A general view of the Milad telecommunication tower (background-L) engulfed by smog in the Iranian capital Tehran on December 19, 2015.

:: Schools were closed for a fourth straight day in Tehran on Wednesday as dangerous air pollution covered the Iranian capital and traffic restrictions failed to clear the thick smog.

Average airborne concentration of the finest and most hazardous particles (PM2.5) was “unhealthy” at 160 microgrammes per cubic metre, slightly worse than Tuesday, authorities said.

That is more than six times higher than the World Health Organization recommended maximum of 25 microgrammes per m3 over a 24-hour period.

Authorities extended traffic restrictions so that only one in two cars could go on the street, depending on their number plates.

Wednesday is the last day of the week for schools in Tehran and authorities have not yet said whether they will reopen on Saturday.

Pollution was also blanketing at least four other major cities: Esfahan in the centre, Orumiyeh in the northwest, Mashhad in the northeast and Tabriz in the northwest.

Schools were closed on Wednesday in all of those cities except in Mashhad, the state broadcaster’s website reported.

Ambulances were also deployed in the streets for possible medical emergencies, it added.

Citizens, especially those with respiratory problems, children and the elderly, were advised to stay at home.