The impact of air pollution on public health is substantial, especially in high-density megacities. Frequent reports of high air pollution concentrations at roadside stations in these cities (e.g., Hong Kong) are the result of both higher anthropogenic pollution emissions in densely populated street canyons and stagnated airflow due to closely packed tall buildings, which result in lower dispersion potential and leads to increased population exposure to air pollutants. Thus, it is critical to design appropriate high-density urban morphologies to lessen the negative impacts of high-density urban living. This chapter addresses the knowledge gap between planning and design principles, and air pollution dispersion potentials in high-density cities, so that air ventilation assessment can appropriately take into account the air pollutant dispersion issues. CFD simulation and parametric study are conducted, and SST κ–ω model is adopted upon balancing the accuracy and computational cost through comparative study. Neighborhood-scale parametric studies are conducted to clarify the effects of urban permeability and building geometries on air pollution dispersion, for both outdoor pedestrian environment and indoor environment of the roadside buildings. Given the limited land resources in high-density cities and the numerous planning and design restrictions for developmental projects, the effectiveness of mitigation strategies is evaluated to optimize the benefits. At the end of the chapter, an actual urban case study is presented to demonstrate how the suggested design principles from the parametric study can be applicable in high-density urban design.