"If ambient level of these pollutants were reduced by an average of around 50 percent, it could have cut down around 3,400 hospital admissions for asthma in children during the study period," a study report concluded.

"There is a need for health policy makers to develop more stringent air quality objectives and take high risk groups into consideration such as those at the extreme of ages, with chronic respiratory or cardiovascular illnesses," it added.

Smog levels in Hong Kong have risen in the past few years to often dangerous levels. Poor air quality reduced visibility to less than one kilometer (about half a mile) on more than 50 days last year.

The government says the problem is mostly due to the industrialisation of southern China's neighbouring Pearl River Delta region, while green groups blame local coal-burning power stations and diesel-powered buses.

Growing concern over the severe increase has put the issue at the top of the political agenda, and the government has been criticised for ignoring it.

Related Links
Hong Kong University
Our Polluted World and Cleaning It Up