The first annual data shows Foshan, Zhuhai , Zhongshan and Guangzhou are cities where pollutant levels often exceed national air quality standards.

Of the four cities, air pollution in Foshan was the worst recorded last year. Its level of respirable suspended particulates failed to meet the national standard for 99 days, whereas sulfur dioxide levels exceeded the national standard for 62 days.

The two governments set up the Regional Air Quality Monitoring Network in 2005 to record air pollution trends for the region. Pollutants recorded include sulfur dioxide, nitrogen dioxide, ozone and respirable suspended particles.

Of the network's 16 air monitoring stations set up in the delta, three are in Hong Kong at Tsuen Wan, Tap Mun and Tung Chung.

The stations in Tsuen Wan and Tung Chung also recorded excessive levels of pollutants for up to 16 days last year.

An Environmental Protection Department spokesman said the report showed cities along the coastal area, such as Jiangmen , enjoyed better air quality. The spokesman said overall concentrations of most pollutants in the region were generally higher from January to March and from October to December, due to the lack of rainfall and southern monsoon to disperse them.

The spokesman refused to comment on the sources of pollution and on which city was the most polluted, saying pollutants could be spread from other cities because of climatic conditions.

He said long-term monitoring was required before a general pattern could be established.

Alexis Lau Kai-hon, an atmospheric scientist at the University of Science and Technology, said the annual data had given a clear picture on the distribution of pollution sources, but agreed that more data would be needed to gauge the changes.

'It gives us the snapshot on the spatial distribution over the past year and I think it is quite an accurate account,' he said, adding that the findings were similar to the data they received from monitoring via satellite.

But Professor Lau said he would like to see the monitoring network expanded to include more cities in Guangdong, while data about pollutant concentrations, instead of just weighted indexes, should be released.

Professor Lau said Foshan might be affected by its ceramics industry but believed the local authorities would 'do something about it' after disclosure of the report.

Last year, Foshan closed or moved 13 of its ceramics producers as well as shutting down two small coal-fired power plants with a total capacity of 208,000 kilowatts, an official with the Guangdong Environmental Protection Bureau said.