Outdoor air purification technologies for urban particulate matter and gaseous pollutant mitigation: A comprehensive review of efficiency, sustainability, and health impacts.

Urban air pollution contributes to approximately 8.5 million premature deaths annually. Outdoor air purification technologies have emerged as near-term complements to source emission controls. However, there is still no comprehensive cross-domain synthesis of evidence on their efficiency, sustainability, and health impacts. A structured search of Scopus, Embase, and PubMed databases was conducted in this study. A total of 266 records were retrieved. After PRISMA-ScR screening, 28 studies that met the full inclusion criteria were included in this comprehensive review. Five technology domains were identified, with the following being the best examples of specific outdoor air purification technologies. Smog towers achieved PM2.5 and PM10 removal efficiencies of 97.8% and 98.4%, respectively, under optimal conditions. Solar-chimney hybrids delivered air purification within 2 to 4% of dedicated systems whilst simultaneously generating electricity. TiO2-modified cementitious composites demonstrated a possibly carbon-negative lifecycle through simultaneous NOx and ozone removal. A roadside hedge applied as an indirect proxy reduced leaf-surface PM2.5 magnetic loading by 82% in a near-school environment. Health-economic modelling of indoor air cleaners targeting outdoor-origin PM2.5, used as the closest available analogue for outdoor systems, estimated 93,200 deaths averted in China at a 35 μg/m3 indoor PM2.5 target, with positive net monetary benefit in 93% of evaluated cities. Hence, the integration of active and passive outdoor purification technologies within urban air quality management frameworks can reduce population-level exposure. Priority research needs to include standardised performance metrics, long-term field validation, and equity analysis of technology deployment.