Urban spaces are increasingly exposed to air pollution, which critically influences the health and well-being of cities’ inhabitants. Urban green areas provide recognised important environmental and social functions. Providing, planning, managing, and protecting urban green areas are on the agenda of sustainability and liveability of cities. In this paper, urban green areas are classified as Sensitive Areas in Urban Spaces (SAUS), corresponding to critical environmental and social spaces that should be preserved from surrounding air pollution sources. The concept of SAUS was tested in two urban green areas of Guimarães, a medium-sized Portuguese city, by defining a buffer of 300 m (m) around each area. Industrial plants and traffic were the sources of air pollution analysed by considering their emissions of NOx, PM10 and SO2. Horizontal maps of NOx, PM10 and SO2 were produced by using the model AUSTAL2000. Results showed that more than 97% of the SAUS had concentrations equal to or below 5 μg/m3 of NOx, 0.5 μg/m3 of PM10 and 0.1 μg/m3 of SO2. Besides the globally low levels of emissions, the sources located in the two buffers had environmental impacts and affected the quality of the air at both SAUS. As the proposed concept was useful for identifying the main air pollution sources and their impact on the air quality of urban green areas, the concept of SAUS could be helpful for support the adoption of planning actions towards improving the sustainability of these areas and maximising their environmental and social benefits. However, more applications are needed for confirming the efficiency of this concept to improve the air quality of urban green areas.