By Jim Muir in Tehran

Special measures have been announced in the Iranian capital, Tehran, to combat the city's worsening air pollution problem.

For several weeks, Tehran has been enveloped in a blanket of smog that shows no sign of clearing.

Tehran is hemmed in by mountains to the North, so that the ever-mounting volume of pollutants are often trapped when the wind is not strong enough to blow them away.

At this time of year, the problem is aggravated by atmospheric inversions 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 of them have now reached well above the crisis point.

Ailing Tehranis

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.

The elderly and those with lung or heart problems are now being urged to stay at home.

Radio stations are also calling on people not to use their cars unless it's strictly necessary; that's an appeal that's apparently little heeded.

Motor vehicles are the main culprit. There are over two million in the city; their average age is over 15 years. They are officially believed to be responsible for more than 70% of the pollution.

For some years now, a scheme has been in operation whereby cars need a special pass to enter the city centre on weekdays, but that is clearly not enough.

A temporary measure resorted to recently, with some success, was the closure of all primary schools, but that is hardly a solution.

In response to growing public concern, the Tehran municipality, in co-ordination with the government, has announced a 15-year plan to combat pollution.

It will not get underway for another two months and officials warn it will take time for the benefits to be felt.