A woman waits for a bus in the haze in Beijing in December 2011.

The Chinese capital currently bases its air quality information on particles of 10 micrometres or larger, known as PM10, and does not take into account the smaller particulates that experts say are most harmful to human health.

But authorities have come under pressure from the public to change the way they rank air quality after thick smog blanketed Beijing late last year, forcing the cancellation of hundreds of flights and triggering a surge in face mask sales.

Official data showed the air was only slightly polluted, while figures published by the US embassy in Beijing, which measures the smaller particles, ranked it as "very unhealthy".

On Friday the Beijing Environmental Bureau said it would start publicising measures based on the smaller particles -- known as PM2.5 -- ahead of the Lunar New Year, or Spring Festival, which starts on January 23.

"The authorities plan to release air-quality monitoring data using PM2.5 before Spring Festival," a bureau official surnamed Jiang told AFP.

"The government has to consider the pleas of the people, so yes, the anger of Beijing citizens these days is a big contributor to our action."

The new measurements were ordered by Beijing's top communist official Liu Qi in a speech published Friday, Jiang said.

The data, previously only available for laboratory use and not disclosed to the public, will be released every hour via the bureau's website and other media, according to the state Xinhua news agency.

International organisations including the United Nations list Beijing as one of the most polluted cities in the world, mainly due to its growing energy consumption, much of which is still fuelled by coal.

Authorities in Beijing went to huge lengths to clean up the city's air ahead of the 2008 Olympics, shutting down coal-fired power stations and restricting the number of cars on the roads, but air quality in the city remains poor.