Our Geo Quiz takes us to one the largest cities of western Asia. It may be a world capital but the government is asking residents to clear out.
Schools and universities are closed until the weekend and most government offices are shutdown. The problem is in the air, says the BBC's Mohsen Asghari.
"If you go outside to a higher location, if you go to the mountainous area, and look out at the city, you'll see that a very thick gray cloud is hanging over the city like a monster," Asghari says.
Air pollution is a big problem in this world capital. It's coming from too many automobiles, and smokestacks. The city's geography is also partly to blame. The city's surrounded by the Alborz mountains that trap the foul air.
The massive Alborz mountain range that basically blocks the winds that blow down off the Caspian Sea.
So can you name this western Asian city shrouded in smog?
There's been a dense fog lingering over Tehran, Iran.
It's more than just a matter unpleasant weather. It's a public health menace.
The BBC's Mohsen Asghari says the smog is bad enough to cause headaches and breathing problems, and to prompt the government to encourage the residents of Tehran leave the city for couple of days.