Dangerously poor air quality has forced Iran's government to close all primary schools in the capital, Tehran, a city home to 10 million people.
A thick smog is hanging over the city, caused by heavy traffic and factory pollution. It's been worsened by a lack of wind and rain in recent days.
On Tuesday, the official state news agency IRNA cited Anoushiravan Bandpay, the governor of Tehran province, as announcing the following day's school closures.
Tehran is among the world's worst cities for air quality. Health experts say many Iranians suffer serious health problems as a result.