Beijing's two recent red alerts for air pollution led to the suspension of classes at primary and middle schools in the capital, but online education platforms experienced a boom.

According to 17zuoye.com, an Internet-based K12 homework service, the number of newly registered users on Dec 8 was almost three times as many as the day before, with 3,000 teachers and 160,000 students in Beijing assigning and doing homework there.

Users of Homework Box, a mobile app that has similar functions to 17zuoye.com, hit a new high in the number of questions downloaded and worked on by students from Dec 7 to 9, when Beijing issued its first red alert for air pollution and primary and middle schools of the capital suspended classes for three days.