Record-high air pollution in Pakistan’s cultural capital of Lahore is sending more people to hospitals and private clinics and the city authorities warned a complete lockdown could be imminent if residents fail to don face masks and follow other guidance related to smog.

Pakistan's second city of Lahore will close primary schools for a week over record pollution, government authorities said Sunday, to avoid exposing millions of children to smog several times above levels deemed dangerous.

Toxic smog has shrouded Pakistan's eastern city of Lahore for days as surging pollution spurs authorities to curb activities and set up a panel to oversee preventive efforts. Here is a look at what causes the hazardous air,