The World Health Organization standards suggest that a normal healthy Air Quality Index is between zero and 50. Air pollution in Iran's capital hits its worst level, for a day without a sandstorm, in at least nine months on Monday, December 14, 2015. The Air Quality Index showed an average level of 162, within a 'red status' that means the air is unhealthy for everyone in. This has prompted warnings that all citizens should stay indoors.
The city's transport and traffic department said, "Tehran's air is unhealthy for all. Citizens, and especially sensitive groups, are asked not to leave their homes unless urgent, referring to children and the elderly."
Visibility was low as a grey fog blanketed the Tehran, a megacity with an estimated 14 million people within its metropolitan area.
The meter hit 252 during an isolated dust storm in September 2015.
Air Quality Control Organization chief Vahid Hosseini said, "Floating particles from car emissions were the worst pollutant Monday - registering 'seven times the standard level'."
The deputy for Tehran's environmental monitoring department said, "Officials may move to shut down schools should the index reach 200."
Peak pollution hit 180 in some areas of Tehran, whose poor air is worsened because of high altitudes - between 1,100 and 1,700 meters (3,600-5,600 feet) - above sea level in a land basin surrounded by mountains.
Masoumeh Ebtekar, a vice president responsible for environmental protection, said, "Iran's government has tried to cut pollution by supplying Euro 4 standard fuel in large cities."
Still, the exhaust fumes of five million cars make up 80% 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.
In December 2014, almost 400 people were hospitalized with heart and respiratory problems caused by heavy pollution in Tehran, with nearly 1,500 others requiring treatment.