When planning new buildings, it is necessary to choose their geometric properties such as heights, lengths and distances between them. These choices, usually made by architects and planners, are often made on the basis of available space and the future use of the buildings. In densely populated cities, the possibilities are limited, and it generally results in high buildings with small distances between them also called street canyons. However, this type of buildings implantations often results in poor ventilation conducting to a stagnation of atmospheric pollutants and high concentrations. To better understand how evolve atmospheric pollutant concentrations in street canyons as a function of buildings geometric properties, RANS-CFD (Reynolds-Averaged Navier-Stokes Computational Fluid Dynamics) simulations were conducted. The study is limited to step-down street canyons where the ratio of upwind building height (H1) over downwind building height (H2) is between 1.0 and 2.0. The ratios of the distance between buildings (W) over downwind building height used in this work are between 0.6 and 1.4. For these ratios, three different flow regimes could appear. The pollutant concentrations in the canyons are highly relying on the regime established and, for a particular regime and a given distance between buildings, the mean concentrations do not change at all. With these new information, it is possible to better understand how the geometric properties of buildings affect atmospheric pollutant concentrations in street. Moreover, architects and planners have now keys in order to achieve better sustainable constructions in terms of atmospheric pollution by choosing the best compromises between H1/H2 and W/H2 to avoid too high concentrations in streets.