On Friday, China's environmental watchdog issued a five-day warning about choking smog spreading across the north and ordered factories to shut, recommended residents stay indoors and curbed traffic and construction work.

Red alerts are issued when the air quality index (AQI), a measure of pollutants in the air, is forecast to break 200 for more than four days in succession, surpass 300 for more than two days or overshoot 500 for at least 24 hours.

The Beijing Municipal Environmental Monitoring Centre showed an air quality reading of 297 by Saturday afternoon as haze started to envelop the capital, after an earlier reading of around 120. Levels in the 301-500 band are considered hazardous to health.