The Environmental Protection Department has recorded higher than normal pollution levels and advised people to reduce physical exertion and outdoor activities.
At 4pm today, the Air Quality Health Index at some general stations had reached 8 or above, corresponding to the "Very High" health risk category.
The department said Hong Kong is being affected by an airstream with higher background pollutant concentrations, with light wind hindering effective dispersion of air pollutants.
Intense sunshine also enhances photochemical smog activity and ozone formation, resulting in high ozone concentrations in the Pearl River Delta region.
The department warned the health index at some monitoring stations may reach the "Serious" level later today.
The Hong Kong Observatory expects pollution will remain higher than normal until the wind strengthens.
Click here for index readings.
