Authorities on Monday urged members of the public - especially children, the elderly and those with underlying illnesses - to refrain from outdoor activities, saying the local air pollution level may reach the "serious" category later in the day.

“Under the influence of a dry continental airstream, the weather in Hong Kong today is fine and very hot, with light to moderate northerly winds, which is unfavourable for pollutant dispersion and results in rising regional pollution level,” the Environmental Protection Department said in a statement.

“The intense sunshine enhances photochemical smog activity leading to rapid formation of ozone and fine particulates in the Pearl River Delta region. The high ozone level also promotes the formation of nitrogen dioxide.”

The department warned that the high pollution levels are expected to persist for a couple more days, and urged people to stay vigilant.

“With a health risk category in the ‘Very High’ or ‘Serious’ levels, children, the elderly and persons with existing heart or respiratory illnesses are advised to reduce physical exertion and outdoor activities to a minimum or avoid such activities,” it said.

Officials also advised outdoor workers to take appropriate preventive measures.