Our health depends on many factors, some of which are unique to each individual (behavior, genetic factors) and others that are more widely shared within a given population. Thus, addressing environmental risk factors such as air pollution, noise, or heat, and fostering protective and health-promoting factors such as physical activity or nature in the city can help protect and improve the health of the population. These actions also contribute to adapting to climate change, reducing greenhouse gas emissions, and protecting biodiversity, within a systemic vision of health. Quantitative Health Impact Assessment (QHIA) is a formalized method for illustrating the influence of a health determinant on health. QHIA is based on the state-of-the-art scientific knowledge, as well as on local data and scenarios regarding the evolution of the determinants under study. As part of a pilot study, Santé publique France conducted QHIA studies focusing on improving health-promoting determinants (urban green spaces, active mobility) or reducing health-damaging determinants (air pollution, noise, heat) in three French metropolitan areas. This report highlights the main findings for the Rouen Normandy Metropolitan Area (MRN). The study covered nearly 490,000 residents. During the study period (2015–2017), an average of more than 4,300 deaths were recorded each year in the metropolitan area. The results of these EQIS studies highlight that taking action on green spaces, mobility, air pollution, or noise could prevent several hundred deaths per year in each case (compared to “current” conditions). This represents major health benefits that would be observed across the entire region, and for some, these benefits would be greater in socially disadvantaged areas. For example, if all the blocks grouped together for statistical purposes had the same level of vegetation as the most vegetated Iris blocks of the same density, nearly 300 deaths would be prevented each year, representing 6.8% of the metropolitan area’s mortality rate; if every resident aged 30 and older in the MRN walked an average of 10 additional minutes per weekday, it is estimated that nearly 150 deaths could be prevented each year, representing 3.4% of mortality. An additional 10 minutes of cycling per weekday would prevent 250 deaths each year, or 5.7% of total mortality; if fine particulate matter (PM2.5) levels met the World Health Organization (WHO) recommended value everywhere in 2021 (5 µg/m³ as an annual average), 310 deaths, or 7.1% of total mortality, would be prevented. This would also prevent 44 new strokes (6.2% of all strokes) and 2 new lung cancers (4.4% of all lung cancers) each year. If nitrogen dioxide levels everywhere met the WHO’s recommended limit (10 µg/m³ annual average), 65 deaths—or 1.5% of total mortality—would be prevented. This would also prevent more than 180 new cases of asthma among children aged 0 to 17, or 7.6% of new asthma cases; if all municipalities complied with WHO recommendations for nighttime road noise (45 dB Lnight), this would result in 1,300 residents of the MRN experiencing fewer significant sleep disturbances; on average each year, heat during very hot days (average temperature ≥18.3 °C) was responsible for nearly 50 premature deaths in the MRN. By translating environmental or behavioral information into health impacts, EQIS are a valuable decision-making tool that can help ensure health is more systematically integrated into public urban planning policies. The remaining methodological limitations and associated uncertainties do not call into question the significance of the expected benefits of transforming environments to make them more health-promoting.