Many in Hong Kong have long liked to blame the city's poor air quality on noxious fumes wafting from the maze of factories across the border in mainland China. However, air quality data for the first-half of this year show that not only have Hong Kong's smog levels worsened, but the fault largely lies with local pollution sources such as the city's ageing vehicles.

Clean Air Network, a Hong Kong-based environmental non-profit organization, says the city's old, dirty vehicles are causing its residents to choke. In the crowded commercial districts of Central and Western, which include the city's financial center as well as some of its most congested streets, levels of nitrogen dioxide, a key air pollutant, jumped 22%, according to their review (in Chinese) of the city's air quality. Their analysis found that such levels of pollution exceed World Health Organization-recommended safe limits by more than 60%.

Clean Air Network Chief Executive Officer Sum Kwong says pollution levels in Tung Chung--an outlying area that she says is influenced more by the overall regional air quality in the Pearl River Delta--have actually improved across the board, dropping by as much as 13% for certain types of pollutants.

'Basically this shows that the pollution nowadays in Hong Kong is definitely roadside and definitely from local sources, and so the Hong Kong government has to do something,' said Ms. Kwong.

Earlier this month, a study in mainland China found that poor air quality cut lifespans by more than five years. Even short-term exposure to high levels of air pollution can help trigger heart failure, arrhythmias and stroke.

Old, diesel-fueled lorries spew out some of the city's most toxic air, and Hong Kong's government last year proposed a $1.3 billion scheme to help truck drivers buy new ones. But such a proposal doesn't address the city's aging taxi fleet, which has also fueled the problem. While Hong Kong recently introduced a dozen electric cabs, much of the city's iconic red taxis are in dire need of an upgrade, Ms. Kwong says.

A decade ago, Hong Kong's old diesel cabs were scrapped in favor of those using cleaner-burning LPG fuel, a move widely praised at the time. 'Today they're a lot better, more environmentally friendly,' says Wong Cheung Keung, president of the Hong Kong Taxi and Public Light Bus Association.

However, Ms. Kwong says many of the catalytic converters attached to such vehicles have been in use for years and need to be replaced, says. Otherwise, such devices--which filter pollutants from vehicle exhaust and typically last only a handful of years--will 'just have no function, ' she says.

The government agrees. Last month, it said it plans to subsidize taxi and bus owners in a one-off scheme to replace such converters starting later this year.

In the meantime, both Ms. Kwong and Mr. Wong say the government should do still more to clean up the city's air. 'The government should provide us with more incentives to support taxi drivers to change,' said Mr. Wong, adding that the government should offer subsidies to encourage taxi drivers--and not just truck drivers--to purchase newer vehicles to help keep the city's air clean.