The air pollution was so bad in Harbin, in China, on October 21 that the city was forced to close roads and schools, and to cancel hundreds of flights from its airport.

News reports said the smog (a mixture of smoke and fog) was so thick that people couldn’t see more than 10 or 20 metres in front of them.

According to unofficial reports, people could not even see the person standing next to them.

The city’s website said: “You can’t see your own fingers in front of you.”

Residents said there was a “burning” smell in the air.

Many people covered their noses and mouths with scarves or masks.

The pollution was blamed on several factors.