https://www.sciencedaily.com/releases/2023/02/230201084329.htm > From 2015 to 2021, the impact of winter heating on China's capital and 27 other cities saw concentrations of fine particulate matter (PM2.5) from heating activities reduce by 41.3% -- compared with a drop of 12.9% in other northern Chinese cities which use lower levels of clean fuels than the '2+26' cities. > China's centralised winter heating strategy is one of the world's largest energy-consumption systems -- providing free or heavily subsidised heating to urban residents. The system is usually switched on from mid-November to March.