The Environmental Protection Department has advised people to reduce physical exertion and outdoor activities due to high pollution levels.
At 3pm today the Air Quality Health Index at some general air monitoring stations reached "10", corresponding to a "very high" health risk.
The department said Hong Kong is under the influence of an airstream with higher background pollutant concentrations from the Pearl River Delta.
Sunshine enhances photochemical smog activity and ozone formation, resulting in nitrogen dioxide accumulation, particularly in urban areas and at the roadside.
It said the index at some monitoring stations may reach the "serious" level later today.
The Hong Kong Observatory said a fresh easterly airstream affecting the Guangdong coast in the next few days will reduce the pollution.
Children, the elderly and people with heart or respiratory illnesses are advised to stay indoors.
Click here for the index readings.
