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 air monitoring stations reached 8 or above, corresponding to a "very high" health risk.
The department said Hong Kong is being affected by an airstream with higher background pollutant concentrations. The sunshine enhances photochemical smog activities and ozone formation during daytime, and the high level of ozone has promoted the formation of nitrogen dioxide, particularly in urban areas and at the roadside.
It is expected that the AQHI at some general and roadside air monitoring stations may reach the "Serious" level later today.
The Hong Kong Observatory said the weather in the next few days will be slightly cooler. Pollution is expected to remain higher than normal until then.
Children, the elderly and people with heart or respiratory illnesses are advised to reduce physical exertion and outdoor activities.
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