0342 GMT January 27, 2020
The pollution level in the capital was "unhealthy for sensitive groups" and officials warned the young, elderly and people with respiratory illnesses to stay indoors, with sporting activities suspended.
The decision to shut schools in Tehran was announced late Saturday by the deputy governor, Mohammad Taqizadeh, after a meeting of an emergency committee on air pollution, according to AFP.
"All of (Tehran) province's schools except for Firuzkuh and Damavand counties are closed for Sunday," he said as reported by the official IRNA.
Schools and universities in the capital city stay closed on Monday, the third day of the Iranian working week, he added.
A traffic scheme based on vehicles' registration numbers was imposed to ease the capital’s traffic load, IRNA added. Trucks were banned outright in Tehran Province.
Taqizadeh also noted that all activities at Tehran Province's numerous sand quarries would be halted. Heavy smog hung over Tehran on Sunday, obscuring the view of the mountains overlooking the city.
Average airborne concentration of fine and most hazardous particles (PM2.5) was at 145 micrograms per cubic meter for the 24 hours until Sunday noon, according to government website air.tehran.ir.
That is close to six times the World Health Organization's recommended maximum of 25 micrograms per cubic meter.
Schools were also closed in the northern province of Alborz and in the central cities of Qom and Arak.
Air pollution was the cause of nearly 30,000 deaths per year in Iranian cities, state media reported earlier this year, citing a health ministry official.
The problem worsens in Tehran during winter, cold air and a lack of wind traps hazardous smog over the 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.