Fa?ade Greening for Healthy Urban Air: An Umbrella Review on Particulate Matter Reduction, Challenges, and Future Directions

Particulate matter (PM) exposure in urban areas is a continuous threat to human health and requires sustainable solutions for effective reduction. The aim of this study is to provide a comprehensive overview of facade greening and its effects on PM and to identify knowledge gaps. We systematically screened, analyzed, and re-evaluated previous reviews according to the criteria established for an umbrella review. As a result, this study covers and discusses common measurement methods, influencing factors, the effectiveness of facade greening, leaf characteristics, and plant species from different subtopics. The main findings are that inconsistent measurement methods limit the comparability of the analyzed reviews, and different climatic conditions affecting the use of plant species lead to different international effects. Despite the existing studies reporting a positive effect on the reduction of PM, comparability is difficult due to methodological differences and external influences. Given the complexity, interdisciplinary collaboration with experts is essential to increase the effectiveness of facade greening for PM accumulation for healthy urban air. We suggest that this umbrella review provides a foundation for future research and practical implementations, such as a suitable plant selection or facade design, by integrating the results.