Air quality in Rennes (France) is treated as both a public health and climate issue, and it is considered below the objectives set by European regulations, with regular episodes of fine-particle pollution influenced by the city’s geography, climate, and urban activities. As the largest agglomeration in Brittany (around 450,000–474,000 inhabitants across 43 municipalities, with high density inside the ring road), Rennes has established reinforced air-quality monitoring, including four fixed measurement stations in different environments and one indicative site near traffic. These stations track key pollutants such as nitrogen oxides and fine particles (PM10/PM2.5), linked to road traffic, wood heating, and industrial activity, and the monitoring has expanded to include pesticides, systematically tracked since early 2022 for a five-year period. Alongside institutional monitoring, Rennes has mobilised residents through the Ambassad’Air citizen-measurement initiative (in partnership with the City of Rennes, the House of Consumption and Environment, and Air Breizh), deploying open-source micro-sensors since 2018. Policy-wise, Rennes has developed successive Atmosphere Protection Plans (2005, 2015, and a third initiated for 2022) and has tested a limited traffic zone in the historic city centre from February 2023, aiming to reduce pollution while improving urban liveability.