Despite success in slashing the number of heavy pollution days in the city this year, Beijing still faces an environmental debt from its mounting traffic congestion, air pollution specialists warned yesterday.

Zhu Tong, an environment professor at Peking University, said it was too early to reach positive conclusions about Beijing's air quality because the car population is about to hit four million this week.

"Beijing's air looks to be improved from the government numbers after the Olympics," said Zhu.

"But research should also be conducted to assess how the increased congestion from the growing car populations have contributed to the car emissions and air pollution."

The more time motorists spend on pulling their vehicles through heavy congestion on the roads, the more emissions they produce, he told METRO yesterday.

The government last month made a statement that Beijing still had "more room" for the car population to grow after the environmental protection bureau removed more than 100,000 heavily-polluted vehicles from the roads, whose emissions are equivalent to two million conventional cars.

Beijing's four million cars will put out about one million tons of pollutants each year, taking up half of the city's total emissions.

Despite using a controversial air pollution monitoring system used for more than a decade, the city last month announced Beijing's air quality had reached its highest level in decades after meeting its annual target for blue sky days one month ahead of schedule for two consecutive years.

However, an air pollution monitor station set up by the US embassy in downtown Beijing has recorded the city's air quality as "unhealthy" for most of the days so far this year.

Zhu, who chaired an academic panel to monitor Beijing's air pollution during last year's Olympics, has repeatedly urged authorities to include the index for pollutants such as ozone, a key pollutant from car emissions, in its air pollution monitoring.

Zhang Jianyu, China program head of the US-based Environmental Defense Fund, said it is difficult to estimate the impact of cars on Beijing's air quality.

"It's very hard to estimate a number saying how many cars the city of Beijing can hold up in terms of air pollution," he told METRO.

"But it's high time for the Beijing government to take the U-turn."

Experts said restricting car ownership is not the answer to reducing the city's car emissions and congestion. They suggested taxing fuel or utilizing cleaner energy for cars.

Zhang said only by adding a fuel tax would the government succeed in "guiding motorists away" from the roads and changing to public transport.

"Apart from accelerating public transport efficiency, switching to zero-emission vehicles should be the ultimate goal for all drivers in Beijing," said Zhu.

"But that is just an ideal model, which still seems pretty surreal in the Beijing we now see."