In Iran this winter, hospital emergency rooms have been full, schools regularly closed, and many people unable to work or even leave their homes due to toxic smog, which a leading United Nations expert is calling an "air pollution catastrophe".
Dirty air is a chronic problem in Iran. Many of its cities including Tehran regularly rank among the world's most polluted, due to emissions from millions of older vehicles on the roads, and from refineries, power plants, and factories.
