More than two weeks of heavy pollution led Iranian officials to ban all outdoor sport and announce new traffic restrictions Wednesday as persistent cold weather exacerbated Tehran’s air quality problems.

In the worst concerted period of pollution for three years, primary schools and nurseries were closed and new car exclusion zones imposed in the capital.

Tehran’s air quality index averaged 159 on Wednesday, up two from the previous day, and more than three times the World Health Organization’s advised maximum of between zero and 50.

At such levels people are advised not to leave home unless absolutely necessary.

One area in northeastern Tehran peaked at 238 on the pollution index and President Hassan Rouhani addressed the issue at a meeting of his cabinet as concern rises about health risks.

“The problem has been around for years and cannot be entirely tackled in a short time,” he was quoted by the official IRNA as saying.

IRNA reported that Wednesday was the 18th straight day of dangerously bad air while newspapers quoted officials casting blame on each other for the problem and failure to tackle it.

“Our preference was to close all schools but the education ministry insisted on keeping high schools open because of final term exams,” said Mohammad Heydarzadeh, secretary of Tehran’s emergency air pollution committee.

The cold weather is causing climate inversion – where emissions from car exhausts hang in the air rather than rising into the atmosphere above.

A decade-long central restriction zone based on car number plates was in place across the city on Wednesday, traffic police announced.

Vehicles with plates ending in an odd number can’t go out on Saturday, Monday and Wednesday, while cars with even numbered plates are banned on Sunday, Tuesday and Thursday.

Sand and cement factories around the capital have been banned from operating until Friday, the end of the Iranian week, and all outdoor sport including professional football league matches has been called off.

Exhaust fumes from the five million cars and almost as many motorcycles on Tehran’s roads account for 80 percent of its pollution, officials say.

Two permanent zones of traffic restrictions introduced in 1979 and 2005 have failed to rectify the sprawling city’s poor air quality.

While Tehran is the epicentre of the problem, primary schools in other major cities including Isfahan, Qom, Arak and Tabriz were also forced to close.

Weather forecasters predicted that air quality would improve after expected rainfall on Wednesday evening.

Last December, almost 400 people were hospitalised with heart and respiratory problems caused by heavy pollution in Tehran, with nearly 1,500 others requiring treatment.

In 2012, pollution contributed to the premature deaths of 4,500 people in Tehran and about 80,000 in the country, the health ministry said.