The Environmental Protection Department recorded higher than normal pollution levels today and warns it may get worse.
At noon 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 Severe Tropical Storm Hato.
Intense sunshine enhances photochemical smog activities and the formation of ozone, resulting in high ozone concentrations in the Pearl River Delta region, it said.
It added 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.
It forecasts pollution levels to remain higher than normal until Hato reaches the south China coast.
