Background Outdoor air pollution can cause serious illness, death and exacerbation of health inequalities on a wide scale and thus is a serious public health concern in the UK. A comprehensive, up-to-date evidence synthesis of neighbourhood-level outdoor air pollution reduction interventions is needed to aid local authority policy and decision-making, for instance, for use in developing Local Authority Air Quality Management Strategies.

Methods We undertook a systematic review synthesising data examining the effectiveness of neighbourhood-level interventions for reducing outdoor air pollution. We searched 10 databases (MEDLINE, Embase, EconLit, CINAHL, GreenFILE, Cochrane and Web of Science), reference lists, citations of included papers, and UK grey literature. We extracted and tabulated key data from included qualitative studies and appraised study quality using the JBI scale for quasi-experimental studies or the interrupted time series checklists and the AACCODS checklist for grey literature. We synthesised the data using narrative synthesis combined with best fit framework synthesis, informed by an a priori conceptual framework to examine the impacts of interventions designed to prevent, mitigate and avoid air pollution on air pollution levels and known precursors, and health and wellbeing of residents. Extraction and synthesis is ongoing at the time of submission.

Results We examined 8912 records and 109 full texts from databases, 103 full texts from grey literature sources, four full texts from reference lists and 20 full texts from citation searches, and included 26 studies. We identified a range of interventions, implemented by a range of stakeholders. Low traffic neighbourhood and school streets interventions reduced air pollution levels and traffic counts overall, with little evidence of spillover onto surrounding streets. Anti-idling messages reduced idling traffic. Walking school bus interventions encouraged modal shift towards active travel and away from car travel. Green wall interventions reduced air pollution overall, although this might depend on wind direction. Street adaptations had mixed effects on air pollution but reduced polluting traffic. A light-touch traffic calming intervention reduced traffic levels. An early warning system had a mixed impact on air pollution. Some studies lacked comparison sites and some may have been confounded by the COVID-19 pandemic.

Conclusion Some neighbourhood-level interventions have the potential to reduce outdoor air pollution locally. The most impactful appear to be low traffic neighbourhoods, school streets, anti-idling messages and green walls. Local authorities, schools and community organisations wishing to address outdoor air pollution could choose from a range of interventions.