According to Tehran's official Air Quality Control Unit, Saturday's Pollutant Standard Index (PSI) -- a standard measurement that incorporates carbon monoxide, dust and other pollutants -- reached 168, or close to "very unhealthy".

"There was a lack of any parameter in the atmosphere to decrease the pollution," a Tehran pollution official, Elmira Parto, told AFP in reference to the current season marked by low winds.

Following the order to close elementary and junior high schools for the day in a bid to reduce traffic, the PSI for Tehran was reported to have fallen to 113 -- closer to Saturday's level in other cities such as Beijing and Shanghai.

By comparison, Saturday's PSI in New York was 52 and in Bangkok 57.

The soaring pollution in Terhan -- often blanketed in a veil of thick brown smog -- has been attributed to a rapid increase in traffic and congestion.

Half of the four million vehicles in the city of 10 million are more than 20 years old and, according to official figures, the average petrol consumption is 17 litres per 100 kilometres, more than three times as much as for European cars.

According to a recent study, each resident of the Iranian capital -- now considered one of the world's most polluted cities, inhales an estimated 7.1 to 9.3 kilogrammes of dust every year.

The poor air quality has been blamed for an increase in respiratory ailments, and asthma is increasingly common.