LAHORE: Dangerously poor air quality has forced Pakistan's government to close all schools in Lahore, the capital of eastern Punjab province and home to 11 million people.

Thick smog is hanging over the city, caused in part by widespread crop burning in the surrounding province. It's a popular practice among poor farmers, who set fire to remnants of the previous season's crop to prepare the land for their next planting. Punjab is considered Pakistan's breadbasket.

Heavy traffic, open fires for cooking and warmth as well as years of cutting down trees have all contributed to the city's poor air quality.

Lahore's air quality index was 114 on Thursday morning, which is considered hazardous, particularly for sensitive groups like children.