More than 400 flights to and from Beijing airport, including around 35 international services, were cancelled or delayed on Saturday due to thick fog and strong air pollution covering the city.
Almost 250 flights were cancelled, including some 15 international services, according to the website of the international airport in the Chinese capital.
More than 180 flights were delayed, including about 20 international services, the site said.
Visibility at the airport early Saturday was at less than 200 metres (650 feet), official state news agency Xinhua said, blaming "widespread fog" for the disruption.
But the US embassy, which has its own measuring system, said pollution in Beijing had reached the "hazardous" level early Saturday, before dropping one notch to "very unhealthy" later.
China said this month that two-thirds of its cities currently fail to meet new air-quality standards introduced this week that are based on the pollutants most harmful to health.
The new targets were introduced to placate growing anger among the Chinese public, which is increasingly concerned about health issues.
Explore further: Oil globs close Los Angeles-area beaches to swimming (Update)