The government of Pakistan’s most polluted province has launched a new policy to address a protracted smog crisis, after the world’s worst air shuttered schools and saw 900 people seek hospital treatment in a single day.

The U.N. children’s agency is warning that the health of 11 million children in Pakistan’s eastern Punjab province is in danger because of air pollution that experts say has become a fifth season in recent years.

Record-breaking levels of thick, toxic smog that have shrouded eastern Pakistan and northern India since last month can be seen in striking satellite imagery.

Pakistan's province of Punjab set up a "smog war room" to tackle severe pollution, officials said, as poor air quality turned its capital of Lahore into the world's most polluted city and forced the closure of some educational institutions.

The United Nations Children's Fund (UNICEF) has raised concerns about the health risks facing 11 million children in Pakistan's eastern Punjab province due to worsening air pollution, which experts say has effectively become a "fifth season" in recent years.

Punjab authorities have banned most outdoor activities and told some shops to shut early as smog-induced illnesses soar. Schools have also been closed to protect children, who are at high risk from the pollution.