TEHRAN, Iran (AP) — All schools in the Iranian capital of Tehran will close for a day because of dangerously poor air quality, the country's state television reported on Tuesday.

Primary schools have been closed for the past two days in Tehran, which has over 10 million residents, but now, increasing pollution is forcing the closure of all schools on Wednesday, the TV said. Classes will continue online.

Similar measures were announced for Tehran's neighboring Alborz province. the school week in Iran is from Saturday to Wednesday.

Also Tuesday, authorities urged the elderly, those who are sick and children to take precautions.

Tehran’s air is among the most polluted in the world and school closures because of poor air quality are a regular occurrence. The smog is mostly caused by heavy traffic as well as factory pollution and typically worsens during the cold season.

Inversions commonly hover over Tehran in the winter. The city is surrounded by tall hills and mountains on three sides. As cold, stagnant air settles in the valley, it traps car and other emissions that have no way of escaping.

In July, schools and government offices closed in Tehran and several other cities in the country after a sandstorm blanketed the Iranian capital and the surrounding region.