China’s meteorological center has issued a yellow alert¹ for air pollution this morning, warning of a new round of heavy smog.

The smog will affect Beijing, Tianjin municipalities, Hebei and Shanxi provinces in northern China, parts of Henan province in central China, Shandong province in the east, and Shaanxi province in the northwest.

Beijing had only seven days of blue skies in December, with two rounds of red alert—the highest air pollution warning. Levels of PM 2.5, the smallest and deadliest airborne particles², topped 500 during those days. This is 20 times the level considered safe by the World Health Organization.