People with heart or lung diseases, the elderly and children should avoid long or heavy outdoor activities and other people should reduce them.
Every year, from mid-October until mid-February, Tehran’s skyline is covered by a thick layer of smog which is largely blamed on a phenomenon known as inversion and the use of environmentally harmful mazut to fuel some power stations as the consumption of natural gas by residential areas increases dramatically.
In recent years, other large Iranian cities such as Isfahan and Mashhad have also experienced increasing air pollution.
Iranian authorities say about 5,000 people lose their lives to air pollution in the country annually. The problem also forces the closure of schools for several days every year.