# Sustainability Assessment on an Urban Scale: Context, Challenges, and Most Relevant Indicators
## 3 Results and Discussion Regarding the Selected Indicators within the Potential Categories
### Outdoor Environmental Quality  
This category focuses on analysing the issues related to outdoor environmental quality and is addressed through 4 indicators (Figure 9). The growth of cities and the expansion of built-up areas lead to many environmental issues, including the urban heat island (UHI) effect, which can potentially increase the air temperature by 2 \({}^{\circ}\)C to 5 \({}^{\circ}\)C in urban areas, as well as affecting air quality and stormwater run-off [59]. Comfortable outdoor spaces have a substantial impact on the comfort perception of the indoor environment, while natural ventilation improves the indoor air quality of buildings by reducing pollutants [60]. SDG 11.6.2 and ISO 37120 promote an assessment of air quality through yearly mean levels of fine particulate matter in metropolitan areas, in order to make cities and human settlements inclusive, safe, resilient, and sustainable. Moreover, considering the analysed methods, developing methodologies to evaluate the thermal perception and outdoor thermal comfort in cities is necessary. Another critical issue is to assess external noise, which is promoted by ISO 37120, and light pollution, which affects wildlife and people as a consequence of urban developments. This category is promoted through the following indicators:  
Adaption for ambient air quality: This indicator assesses the long-term ambient air quality and associated emissions from primary energy used in building operations, street infrastructure, and private vehicles in the local area. It is covered by SBToolPT_Urban (2018), SNTool (2020), BREEAM-C (2012), and supported by SDGs. Major sources of particulates are pollutants emitted from residential wood combustion and forest fires, gasoline or diesel-powered motor vehicles, coal-fired power stations and industry, and natural dust and salt [61; 62; 63; 64]. Under the Clean Air Act, the EPA [65] establishes national air quality guidelines for PM and five other pollutants hazardous to human health and the environment. Air quality monitoring can determine PM concentrations in metropolitan areas to ensure that PM in the air is safe for people and the environment. On the other hand, the results can help to adapt the strategies, which encourage the use of clean energy in terms of transport, therefore impacting the quality of the air being breathed [23]. Heat island effect in the local area: This indicator aims to improve the comfort of inhabitants in the outdoor spaces of the site by reducing the heat island effect and thermal comfort in the local area. This indicator is addressed by SBToolPT_Urban (2018), SNTool (2020), BREEAM-C (2012), and LEED-ND (2018). Urban structure, hard surfaces, urban fabric (mass and bulk), and the shortage of vegetation cover in cities are recognized as the major contributors to the artificial temperature increase in cities, commonly known as the urban heat island (UHI) effect [66]. The worst causes are dense urban areas with a high level of re-radiation between buildings with low-albedo surfaces and the absence of adequate air circulation in the urban mesh [67]. Taking advantage of the evapotranspiration from urban vegetation and water bodies, the adequate design of urban areas to promote air circulation, street shadowing using deciduous plants, and the use of cool materials with high albedo in the external surfaces of the building envelopes, green roofs, as well as permeable, light-colour, and reflective road surfaces are some mitigating strategies for the UHI effect. In this regard, the objective of this indicator is to estimate the extent of the urban heat island effect in a local area. Ambient noise conditions: This indicator aims to assess the acoustic comfort of the site and, if necessary, promote the attenuation of on-site noise. The indicator is covered by SBToolPT_Urban (2018), SNTool (2020), BREEAM-C (2012) and supported by ISO 37120.