Eastern China plunged into a thick haze as air pollution in the city reached extreme levels on Tuesday, forcing airlines to cancel flights and prompting authorities to warn citizens to stay indoors.

Beijing was forced to temporarily shut factories and refrain using government cars in a bid to curb the pollution as visibility dropped to less than 100 meters (100 yards) in parts of the capital, according to the official Xinhua News Agency.

Pedestrians who braved the toxic smog donned masks while several workers worried that the smog could penetrate indoors took extra precautions and wore protective headgear at their office desks, AFP reports.

Thousands of passengers were stranded after more than 100 flights were cancelled across eastern China because of the poor visibility amid China’s peak traveling season ahead of the Chinese New Year.

The US Embassy reported an hourly peak level of PM2.5 at 526 micrograms per cubic meter – more than 20 times higher than World Health Organization safety levels.

Meanwhile, the Beijing Municipal Environment Monitoring Centre recorded a 406 on the air quality index, indicating the capital’s air was “severely polluted”.

Tuesday’s “off-the-scale” smog had been the second time air quality worsened in about two weeks, shrouding a total area of 1.3 million square kilometers (520,000 square miles), the Ministry of Environmental Protection said.

Several organisations have reportedly suggested banning or regulating fireworks for next month’s huge celebration of the New Year.