Chengdu Shuangliu International Airport’s runway was closed for up to 10 hours as the city’s air quality clocked in at 280, a level considered to be in the “very unhealthy” category. About 20,000 passengers are still stranded at an airport in Chengdu, capital of Sichuan Province.

Authorities at Chengdu Shuangliu International Airport lifted the red alert for flight delays at 3:23 p.m. after issuing it at midnight, but the number of delayed flights are still increasing. As of 3:30 p.m., 58 flights had been canceled, according to the airport authority.

Beijing and the nearby Hebei province were placed on high alert, registering levels as high as 875, considered to be ‘extremely hazardous,’ according to the World Air Quality Index.