"The schools will remain closed for another week due to the hazardous air quality. Institutes including universities would switch to online classes to ensure the safety of students," Marriyum Aurangzeb, Punjab's environment minister, told a news conference.

Air quality in Lahore was deemed "hazardous", according to data by from IQAir, with the concentration of deadly PM2.5 pollutants -- fine particulate matter in the air that causes most damage to health -- around 30 times the level deemed acceptable by the World Health Organisation.

"More than 35,000 patients have been reported in the hospitals of Lahore due to smog related health issues in almost a month," Aurangzeb added.

She also urged residents to "stop roaming around on bikes and in cars and to wear masks".

The province, home to more than half of Pakistan's 240 million people, had previously closed all schools until November 17.

A mix of low-grade fuel emissions from factories and vehicles, exacerbated by agricultural burning, blanket the city each winter, trapped by cooler temperatures and slow-moving winds.

The UN's food agency says transport is the main cause of air pollution in Pakistan, followed by industry and agriculture.

Children are often hardest hit, with UNICEF noting that "prior to these record-breaking levels of air pollution, about 12 percent of deaths in children under five in Pakistan were due to air pollution".