More than 400 people have been killed by smog in under a month after a lethal blanket of pollution smothered the capital of Iran, it has emerged.

Schools have been shut while millions have been forced to wear face masks or retreat indoors after choking levels of pollution descended on Tehran.

As many as 412 citizens have died in the past 23 days and authorities have rolled out a series of unprecedented emergency measures.

More than 400 people have been killed by smog in under a month after a lethal blanket of pollution smothered the capital of Iran, it has emerged

Exhaust fumes from millions of cars and motorcycles that ply Tehran's roads account for 80 percent of its pollution, which increases in winter as emissions fail to rise above cold air

All of the city's schools were due to have been shut today while tourism bosses have even apologised to foreign visitors for the pollution.

A blanket of brown-white smog descended on the capital on Sunday, blocking views of the mountains that line its northern edge and forcing many of its 14 million residents to stay indoors.

The pollution in Tehran hit 156 on the Air Quality Index of deadly airborne particles, over the 150 considered 'unhealthy' for the general public. In tourist hotspot Isfahan the level hit 167.

Officials apologised to foreign visitors for the bleak conditions.

'We hope our people's hospitality wipes the grey image of Tehran's beautiful attractions from their minds,' the capital's tourism boss Rajab Ali Khosroabadi told the ISNA news agency.

Kindergartens and primary schools in Tehran were ordered to stay closed Monday and Tuesday, and traffic restrictions were tightened.

A blanket of brown-white smog descended on the capital on Sunday, blocking views of the mountains that line its northern edge and forcing many of its 14 million residents to stay indoors

Schools have been shut while millions have been forced to wear face masks or retreat indoors after choking levels of pollution descended on Tehran

Ambulances were deployed to wait in the busiest and dirtiest areas amid warnings that children, the elderly and those with existing health conditions were at particular risk.

'Since no one does anything, every year the problem gets worse. The government should block old cars. We must improve public transport,' Zeynab Nazari, a first-year sociology student, told AFP.

Every year, Tehran suffers some of the worst pollution in the world when cool autumn temperatures cause an effect known as 'temperature inversion'.

The phenomenon creates a layer of warm air above the city that traps pollution from some 10 million cars and motorbikes.

The latest smog cloud is expected to hang over the city until today when forecasters hope winds will move the stagnant air, an official told state television.

Tehran mayor Mohammad Bagher Ghalibaf rode the metro to work Sunday in a bid to encourage people to use public transport.

The pollution in Tehran hit 156 on the Air Quality Index of deadly airborne particles, over the 150 considered 'unhealthy' for the general public. In tourist hotspot Isfahan the level hit 167

The phenomenon creates a layer of warm air above the city that traps pollution from some 10 million cars and motorbikes

Despite having around 100 stations, Ghalibaf says the metro is not sufficiently funded by central government, forcing millions to rely on cars.

Pollution has become a political football in recent years, with conservatives and reformists blaming each other for the problem.

Hardliners accuse reformist vice-president Massoumeh Ebtekar, who heads the environmental protection agency, of not doing enough.

Ultra-conservative daily Vatane Emrooz said Monday that 70 percent of deaths in Tehran were linked to pollution.

Ebtekar, in an Instagram post, said various measures to reduce factory pollution and provide cleaner petrol had led to 'significant' improvement.

Still, weather conditions and heavy congestion remain a blight.

Two traffic restriction zones -- introduced in 1979 and 2005 -- have done little to solve the problem.

Local carmakers have been reluctant to introduce cleaner engines, while foreign firms have been kept out by international sanctions.

In 2014, almost 400 people were hospitalised with heart and respiratory problems caused by pollution in Tehran. Nearly 1,500 others required treatment.