Air that is cleaner inside your car than outside - in smog-weary China it is an attractive sales pitch, and the world's biggest carmakers 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 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 adding in air purifiers to lure Chinese drivers. Volvo Cars, the Swedish subsidiary of Chinese carmaker Geely, launched a publicity campaign last year showcasing a new air-cleaning system that filters out 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. Wang Jiran, a visitor to the Beijing auto show, said he now takes an interest in whether or not a car has an air purifier. "It is definitely an important index for people who are looking to buy a car," he said. "For the sake of family members' health." Nissan's Forest AC system has another selling point in China - the nation's smokers have been told the system can suck away all traces of cigarette smoke and odour in a mere five minutes. French 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 the competition, 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 per cent 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. Namrita Chow, analyst for IHS Automotive, said: "If you can highlight you've got a safer car with cleaner air … you're going to attract more buyers."