From indicators to actions: design and planning protocols for built-up and green spaces to improve urban air quality

Air pollution remains a persistent challenge for cities. However, practical guidance on built-up and green spaces that can reduce pollution across multiple scales is limited. This study builds on a prior systematic review to develop actionable, multi-scalar urban planning and design protocols informed by spatial indicators. Indicators were selected based on their documented air quality impacts and spatial applicability and were aggregated by spatial function to reduce redundancy. Where findings across studies were inconsistent, for example, regarding green space density, we examined the underlying mechanisms (e.g., obstructed airflow, BVOCs emissions) and integrated context-specific indicators (e.g., species selection) to develop more nuanced protocols. All protocols were categorized by certainty level: high, medium, or low, based on the consistency of the evidence reported in the literature and spatial actionability. Protocols were applied across five urban units (facet, urban canyon, block, neighborhood, and city and urban agglomeration) and linked to four key air quality processes: emission, dispersion, deposition, and exposure. The study also identifies pollution hotspots to prioritize target areas for intervention, such as poorly ventilated areas and dense zones with high exposure risk. The resulting framework provides practical, evidence-informed guidance for urban planners and designers.