The Environmental Protection Department warned that the Air Quality Health Index may reach the "Serious" level later today.

The department has recorded a gradual increase in pollution levels since this morning.

It said higher than normal levels of ozone and particulates have been recorded in the city, and Hong Kong is being affected by an air mass with higher background pollutant concentrations.

The high level of ozone has promoted the formation of nitrogen dioxide, particularly in parts of the urban areas and at the roadside.

The Hong Kong Observatory forecasts that there will still be light winds for a couple of days.

Pollution levels are expected to remain higher than usual until the wind strengthens next week.

Click here for the readings.