ZHENGZHOU/JINAN -?Ongoing heavy smog in large parts of China is disrupting schools and causing holiday travel problems.

Monday is the last day of the New Year holiday in China, but authorities in Zhengzhou, capital of central China's Henan Province, ordered students of primary and high schools as well as kindergartens to take Tuesday off as the city is on red alert for smog.

China has a four-tier alert system for pollution, with red the highest, followed by orange, yellow and blue.

According to an urgent notice issued by the city's educational bureau, primary and high schools may use weekends or holidays to make up the missing classes.

In neighboring Shandong Province, the provincial weather station Monday issued orange alerts for fog and smog that has led to visibility of less than 200 meters in 11 cities in the province.

Around 50 flights were affected in the provincial capital Jinan as of 10 a.m. Nearly 300 toll gates along 30 expressways have been closed, according to the provincial transport department.
Heavy fog was also seen in Chengdu, capital of southwest China's Sichuan Province, stranding over 8,000 passengers at the airport.

A runway at Chengdu Shuangliu International Airport was closed for four hours, leaving over 80 flights delayed and seven canceled.

Flights did not resume until 11:40 a.m., the airport said.