The Environmental Protection Department today warned that the Air Quality Health Index will remain higher than normal this week.

The announcement was made after the index reached the "Serious" level at the Tuen Mun Air Quality Monitoring station at 4pm.

Hong Kong is being affected by an air mass with higher background pollutant concentrations, the department said, adding sunshine is enhancing the photochemical smog activity and the rapid formation of ozone and fine particulates in the Pearl River Delta region.

The Hong Kong Observatory forecasts that there will be an easterly airstream next week.

Pollution levels will remain higher than normal until the arrival of the airstream and stronger winds.

Click here for the readings.