The city of Linfen in Shanxi province has put stricter controls on emissions of sulfur dioxide, including suspending production at many coal-consuming plants, after residents experienced another bout of alarmingly high pollution on Saturday.

The high level of coal consumption is considered a major source of sulfur dioxide, experts sent by the provincial government and the Ministry of Environmental Protection said after they conducted three days of thorough investigations at plants and heating service providers.

Annual coal consumption increased to 36.6 million metric tons in 2016, up from 30 million tons in 2013, they said, despite many other northern cities cutting consumption.