The Iranian capital Tehran is grappling with severe pollution as a pall of thick black smoke descended on many parts of the city following a clear weekend, according to local reports.
Large parts of mountains in the northern region of Tehran have been blanketed out by the smoke.
In November, schools were shut down due to severe pollution in Tehran. However, it is not clear whether such measures have been taken this time.
The scale of the pollution is also not known immediately as official statistics are not available.
Many factors, ranging from substandard parts manufactured by auto companies to low-quality fuel, are being attributed to the air pollution in Tehran.
Apart from the Iranian capital, other major cities have also been affected by the problem. Four major Iranian cities feature in the world's 10 most polluted cities.
The state-run Mehr News reports that excessive use of private vehicles and Iranians' increasing indifference towards environmental issues have contributed to the growing atmospheric pollution.
Successive governments have also not addressed the issue sufficiently, said the report.
Alternate-day driving restrictions, based on the number plates of private vehicles, have already been in place in Tehran, which has a population of about eight million.
According to statistics released by Rahmatollah Hafezi, the chief of the health committee of the Tehran City Council, about 227 Iranians die every month due to air pollution. He added that poorly-maintained private vehicles are responsible for nearly 70 to 80% of the pollution.
In 2012, some 80,000 people across Iran, at least 4,500 in Tehran alone, are estimated to have died because of pollution, said Iran's health ministry.