Days of heavy smog shrouding swathes of northern China pushed pollution to more than 20 times recommended limits 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 recommended daily maximum average 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.

Levels were beginning to drop on Friday afternoon but 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.

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.

"The Beijing government must do what it can," the government-run China Daily newspaper said in an editorial. Otherwise, it said, "the mayor's promises will turn out to be empty words".

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 capital hosts a Brazil-Argentina football friendly and cycling's Tour of Beijing.

Brazilian defender David Luiz, of Paris Saint-Germain, told reporters from his home country: "We hope that (the pollution) can change, because the Chinese people don't deserve to live like this."

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 and give public sector employees a six-day holiday, while requesting neighbouring areas to shut down polluting facilities.

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."