Hong Kong's concentration of the pollutant - a byproduct of burning fossil fuel - rose from 127.6 micrograms per cubic metre in 1996 to 166.5 in 2005. In Guangzhou, the level fell from 151 to 68.

Authors of the paper - 'Evaluation of Ambient Air Quality in Guangzhou, China' - include Zhou Kai from Guangzhou's South China Botanical Garden and Ye Youhua of the Graduate School of the Chinese Academy of Sciences in Beijing.

Researchers found the concentration of sulfur dioxide in Hong Kong rose slightly in the 10 years, but Guangzhou cut the pollutant by 18 per cent over the period.

Hong Kong's total suspended particulate level also rose over the 10 years, but Guangzhou cut its by 32 per cent from 1996 to 2003. (Figures for the pollutant in Guangzhou in 2004 and 2005 were not given.)

Hong Kong outdid Guangzhou only in limiting the growth of respirable suspended particulate levels between, according to the paper.

Alexis Lau Kai-hon, an air pollution expert at the University of Science and Technology, said the high level of nitrogen oxides showed the effort to reduce vehicle emissions had not been enough.

'We are seeing some reduction of nitrogen oxides from 1999, at a very slow pace. We have to reduce emissions by cutting the number of cars,' he said.

The figures emerged as the high pollution levels of the past few days were blamed for sending nearly 40 elderly people to hospital with breathing difficulties.

Yesterday alone, 1,170 elderly people pressed their alarms to call for help from the Senior Citizen Home Safety Association as pollution levels remained high or very high for the fourth consecutive day.

Association chief executive Timothy Mak Kam-wah said: 'The figures have significantly gone up in the past few days as air quality deteriorated.'

By last night at least 13 elderly people had been taken to hospital with respiratory problems after raising the alarm. There were seven on Sunday and 16 on Saturday.

The pollution, trapped by light winds and hot sun, is expected to remain until wind speed picks up later in the week.

The bad air yesterday also caught the attention of California Lieutenant Governor John Garamendi, on a visit to Hong Kong to give a talk on climate change.

'Is that a San Francisco fog?' he asked while addressing a lunch. He said air quality, like climate change, needed an international effort.

Director of Environmental Protection Anissa Wong Sean-yee said the government would soon start consultation on a proposal for a law to ban idling engines and pledged to encourage more vehicle owners to switch to cleaner vehicle models.