Hundreds of planes were grounded in Beijing on Monday and several freeways temporarily closed because of heavy smog and poor weather conditions, underscoring the growing cost of Beijing’s chronic air pollution.

By Monday evening, nearly 500 flights out of Beijing had been cancelled or delayed due to weather, according to the Beijing International Airport. Monday’s weather in Beijing was cloudy with a light snow and pollution readings described as “hazardous” by the US embassy’s pollution monitoring system.

Pollution has been a growing concern for China, which is home to 21 of the world’s 100 most polluted cities, as environmental degradation inflicts an ever higher economic and social cost. This year has been marked by a series of protests over pollution concerns, the largest of which in Dalian attracted more than 10,000 people.