In the year to March 20, there were 125 days when the quality of the air was deemed critical or even dangerous.

"According to international standards, air pollution should not pass the critical threshold more than once a year," Said Abol-Hassani, a spokesman for the centre said.

"This situation is dangerous for the vulnerable section of the population, particularly children and old people," he added.

Without giving an indication of the levels in question, he blamed the bulk of the pollution in the Iranian capital on cars.

Half of the four million vehicles in the city 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. Even new cars do not meet accepted norms.

Considered one of the most polluted urban areas in the world, Tehran and its satellite towns has a population of some 12 million people.

SPACE.WIRE