BEIJING -?Grey smog fell over northern China Sunday, prompting the national observatory to issue a yellow alert, the third-highest level warning.

Smog was forecast to blanket Beijing, Tianjin, Hebei and Shaanxi over the next two days, according to a statement on the National Meteorological Center (NMC) website.

The NMC advised the public to take precautions, and patients with respiratory diseases were told to reduce outdoor activities.

China has a four-tier color-coded system for severe weather, with red being the most serious, followed by orange, yellow and blue.