The Iranian capital experiences such days quite often, forcing schools to close and people with heart and lung problems to stay home. According to official figures, Tehran had over 125 days of unhealthy air pollution over the last year.
And that is having serious consequence on people's health. Some 4,600 Tehran residents reportedly die each year from pollution-related illnesses.
Alborz Maleki is a Tehran-based journalist. He says respiratory problems as well as eye and throat irritation have become commonplace.
"It causes headaches, burning eyes, especially children and elderly people are facing many [health] problems. On Saturday afternoon after walking for only 15 minutes in Vanak Square [in the center of Tehran], I got a terrible headache. People suffer from eye allergies; nose irritation is very common. In the last week, it was announced that some 360 people in Tehran have sought emergency medical help because of respiratory problems and some 300 persons because of heart problems," Maleki said.
Cars are chiefly to blame for Tehran's heavy pollution. Most of the city's more than 2 million cars are at least 20 years old and do not have filters such as catalytic converters to reduce pollutants.
"The children were shouting, 'We also want to live,' 'Why do you make the air so polluted?' and 'Do something so that after you, we can live too.'"
Experts say the geographical position of the city, which sits in a basin surrounded by mountains, worsens the pollution.
Tehran is usually enveloped in a cloud of smog and, according to a recent study, each resident inhales between 7 and 9 kilograms of dust per year. In recent years, due to the increase in the population, the pollution problem has worsened.
The Iranian government has announced various steps to combat the problem, but none so far have proven effective.
In 2001, a 10-year program was launched to reduce air pollution by phasing out old motor vehicles. Construction on a subway line was begun in Tehran. And every year a "National Clean Air Day" is held to urge city residents not to use their cars on that day unless necessary.
Hossein Abolhassani, a member of the Women's Society Against Environmental Pollution, says other immediate steps should be taken to fight the pollution.
"The first thing they should do is that they should immediately apply the odd-even license plate system so a big number of cars are not allowed into the city. They can also ban the passage of cars with one passenger. They should also improve public transportation and clean fuel should be used. There have been talks for a long time about phasing out old smoke producing cars; they should do it as soon as possible," Abolhassani said.
Environmental experts also say that the government should subsidize the purchase of new cars, enforce emission controls from old cars and increase green spaces in the city.
Abolnassani says that environmental NGOs are playing an important role in the fight against pollution. The Women's Society Against Environmental Pollution has organized several demonstrations against pollution with the participation of children.
"The children were shouting, 'We also want to live,' 'Why do you make the air so polluted?' and 'Do something so that after you, we can live too.' They were small children. Such steps are efficient and from the time that NGOs began their activities in that regard, some 30,000 to 40,000 taxis are using [clean] gas as their fuel. And the government has announced that buses will also use [clean] gas. These are things that are being done and it's all because of the cries of the people," Abolhassani said.
Maleki, the Tehran-based journalist, says people are becoming more aware of the danger of pollution and government warnings are being taken more seriously.
"People really felt the danger yesterday with the announcement regarding the closure of schools. It does not happen very often. I think people have taken the warning seriously. As I witnessed myself yesterday afternoon -- when it's usually the peak of Tehran's traffic -- the streets were less crowded, there were fewer cars," Maleki said.
Experts say it will take several years before Tehran's pollution level is decreased or at least kept at the same level. Meanwhile, the capital's residents are taking their defense into their own hand -- by wearing masks.
"People were masks even though experts say wearing mask is not very efficient. At the same time, these masks are a real help because they absorb some of the pollution and prevents some of the carcinogens from entering the respiratory tract," Maleki said.
According to Tehran's official Air Quality Control Unit, the 1 January Pollutant Standard Index (PSI) -- a measurement that incorporates carbon monoxide and other pollutants -- reached 168, or close to "very unhealthy."
By comparison, the PSI in New York City was 52 and in Bangkok 57.