Hong Kong  - Hong Kong residents with health problems were advised to avoid the city centre streets Thursday as roadside pollution levels hit peak levels for the fourth day running.

The warning, issued by the Environmental Protection Department, was aimed at people with respiratory problems or heart conditions.

Roadside monitors in the city of 7 million have been registering increasing pollution levels since late Monday.

A department spokesman said the major component of the pollution was nitrogen dioxide from traffic which had become trapped in the central area by high-rise buildings and a north-easterly winds.

He added that the very high pollution levels were expected to continue in the coming days as the weather remained fine and dry.

Air quality in Hong Kong has deteriorated significantly since the early 1990s, largely because of factory pollution blowing into the city from neighbouring heavily industrialized southern China.

Hong Kong's Beijing-appointed leader Donald Tsang has pledged to tackle the issue, describing it in one speech as "a matter of life and death," but air quality in the high-rise city has continued to deteriorate.

Government statistics show the number of days in which roadside air quality in Hong Kong reached potentially dangerous levels was up almost one fifth in 2008 compared to 2007.

A university survey earlier this year found that more than one million people are considering leaving Hong Kong because of its worsening air pollution. (dpa)