BEIJING -?Vehicles in Beijing aren't just a means of transportation, they're also a difficult part of the "race" against pollution for Yu Jianhua.

Yu is the official in charge of air pollution management with the Beijing Municipal Environmental Protection Bureau.

"Our emissions reduction efforts have outperformed in the race over recent years, but the competition is getting increasingly challenging for us," Yu said.

Heavy smog has shrouded the capital many times since January, raising health concerns among residents. But the threat is one that doesn't seem to be going away any time soon.

According to official statistics, in 2012, the number of long-term residents in Beijing increased by 507,000 year on year, hitting 20.69 million, and energy consumption was up 84 percent from the previous year.

These additional pressures are equivalent to adding a small or medium-sized city to Beijing, according to government estimates.

Increased population and energy consumption aside, vehicles in the city are a major contributor to Beijing's pollution -- and one that keeps growing.

A report issued by a research team under the Chinese Academy of Sciences in February revealed that vehicle exhaust is among Beijing's top sources of air pollution, contributing to up to one-quarter of the PM2.5, airborne particles measuring less than 2.5 micrometers in diameter, in the city.

By the end of last year, about 5.2 million cars were registered in Beijing, and the number is expected to top 5.3 million this year, adding more pressure to the city's environmental protection efforts, Yu said.

Despite the municipal government's policy that caps the amount of new cars registered in the city at 240,000 annually, the total number of vehicles is still growing, Yu said, calling on citizens to reduce the use of private cars and opt for public transportation, instead.

Though it costs as little as 0.4 yuan (0.65 U.S. dollar) to ride a bus or 2 yuan to hop on the subway, many residents only use public transportation reluctantly.

"I have to meet at least three clients every day, travelling across the city, and using public transport is still much more time-consuming," said Henry Ma, the manager of a venture capital company in Beijing. "Beijing is too large."

Moreover, private cars remain an important status symbol in China.

"If I take a bus to see my client, he may doubt my competence in doing business," Ma said.

To reduce people's reliance on private cars, Hao Jiming, a member of the Chinese Academy of Engineering and a professor with Tsinghua University, has suggested creating more favorable conditions for public transport, such as more dedicated bus lanes, and non-motor vehicles.

Moreover, air pollution control efforts should be integrated into city planning, economic restructuring and ecological progress campaigns, said Niu Wenyuan, an expert on sustainable development as well as a consultant for the State Council, China's cabinet.

He suggested moving some administrative agencies, prestigious universities, hospitals and big companies to some suburban areas of Beijing, or implementing a trend of "reverse urbanization" to reduce pressures on the city's already strained downtown resources and space.

In April, the Ministry of Environmental Protection unveiled the theme for World Environment Day, which falls on June 5 this year, as "Breathing and Working Together," calling on every citizen to do his or her part in improving air quality.