Smog forces schools shut again in Iran's Tehran province

Schools in Iran's Tehran province will be closed for the next two days over rising levels of air pollution, authorities announced on Friday, following similar measures last week.

The decision was made in a meeting of an emergency committee on air pollution and announced by deputy governor Mohammad Taghizadeh, according to state TV.

An "odd-even" traffic scheme -- based on the last number on vehicle registrations, with odd numbers allowed access one day and even numbers the next -- will be imposed to restrict traffic, Taghizadeh said.

Trucks will be banned outright and construction and mining activities halted.

"Considering the level of air pollution, citizens are asked to refrain from unnecessary movements across the (capital) city (of Tehran) and use public transport," Taghizadeh said.

Average airborne concentration of the finest and most hazardous particles (PM2.5) was at 109 microgrammes per cubic metre for the 24 hours until Friday evening, according to government website air.tehran.ir.

That is more than four times the World Health Organization's recommended maximum of 25 microgrammes per m3.

Pollution levels are expected to be "unhealthy for all groups" with average airborne concentration of 150-200 PM2.5 for the first two days of the working week that starts on Saturday, state news agency IRNA said.

The situation could persist for the rest of the week if urban pollutant sources are not checked, it added.

The problem worsens in Tehran during winter, when cold air and a lack of wind traps hazardous smog over the capital city for days on end, a phenomenon known as thermal inversion.

Most of the city's pollution is caused by heavy vehicles, motorbikes, refineries and power plants, according to a World Bank report released last year.