During the first weeks of 2013, Tehran was often blanketed in a stagnant, brown layer of smog so thick and obtrusive that it was difficult to make out the conspicuous mountain ranges that encircle the city. After trying to regulate the number of cars on the streets, a measure that failed to reduce the noxious haze, the capital city’s government shut down its offices as well as banks, universities, and schools.

Sadly, this was not an anomalous incident: Tehran, along with a few other cities in Iran, face dangerously high levels of air pollution during the winter months. In the two weeks of January that I was there on a family visit, the days with high levels of air pollution outnumbered the ones when the air seemed relatively clean

Radio and TV announcers urged the elderly and those with breathing and heart problems to stay indoors. And for good reason: on one such day I took a twenty-minute car ride that left me with a sore throat and shortness of breath--I could feel my lungs laboring for air. Others who had ventured out tackled the smog with face masks or scarves wrapped around their nose and mouth, even when sitting inside cars.

In a last ditch effort to stem climbing pollution levels, the government intervened: cars alternated on the road depending whether a license plate ended in an odd or even number. The inadequacy of this measure resulted in the whole city shutting down for three consecutive days. The mountains became visible again for a couple of days, but breathing was still unpleasant.

Tehran resides on a plateau surrounded on almost all sides by mountain ranges. In the winter months, it experiences temperature inversions in which cold air rests below warmer layers at higher altitudes. The effect of a temperature inversion is that it forms a cap over the city, reducing air convection between cold and warm air masses so that polluted air stays close to the ground and the lungs of the city’s denizens.

The World Health Organization (WHO) tracks the air quality of cities around the world, measuring how many particles in the air are 10 micrometers or less, PM10 are tiny enough to enter the lungs and bloodstream, leading to respiratory problems, heart disease, lung cancer, and asthma. The maximum recommended levels for PM10 particles are 20 micrograms per cubic meter of air. Based on global measurements taken in 2009, Tehran’s air quality for PM10 reaches 96 micrograms per cubic meter.

Worst still, Tehran isn’t even the worst city in Iran. Three cities in the country ranked in the WHO’s top 10 cities worldwide with the highest PM10 levels, but not Tehran. Ahwaz, an oil-mining city in western Iran, ranks number one in the country, with PM10 levels measured at 372 micrograms per cubic meter.

A little context: 2009 measurements in Los Angeles found the PM10 air quality to be 25 micrograms per cubic meter, and 52 for Mexico City. Besides the topography, another contributor is the city’s bursting population that translates into chronic traffic congestion. Population estimates for Tehran range from over 7 million to almost 12 million people, depending on where city limits are calculate. And the buses, private cars, cabs, and motorcycles--over two million and counting--that occupy the highways and streets of Tehran, compete with the worst traffic jams experienced in Los Angeles (a 25 minute ride at 11 pm one night took over an hour and half at 6 pm that same evening). Another reason for the burdening pollution is the type of gasoline used to fuel vehicles. Though Iran produces its own oil, it lacks much of the infrastructure and technology to refine gasoline.

Due to strict sanctions, especially recent ones imposed by the US in 2010, Iran has a paucity of clean gasoline. To fill this fuel gap, an improvised “bathtub” gas, high in carcinogens, is now commonly used. Though the role of dirty gasoline in air pollution is sometimes disputed, the health ministry reported in 2011 that the country had only 150 “healthy” air quality days, whereas in 2009 that number was as high as 300.

Once government offices, banks, and schools re-opened, the streets of Tehran returned to their usual near standstill, with cars, motorcycles and people all veering in and out of each other’s way, tied together by the relentless noise of honking. Within a few days, the smog levels rose again, and the government prepared for yet another citywide shut down.