Schoolchildren in Pakistan’s second-largest city of Lahore have been banned from outdoor exercise until January because of hazardous smog levels, officials said Friday.

The eastern megacity near the border with India regularly registers among the world’s most polluted cities, this week recording more than 20 times the level deemed safe by the World Health Organization (WHO).

Smog is particularly bad in winter as a result of low-grade fuel from factories and vehicles in the low-lying megacity of 14 million, where denser cold air traps emissions at ground level.

Seasonal crop burn-off by farmers on the outskirts of Lahore is also a major contributing factor.