This case study is part of the SEI Urban Toolbox for Liveable Cities which has been developed by the SEI Initiative on City Health and Wellbeing. The Urban Toolbox is a collection of tools, developed within SEI or in coordination with SEI, aimed at supporting planning and decision-making for improving the health, well-being and resilience of city residents and urban systems more broadly. It demonstrates how a co-created citizen science approach can be used to assess air quality in an urban area.