Students and civil servants in parts of Iran were ordered to stay home on Wednesday and Thursday due to "unhealthy" air pollution, state media reported.
Tehran, where high pollution levels are common, has been engulfed in smog in recent days.
The capital is located in the southern foothills of the Alborz Mountains, which tower over the city and trap polluted air.
This phenomenon, known as thermal inversion, is most pronounced during winter, when cold air and a lack of wind can keep hazardous smog over the city for days.
On Wednesday, local media announced the air quality in Tehran had reached "unhealthy" levels, with an average air quality index (AQI) of 170.
All museums and banks, except for a few specific branches, were closed in the capital.
The landmark Milad Tower, which stands at 435 metres, was barely visible on Wednesday morning.
Pollution was also impacting other major cities, including Isfahan in central Iran and Tabriz in the northwest.
Ahvaz in the southwest has also been experiencing high pollution levels, as reported by state television.
Local media have attributed the pollution to the dilapidated state of some industrial infrastructure, an ageing vehicle fleet and poor-quality gasoline.
In recent weeks, the government has also singled out the fuel used in certain power plants as a contributing factor.
According to Iranian Health Minister Mohammad-Reza Zafarghandi, air pollution is responsible for the premature deaths of approximately 50,000 people across the country each year. In recent years, Iranian authorities have repeatedly closed schools in Tehran, including a week-long closure in 2019, due to air quality concerns.
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