints, noted that calcium carbonate particles are widely used in advanced radiative cooling coatings and even fabric treatments to block UV and infrared rays, helping keep wearers cool.