This paper presents a numerical investigation of air pollutant dispersion within urban environment, containing street canyons of different aspect ratios and buildings with a wide variety of different geometries. The numerical methodology is based on the finite-volume numerical solution of the Navier-Stokes equations, using a CFD commercial code (FLUENT), which is validated with a series of available experimental data from the recent literature, with quite good agreement. However, the simulation seems to overestimate the air pollutant concentration, especially close to the ground, for the cases with very narrow street canyons (B/H = 1/2). Furthermore, the findings of this validation process are applied into a real complex three-dimensional geometry; the Campus of the Technological Education Institute (T.E.I.) of Piraeus, Greece. The behaviour of pollutants, emitting from the roads within the campus, under certain wind values and directions, is investigated, reaching significant conclusions about air pollution dispersion and recognising the most vulnerable areas within the TEI campus, which are usually observed in the areas of the roads, which are very close to high buildings and the wind has low velocity.