All schools and nurseries in the Iranian capital, Tehran, have been closed for two days because air pollution has reached dangerous levels.
Tehran is believed to be one of most polluted cities in the world
Two million school children have been given an unexpected day off.
The authorities have advised children, the elderly and people suffering from heart and lung disease to stay indoors.
There are three million cars in Tehran, of which two-thirds are more than 20-years-old and lack modern exhaust filters. Petrol is heavily subsidised.
It is estimated that up to 5,000 people die every year from air pollution in the city.
Tehran has been suffering from severe smog since the beginning of December.
The authorities have blamed vehicle pollution and have urged people to stop driving and to use public transport instead.
'Panting for breath'
State television has not given the exact level of pollution, but did say that levels of carbon monoxide, suspended particles and other pollutants had become critical in Tehran and the southern district of Shahr-e Rey.
Pollution tends to hang over the mountain-surrounded city
The BBC's correspondent in Tehran, Frances Harrison, says one hospital specialising in respiratory problems has received a lot more patients in recent days, suffering because of the poor air quality.
The geographical location of Tehran, wedged between mountains, means if there is no wind or rain the dirty air gets trapped, our correspondent says.
Newspaper headlines say Tehran is panting for breath and carry pictures of grey smog hanging over highways packed with traffic.
One newspaper has published a map indicating unhealthy air in central and northern Tehran, with some spots designated as very unhealthy.