The province, home to more than half of Pakistan's 240 million people, last week closed all schools until November 17 in major cities enveloped by smog.

Air quality in Lahore was deemed "hazardous", according to data by 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.

"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.

Breathing the toxic air has catastrophic health consequences, with the WHO saying strokes, heart disease, lung cancer and respiratory diseases can be triggered by prolonged exposure.

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

- 'Detox Lahore' -

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".

Last month, authorities banned schoolchildren from outdoor exercise until January and adjusted school hours to prevent children from travelling when the pollution is most punishing.

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

Tuk-tuks equipped with polluting two-stroke engines have been banned, as are restaurants that barbecue without filters, while building construction has been halted.

Aurangzeb also urged residents on Friday to "stop driving motorbikes and cars and to wear masks".

"Citizens play a role in mitigating smog, you need to help us to detox Lahore," she said.

A mix of low-grade fuel emissions from factories and vehicles, exacerbated by seasonal crop burn-off by farmers, blanket the city each winter, trapped by cooler temperatures and slow-moving winds.

Government offices and private companies had half their staff work from home this week.