Air that's cleaner inside your car than on the outside -- in smog-weary China it's an attractive sales pitch, and the world's biggest automakers are racing to cash in.

To drive the point home, thick smog blanketed the Chinese capital when the Beijing auto show opened to the public in the past week, showcasing more than 1,000 vehicles in the globe's largest car market.

The grey haze is a familiar sight over Chinese cities, where an explosion of traffic in recent years has further worsened the pollution belched out by booming factories and coal-fired power plants.

Carmakers have long boasted of their ability to cut their vehicles' harmful emissions -- but increasingly, they are also adding in air purifiers to lure China's army of drivers.

Volvo Cars, the Swedish subsidiary of Chinese automaker Geely, launched a publicity campaign last year showcasing a new air-cleaning system that filters out polluting particles and pollen.

And Japanese rival Nissan has offered its "Forest AC" system since 2010 in its luxury Infiniti range -- which besides filtering the air, can add a hint of leafy aroma to help keep the driver alert.

- 'Intelligent filtration' -

Wang Jiran, a visitor to the Beijing auto show, said he now takes a keen interest in whether or not a car has an in-built air purifier.

"It is definitely an important index for people who are looking to buy a car," he told AFP, "for the sake of family members' health."

Nissan's Forest AC system has another selling point in China -- the nation's legions of smokers have been told the system can suck away all traces of cigarette smoke and odour in a mere five minutes.

French auto group PSA Peugeot Citroen has also jumped on the bandwagon.

Its C-Elysee model already offers an optional air purifier and the company plans to extend the feature from 2016.

This will bring the technology to PSA's luxury DS range, as well as additional models depending on their market positioning and what the competition may offer in the future, according to Patrick Andre, chief of PSA's air filtration research team.

The company's "intelligent filtration" system only kicks into action when the vehicle enters a polluted environment.

A sophisticated filter blocks 90 percent of fine particles -- those smaller than three micrometres -- while a layer of activated carbon, a porous form of charcoal, can be added to "capture pollutive gases", said Andre.

In-car air purification may have been around for 30 years already, but the push to advance the technology has accelerated in recent years as pollution in China reaches crisis level.

- Polluted promised land -

Suppliers to the manufacturing giants are just as anxious to enter the potentially lucrative market.

French parts maker Valeo has been developing a new system in China to replace its traditional air filter, and says it has already secured a customer.

China could become a polluted promised land for parts manufacturers like Valeo.

"The air quality is a general challenge in the country, and Chinese drivers are more and more conscious of it," said Edouard de Piray, the Shanghai-based president of Valeo in China.

Smartphone apps now allow people to constantly check pollution levels, he said.

"Lots of people already have particle filters at home to protect their children," de Piray added, predicting that similar systems in people's cars would be the next step.

Especially after the now notorious images of Chinese cities hit by the "airpocalypse", analysts say the industry anticipates the demand for in-car air purification to grow.

"Chinese consumers are more much aware now of what is safe and what is not in terms of air quality," said Namrita Chow, Shanghai-based analyst for IHS Automotive.

"If you can highlight you've got a safer car with a cleaner air and better safety systems, you're going to attract more buyers."

And even if China is the main target for this new technology, purification systems could also prove a hit amongst European drivers, analysts predict.

"Pollution is also a problem in Europe -- just look at the smog that hit France in March," said Patrick Andre of Peugeot Citroen.

Mass production in China, meanwhile, could bring down the cost of developing the filters, he added.