In developing countries, where few air quality stations and studies exist, measuring the spatial gradient of nitrogen dioxide (NO₂) in urban areas is challenging. Our research explores the linking the of built environment with air quality by developing a model that allows relating NO₂ with transport, land use, socioeconomics, and built environment characteristics. For the model estimation, we installed and quantified 114 diffusion tubes from Gradko© in Barranquilla, Colombia, a Caribbean city. Our results indicated that the lowest NO₂ values occurred in remaining green areas, reduced traffic, and places that favor walking. However, the city design and current conditions of the built environment generate inequalities in exposure to air pollution. Low-income inhabitants are exposed to higher NO₂ values than wealthier people. Therefore, planning sustainable and equitable cities should involve reducing NO₂ concentrations by designing sound strategies for adequate mobility and developing an urban design that promotes walking.