Iran is a very polluted place at the best of times; Tehran and other big cities, particularly, are full of vehicle fumes. Tehran sits in a sort of bowl surrounded by mountains. In the summer no wind comes to take the fumes away and it is common to see people walking around wearing masks.
Right now, however, Iran is choking under a new cloud - of dust, blown from Iraq and Saudi Arabia. The storm affected US Vice President Joe Biden's trip to Iraq last week. There are some dramatic pictures of it here from an 'embedded reporter' in Iraq.
The dust-storm has served as a convenient excuse for the Iranian government to shut government offices and universities. Tomorrow is the 10th anniversary of a major student uprising in 1999, when students at Tehran University, emboldened under the regime of the 'reformist' president Khatami, demonstrated against the closure of a reformist newspaper. Their dormitories were attacked and one student killed. Protests followed throughout the country, and in the crackdown more people were killed and hundreds of student activists arrested. Some of those are still in prison.
President Khatami's response, denouncing the protests, led to great disillusionment among many young people about the possibility that the 'reformists' could deliver real change in Iran, and earned the response 'marg bar Khatami!' - 'death to Khatami!' - from some.
There have been student protests on previous anniversaries of "18 Tir" (18th day of the month of Tir = 9th July). Who knows what will be possible this year, but it is a date on many people's minds.