The Environmental Protection Department recorded higher than normal pollution levels today and warns conditions may worsen.
At 1pm the Air Quality Health Index at some general and roadside stations reached 8 or above, corresponding to the "Very High" health risk.
The department said Hong Kong is being affected by a subtropical ridge.
Intense sunshine enhances photochemical smog activities and the formation of ozone, 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.
The Hong Kong Observatory said the weather in the next few days will remain very hot and there will be showers. Pollution is expected to remain higher than normal.
Children, the elderly and people with heart or respiratory illnesses are advised to reduce physical exertion and outdoor activities.
Click here for index readings.
