Background: The Air Quality Act requires regional air quality plans. These plans must include an assessment of the health effects of air pollution in order to propose air quality objectives. The objective of this study is to assess the health impact of short-term exposure to air pollution in the urban areas of Caen, Nantes, and Rennes. Materials and Methods: A health impact assessment (HIA) approach was implemented. The health impact is calculated based on the proportion of attributable cases for deaths and hospital admissions. Population exposure is estimated using sensors from air quality monitoring networks. Relative risk is modeled using exposure-risk relationships established in temporal epidemiological studies conducted in the general population. Results: In these three metropolitan areas, current exposures are associated annually with 77 (95% CI: 37–126) premature deaths and 108 (95% CI: 42–184) hospitalizations for a population of 650,000 inhabitants. A 25% reduction in pollution levels would have been accompanied, over the study period, by a 40% reduction in premature mortality attributable to pollution. Regarding the O3 indicator, it is shown that levels exceeding 120 μg/m³ account for only 5 to 11% of the impact on premature mortality, depending on the metropolitan area. The health impact is more closely linked to chronic pollution levels than to pollution “peaks.” Discussion: These results demonstrate the significant collective impact of urban air pollution, even at moderate pollution levels. The pollution reduction simulations conducted demonstrate the limited effectiveness of a risk reduction strategy based solely on controlling pollution spikes. Recent advances in pollutant monitoring and understanding of their effects should now prompt an assessment of the long-term health impact of urban pollution associated with prolonged exposure.