Iranian officials closed government offices, courts, schools and universities in Tehran on Monday after a severe sandstorm hit the capital, state media reported.
"The air pollution emergency committee of Tehran province has ordered the closure of all administrative offices and public educational centres today (Monday) due to the spread of dust," state news agency IRNA said.
Visibility was severely limited in Tehran, a city of more than eight million people.
The neighbouring province of Alborz, located west of the capital, also announced the closure of all offices, banks and scientific and educational centres, state television said.
It reported an "increase in the concentration of atmospheric pollutants and dust" in the air.
While the region has always been battered by sandstorms, dust clouds have become more frequent and intense in recent years.
The trend is linked to climate change and associated with overgrazing and deforestation, as well as the overuse of river water and more dams.
In April, Tehran's Air Quality Control Society said dust clouds originated from "countries to the west of Iran".
Iran's western neighbour Iraq has been hit by severe sandstorms, with air pollution in recent months there sending thousands to hospitals with respiratory problems.
Iranian authorities also blamed sand quarries west of Tehran that they said are making the situation worse.
Tehran's Metrological Organisation warn that waves of dust clouds are expected to sweep across parts of the city for the next five days.