# Sensors for Sustainable Smart Cities: A Review
## 2 Literature Review  
A systematic search of relevant scientific literature was conducted in this study through databases such as Scopus, Google Scholar, and IEEE Xplore. The literature search on sensors for smart cities was divided into six main sectors: health, security, water, waste, energy, and mobility. The criteria for selecting and revising relevant papers from each section, followed the PRISMA methodology [13], as well as these principles:  
1. Recent (2010-2020) literature was reviewed to ensure a revision of the current state of the art of the technologies applied to smart cities environment; prioritizing papers published during the 2015-2020 period. Figure 1a shows the distribution of the years of publications of the revised papers for this review;
2. Among the selected literature for each section, the most cited papers, including journal articles and conference proceedings were revised extensively. Figure 1b presents the distribution of the number of references with an increasing number of citations of the revised literature. Papers from Q1 and Q2 journals were given priority over Q3 and Q4 journals; as well as journals with impact factors higher than 1.0. Figure 1 shows (c) the quartiles, and (d) journal impact factor of the revised papers in this this review. Additionally, Figure 2a shows the type of references (journal, conference proceedings, books, webpages, and theses) selected and its percentage;In the six main sectors, preference was given to articles where the main topic was the use of sensors exclusively for the subject evaluated. A wide range of studies from the exploration of theoretical aspects up to practical applications were included. Figure 2b shows the percentage of revised papers under categories \"Health\", \"Security\", \"Mobility\", \"Water\", \"Waste\", \"Energy\", and \"Smart Cities\";
4. For each of the six sectors, different keywords were used to find relevant literature across each field. A list of keywords used for each section is presented as follows: 1. Health: Key terms were searched in the publication title, abstract, and keywords, and include: \"smart city\", \"smart cities\", \"sensors\", \"wearable sensors\", \"body sensors\", \"smart health\", \"smart healthcare\", \"healthcare sensors\", \"healthcare applications\", and \"internet of things\"; 2. Water: The selection of the articles for the survey was carried out using the keywords \"water\" AND \"sensors\" AND \"smart cities\"; 3. Waste: The selection of the articles for the survey was carried out using the keywords \"waste\" AND \"sensors\" AND \"smart cities\"; 4. Mobility: Among the main keywords and combination of keywords used for this search were \"mobility\" AND \"sensors\" AND \"smart cities\". Other keywords such as \"traffic\", \"vehicle\", \"pedestrian\" AND \"sensors\" AND \"smart cities\" were also included; 5. Energy: The following keywords were included in the search: \"energy consumption\", \"thermal comfort\", \"energy-consuming systems\", \"greenhouse gas emissions\", \"HVAC system\", \"lighting systems\", \"buildings energy consumption\", \"urban space energy consumption\", \"Key Performance Indicators\", \"Light Power Density (LPD)\", \"alternative energy source\", \"smart buildings\", \"smart lighting\", \"smart citizens\", \"ecological buildings\", \"virtual sensors\", \"BIM modeling\", \"energy consumption sensors\"; 6. Security: Keywords from this topic include: \"Cybersecurity\", \"Sustainable Development\", \"Environment Security\", \"Society Security\", \"Human Security\" AND \"sensors\" AND \"smart cities\".  
Following the aforementioned principles, a total of 193 references were reviewed in detail; of which 129 come from journals, 46 from conference proceedings, 9 from books, 8 from web pages, and 1 from a Ph.D. thesis.