The Air Pollution Control Ordinance (Cap. 311) sets out Air Quality Objectives (AQOs) and provides for the periodic review of the AQOs at least once every five years to promote the conservation and best use of air in the public interest. Listed below are the standards of Hong Kong air quality standards as compared to the 2021 revised WHO air quality guidelines which offer global guidance on threshold and limits for key air pollutants that pose a major health risk. We see from Fig 11 that the Hong Kong concentration limit (highlighted in red) is comparatively higher than the standards set by WHO.