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.

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.