*3.5. Applications*

In the health section, sensors can be used in different multidisciplinary areas, for example: smart toilets, applications that monitor and direct physical exercise with the final objective of rehabilitation or prevention of injuries, and applications in health institutions [155,156]. For cycling sports, sensors are used most frequently in long-distance running and swimming [157]. There are different methods used to monitor the condition of patients, for example: monitoring the electrical activity of the heart using an array of electrodes placed in a sterile way on the body [158], monitoring of vital signs via wireless [159] for the tracking of the patient's location by issuing alerts if necessary, implementation of user-friendly interfaces to share information, based on wireless ECG sensors and pulse oximeters [159], implementation of a network-based multi-channel frequency EM for rehabilitation patients with exercises [160], design of a system with three sensors to monitor the ECG, body weight, and pulse of the patient [161], portable home health monitoring system using ECG, fall detection, and GPS to monitor people outside [162], e-health monitoring system based on the fusion of multisensory data to predict activities and promote the decision-making process about the health of the person [163]; all of these home health systems allow to monitor patient's activity in their homes.

In the mobility section, the most common ITS applications in reducing road traffic are related to accident detection [164,165] and prevention [40], identifying road traffic events [166] and studying the driving behavior and applying real-time feedback [167,168]. Interesting applications in urban mobility have been proposed as well, such as smart traffic lights [67], smart parking [72], collision prediction and avoidance [66], vehicle WIM detection [65], and mobility visualization through heatmap representations [169]. Heatmaps can also be obtained from analyzing mobility data from vehicles and pedestrians and reflect the behavior of different phenomena, such as average speed in the city [170], traffic jams location, frequency, and duration [171], and combined spatio-temporal traffic clustering and analysis [169].

For the water section, by analyzing parameters like soil humidity and nutrients, the design of efficient irrigation programs, including remote control or automatic irrigation systems, can be done, reducing water and energy consumption [42,127,172]. For example, a system that allows real-time monitoring of surface water quality in different aquatic environments [43], systems for monitoring the quality of the river that crosses a city [173], and the implementation of a system that could be used in pipelines networks to monitor the quality of water [122]. Distribution systems need to be regulated to ensure the required quantity and quality. The use of sensors to collect data in real-time can detect and locate leaks, which can affect the correct supply of water to a city or deteriorate the infrastructure around the leak [14]. The water distribution systems need to be monitored to ensure that correct water quality is distributed and for detecting pollutants [174].

In the waste section, the IoT system permits an onboard surveillance system, which raises the process of problem reporting and evidence good waste collection practices in real-time [135]. The use of mobile apps and software permit that truck drivers receive alerts from the smart bins that need attention, and also to ge<sup>t</sup> the optimal route to collect the garbage, reducing the effort and cost of waste collection [136–138]. With the collection of information about the filling level of the containers, it is possible to determine the best types and sizes of containers, areas that require greater or special collection capacity, and the timing of the collection [134–136,138]. The sensors can improve the automation in identifying and separating waste, allowing an increase in the processing speed for reuse and recycling to convert a smart city to a city with zero-waste [20,137]. All data collected from the bins and the analytics, in conjunction with the use of a GPS to know the coordinate position of bins, dumps, and fleet, can be used to manage in novel ways the collection and segregation of waste [17,129,136,138,141].

Finally, countries are looking to implement innovative technologies focused on minimizing energy consumption and improving their citizens' environment and welfare. For this reason, there are various applications in the areas of buildings, public lighting, and urban space such as counting, movement, and location of people and vehicles, security actions for citizens, fire detection in building enclosures [140], smart homes [135,147,175] and home networks [135,136], Light Emitted Diode-wireless (LED) indoor and outdoor lighting fixtures [18], geothermal technology [137], hygrothermal comfort [17], cybernetic cities, ubiquitous connectivity [176], HPCense (seismic activity), smart thermostat [18], indoor lighting apps [129], microgrids [139], structural health monitoring of buildings [18], ecological buildings [134], among others.
