Days of heavy smog shrouding swathes of northern China pushed pollution to more than 20 times safe levels on Friday, despite government promises to tackle environmental blight.
Visibility dropped dramatically as measures of small pollutant particles known as PM2.5, which can embed themselves deep in the lungs, reached more than 500 micrograms per cubic metre in parts of Hebei, a province bordering Beijing.
The World Health Organization's guideline for maximum healthy exposure is 25.
In the capital buildings were obscured by a thick haze, with PM2.5 levels in the city staying above 300 micrograms per cubic metre since Wednesday afternoon and authorities issuing an "orange" alert.
"It's very worrying, the main worry is my health," said a 28-year-old marketing worker surnamed Hu, carrying an anti-smog mask decorated with a pink pig's nose as she walked in central Beijing.
China has for years been hit by heavy air pollution, caused by enormous use of coal to generate electricity to power a booming economy, and more vehicles on the roads.
But public discontent about the environment has grown, leading the government to declare a "war on pollution" and vow to cut coal use in some areas.
Nonetheless poor air quality has persisted with officials continuing to focus on economic growth, and lax enforcement of environmental regulations remains rife.
In a sign of growing environmental activism, Greenpeace East Asia projected the message "Blue Sky Now!" onto a facade of the Drum Tower, a historic building north of the Forbidden City.
The pollution -- which also hit areas hundreds of kilometres from Beijing -- comes as the city hosts a high-profile cycling tournament, the Tour of Beijing, and a Brazil-Argentina football friendly.
Global heads of state from the US, Russia and Asia are set to gather in the capital for a key summit next month.
City authorities said Thursday that they would place tighter restrictions on vehicle use during the APEC Economic Leaders' Meeting in November, while requesting neighbouring areas to shut down polluting facilities.
But most locals were not wearing protection Friday, and several people said they believed Beijing was being hit by natural haze, rather than pollution.
Even so, sitting in a Beijing park 82-year-old Liu Shuying said: "There are too many cars. I don't wear a mask because I'm not afraid of death."