Though not very dense, it reduced visibility and caused the delay and cancelation of domestic flights to cities including Beijing, Shenzhen, Tianjin, Shenyang and Kunming.

Visibility was just 700 meters at Shanghai's Hongqiao International Airport and 2,200 meters at Pudong International Airport yesterday morning, the city's airport authority said.

More than 60 flights, mostly domestic, were canceled at Hongqiao while 43 were canceled at Pudong, the authority said.

The fog dispersed in the afternoon but many flights still failed to take off because destinations in east and northern China were shrouded in fog.

Meanwhile, an air pollution warning system connected to local primary and middle schools and kindergartens has been established by the Shanghai Education Commission and Shanghai Environmental Protection Bureau.

The bureau will warn the education authority about air pollution and information will be sent to the principals of all 3,053 schools and kindergartens within 15 minutes to guide outdoor activities.

The first message was sent last Tuesday, reminding schools and kindergartens about the pollution and urging them to avoid outdoor exercises.

Experts from Shanghai Center for Disease Control and Prevention said preschoolers and students were more likely to suffer respiratory infections than adults. Children should avoid outdoor exercise in polluted days and wear masks.