•A state of art review of photocatalysis for air purification.•Provides an in-depth analysis of photocatalyst development.•Discusses various aspects of photoreactor modelling.•Presents an overview of possible intensification pathways. Photocatalysis has been extensively investigated for several decades, motivated by the fascinating applications in pollution remediation, chemical synthesis, and energy innovation. However, the practical/commercial/industrial applications of photocatalysis have been restricted in the field of building materials. The low quantum efficiency in solar energy conversion and limitation of low level of pollutants in photodegradation are very difficult to solve. Air purification by photocatalytic oxidation (PCO) of various pollutants, for example volatile organic compounds (VOCs) or inorganic gaseous (NOx, SOx, CO, H2S and ozone, etc) at reasonably low concentrations, appears to be more feasible for commercialization. This review firstly introduces the removal mechanism of these contaminations by PCO, and then provides detailed survey and discussion on both photocatalysts and reactor design. This paper aims to deliver fundamental and comprehensive information for paving the venue of gas-phase photodegradation to commercialized air purification.