# IoT-Enabled Smart Sustainable Cities: Challenges and Approaches
## 3 Improving the Sustainability of a Smart City
### Water and Air Quality  
Water and air quality in a city are strictly related to energy management applied methods, as it is well known that saving energy mechanisms and producing from renewable sources have also a strong impact on the quality of air (both outdoor [82] and indoor [83]) and on water pollution. This is a critical issue that cannot be ignored, as it is well-known that the air pollution has a significant risk factor for human health. Transportation, road traffic, home heating, industrial emissions, and other entropic activities are the major emission sources of toxic gases (e.g., NO\({}_{x}\), O\({}_{3}\), and CO), volatile organic compounds (benzene, toluene, and xylene), greenhouse gases (CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O), and particulate matter (PM\({}_{10}\), PM\({}_{5,0}\), PM\({}_{2,5}\), and PM\({}_{1,0}\)). Air pollution not only has negative effects on public health, but also on the environment, as acid rains, smog, ozone layer reduction, and global warming damage the environment in multiple ways. Air pollution and air quality monitoring, through the introduction of IoT technologies, are strictly necessary in a smart sustainable city to implement reduction strategies and stimulate environmental awareness in people's community [84]. Several techniques and technologies can be used to monitor air quality, as shown in Figure 9. Currently, traditional environmental monitoring methods, such as air quality monitoring stations, are precise but expensive and bulky. These high-cost and high-accuracy air quality monitoring stations, based on chemical analyzers, are used to address the environmental regulations and standards. Generally, they are less densely employed and provide sensing data at low-spatial resolution in the cities. Therefore, air quality monitoring systems based on low-cost and accurate sensors to be integrated in WSNs distributed at high-spatial resolution in the smart cities can be a valid and effective tool for real-time ubiquitous monitoring, in order to support decisions and to keep the general public informed on city status [85]. For example, in the city of Uppsala (Finland), with the GreenIoT project, a vehicular network of sensing nodes has been implemented [86]. Following this distributed approach, the collected data can help to measure the daily individual exposure to air pollutant by combining context-aware information, given by users' smartphones, and urban air pollution levels provided by the sensing layer [87]. Water management is a critical aspect to ensure sustainability, as water scarcity and water stress issues have become clear threats to the global population, as shown in Figure 10. Water is one of the most crucial elements for human life, as it is used for drinking, domestic use, and food production. To this end, it is imperative for municipalities to maintain a good water quality balance; otherwise, it would severely damage the health of the humans and, at the same time, affect the rest of the natural environment. Water pollution is the major global problem which thus requires a fast intervention water resource management in different levels: from international down to individual entities. It has been verified that water pollution is the leading cause of mortality and diseases worldwide [88]. A distributed sensing approach--similar to the one adopted for air quality monitoring--has been extended in the last years also to water monitoring [89]. Traditional methods are based on manual and laboratory-based analyses, in which water samples are analyzed in specific laboratories, with some drawbacks, such as the need of expensive devices, trained personnel, and the absence of real-time monitoring (the sample has to be brought to the laboratory). With the advent of IoT-based solutions, the distributed sensing is appealing also for the smart cities of the future. Even in the case of water and air quality monitoring, a municipality can evaluate the adoption of IoT-oriented solutions to be deployed on both private and public sites.