Air pollution in Iran's capital hit its worst level -- for a day without a sandstorm -- in at least nine months on Monday, prompting warnings that all citizens should stay indoors.

"Tehran's air is unhealthy for all," said the city's transport and traffic department on a municipality-owned website.

"Citizens, and especially sensitive groups, are asked not to leave their homes unless urgent," it said, referring to children and the elderly.

Visibility was low as a grey fog blanketed the capital, a megacity with an estimated 14 million people within its metropolitan area.

Monday's Air Quality Index showed an average level of 162, within a "red status" that means the air is unhealthy for everyone, according to World Health Organization standards.

A normal healthy level is between zero and 50.

Monday's high pollution level was the second highest recorded since the start of the Iranian year on March 21.

The metre hit 252 during an isolated dust storm in September.

Floating particles from car emissions were the worst pollutant Monday -- registering "seven times the standard level," Air Quality Control Organisation chief Vahid Hosseini told official media.

Officials may move to shut down schools should the index reach 200, the deputy for Tehran's environmental monitoring department said.

Peak pollution hit 180 in some areas of Tehran, whose poor air is worsened because of high altitudes -- between 1,100 and 1,700 metres (3,600-5,600 feet) -- above sea level in a land basin surrounded by mountains.

Iran's government has tried to cut pollution by supplying Euro 4 standard fuel in large cities, Masoumeh Ebtekar, a vice president responsible for environmental protection, said on state television on Sunday.

Still, the exhaust fumes of five million cars make up 80 percent of Tehran's pollution, which increases in winter as emissions fail to rise above cold air in the atmosphere.

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

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.