Schools and kindergartens have been closed in Tehran as a blanket of dangerous smog envelopes the city.

Iranian state-radio has urged the city's 10 million residents, especially children, the elderly and the sick, to stay indoors, and for people to use public transport instead of their private vehicles.

Parents of small children welcomed the government's decision to close the schools, but high school teachers and pupils were unhappy that their schools were not closed too.

For several weeks, the dangerously high levels of air pollution have choked Tehran.

Stagnant smog

Many residents with heart conditions and asthma have needed hospital treatment.

The city is hemmed in by mountains to the north, causing the increasing volume of pollutants to become trapped when the wind is not strong enough to blow them away.

At this time of year, the problem is aggravated by atmospheric conditions which push dense clouds of stagnant smog down onto the city.

The result is an insidious haze of mixed pollutants, mainly carbon monoxide and suspended particles. Levels of both have now reached well above the crisis point.

Dirty cars

To help combat the problem the authorities also announced on Sunday a scheme to limit private cars in the city.

Vehicles with odd and even number plates will only be allowed into Tehran on alternate days.

Motor vehicles are believed to be responsible for 70% of the pollution - most are over 15 years old and lack modern exhaust filters.

There are over two million cars in the heavily congested capital - a scheme to restrict their entry on weekdays using special passes has proved insufficient.

The Tehran municipality, in co-ordination with the government, recently announced a 15-year, $2bn plan to combat the problem, but it will not get under way for another two months.

Meanwhile many Tehranis have resorted to wearing face masks while breathing through wads of cloth. Breathing problems and irritations of the skin and eyes have become common.