A visitor wearing a face mask takes a picture of herself in front of the Palace Museum in Beijing during a smoggy day in late December. Wei Xiaohao/China Daily

Pollution grounded 100 flights in Shandong province's capital Jinan and closed all major expressways between Hebei province and neighboring Beijing, plus many throughout North, East and Central China during the return rush on Sunday, Xinhua reports.

International travel agencies from such countries as Singapore adjusted plans for citizens visiting the Chinese mainland in early December.

CTC Travel ordered its Beijing branches to give its 280 Singaporean visitors filtering facemasks, the Straits Times reports.

The newspaper quoted Dynasty Travel spokesperson Alicia Seah as saying: "We are worried. We've instructed our ground operators to stock up on masks and we will replace outdoor activities with indoor shopping trips or acrobatic shows, if need be."

Singapore's Chan Brothers Travel also dispensed masks and publicized an emergency hotline among its customers last month, the paper reports.

Ctrip's top 10 international smog-escape routes around Christmas, according to bookings, were Japan, Thailand, Australia, Canada, Switzerland, New Zealand, the United States, Maldives, Mauritius and the United Arab Emirates.

Reservations for Japan's Hokkaido, Tokyo, Osaka and Okinawa rocketed.