The Environmental Protection Department has advised people to reduce physical exertion and outdoor activities due to higher than normal pollution levels.
At 3pm today the Air Quality Health Index at some general and roadside stations reached 8 or above, corresponding to a "Very High" health risk.
The department said Hong Kong is being affected by the outer subsiding air of Typhoon Mawar and the weather is mainly cloudy with moderate northerly winds.
Intense sunshine enhances photochemical smog activities and ozone formation, resulting in high ozone concentrations in the Pearl River Delta region, it said.
Light winds have hindered the dispersion of air pollutants.
The department warned the health index at some monitoring stations may reach the "Serious" level later today.
Click here for index readings.
