Smog in the northeast Chinese city of Shenyang hit record highs over the weekend as residents were warned to stay inside or don gas masks to leave their homes.

Shenyang, China.

Shenyang, China. Image by nina.bruja / CC BY-SA 2.0

Air quality in China is typically measured by the number of PM2.5 particles (tiny bits of cancer-causing grit released by coal burning, exhaust and factory pollution) in the air – 35mg per cubic metre is accepted as the highest safe level. Shenyang’s air quality reportedly measured was measured by some at up to 1,400mg per cubic metre. The city was photographed covered in a dense grey fog, and visibility in some cases was reduced to only a dozen metres. The United States consulate in Shenyang, which does its own air quality reporting, tweeted that the pollution was ‘Beyond Index’ on Monday. Read more: