Many greenhouse gas mitigation actions also benefit air quality and health. However, progress incorporating co-benefits assessments into climate mitigation planning has been limited. Over the next several years, C40 Cities is working with city governments to develop climate action plans. We aim to develop methods to integrate PM2.5 and associated health co-benefits into the climate action planning tool these cities will use, thereby building a bridge between the scientific evidence on co-benefits to the largest urban climate action planning effort worldwide. Specifically, we will: 1) Develop, evaluate, and integrate a screening-level air quality model (focusing on fine particulate matter, PM2.5) into C40’s climate action planning tool, Pathways, for at least three pilot cities; 2) With local partners, test the tool to explore air quality and health co-benefits of climate action pathways in the pilot cities; and 3) Assess the potential for quantifying additional health co-benefits in Pathways, such as changes in ozone, nitrogen dioxide, physical activity, noise, and green space. Data and tools will be publicly available to support additional research into climate/health linkages. C40 will maintain Pathways beyond the project’s end, creating a platform to study more cities and enabling long-term integration of co-benefits into city climate action planning. Many actions cities can take to reduce greenhouse gas emissions would also improve air quality and therefore also human health. This project develops a decision-support tool that helps cities explicitly recognize the nexus between climate action and air quality and public health benefits. We will add a new screening-level air quality and health assessment capability to an existing greenhouse gas planning tool that is maintained by C40 Cities and used in cities worldwide. We will then work with local officials in three pilot cities to test the tool to explore the air quality and health implications of specific climate action pathways that these cities can take. We will also assess the potential for including additional health co-benefits such as from increased physical activity and green space.