Heavy smog in Tehran broke records this week, reportedly causing hundreds of deaths, and prompting school closings in the region.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured a natural-color image of the smog on November 9, 2016. The pale gray smog shrouds the city of Tehran, just south of the Alborz mountain range.

Fine airborne particles (PM 2.5 ) climbed to concentrations of more than 150 micrograms per cubic meter this week; concentrations between 101 and 150 are considered unhealthy for sensitive groups, including the elderly and children. More than 400 people have died due to air pollution in the city over the past month, according to news reports. The city closed schools on November 17.

“In the past several days or so [it has] been really bad,” said NASA atmospheric scientist, Amin Dezfuli, who lived in the city between 2001 and 2006. “That was one of the reasons I wanted to leave Tehran.”

When Dezfuli lived in the city, air-pollution-related school closings were not so common, he said. Since then, air quality has further declined, with significantly fewer days with good or average air quality, studies show. Emissions standards for cars are weak, he noted. In addition, public transportation, though increasing, is still having trouble keeping up with a growing population. Leading sources of air pollution in the city include fumes produced by cars, trucks, and motorcycles.

In the winter, temperature inversions can exacerbate Tehran’s smog. A layer of warmer air traps the colder, denser, smog-laden air beneath it. Meanwhile, mountains around the city prevent smog from leaving the valley.

NASA Earth Observatory image by Joshua Stevens, using MODIS data from LANCE/EOSDIS Rapid Response. Caption by Pola Lem.