Iran has closed government offices and schools in Tehran for two days because of wind-blown dust that has polluted the capital.
Government authorities have declared Tuesday and Wednesday a public holiday, and Iranian media say many domestic airline flights have been canceled.
Medical authorities are advising people with heart and respiratory problems to stay inside.
Iran's Press TV says Tehran's Air Quality Control Company has found the amount of potentially harmful particulate in the air has reached dangerous levels.
The dust has blown in from Saudi Arabia and neighboring Iraq, where a severe sandstorm recently blanketed the capital, Baghdad.
Sandstorms can be caused by heavy winds blowing across deserts, or across land that has lost its fertile top soil and greenery.