Iran is facing a severe pollution episode in Tehran, where high levels of fine particles have forced schools and kindergartens to close for at least four days.
Many public employees have also been told to stay home.
The smog comes mainly from old cars, motorbikes, factories and the widespread use of low quality fuel.
In winter, the lack of wind keeps dirty air trapped over the city. Tehran sits in a valley surrounded by hills, which makes the problem worse during temperature inversions.
On Tuesday the air quality index reached 164, a level considered unhealthy for everyone. Some areas have recently gone above 200.
Health experts warn that children, older people and those with breathing problems are most at risk, and authorities have begun limiting traffic and shutting some factories.