## 2 Materials and Methods  
Having selected the theme, centered on the aspects related to thermal comfort in bus cabin environments, the efforts turned to define the research intention, guided by the following research questions (RQ). This methodological starting point has already been adopted by a thermal comfort study [23]. RQ\({}_{1}\): What were the approaches adopted by thermal comfort studies conducted in bus cabins? Overall, thermal comfort studies in vehicular environments may adopt approaches in three directions, in isolation, or combinations. The three directions are the thermal satisfaction of passengers and drivers, which can be expressed in terms of well-being, health, and security; productivity, usually linked to the performance of drivers; the fuel/energy conservation of the vehicles [26; 29]. RQ\({}_{2}\): What thermal comfort parameters were investigated in the studies that analyzed bus cabins? The six essential parameters employed by models of thermal comfort are divided into environmental and personal [30; 31; 32]. The environmental parameters are air temperature (t\({}_{a}\)), air velocity (V\({}_{a}\)), air relative humidity (UR), and mean radiant temperature (t\({}_{\text{rm}}\)), while the personal ones comprise metabolic rate (M) and clothing thermal insulation (I\({}_{\text{clo}}\)) [25; 32; 33; 34]. RQ\({}_{3}\): Besides thermal comfort parameters, what other variables were analyzed by thermal comfort studies conducted in bus cabins? RQ\({}_{3}\) intends to complement the second research question. As mentioned in the introduction, the comfort perceived by the bus's occupants may encompass other types of comfort other than thermal [4; 5]. RQ\({}_{4}\): What international standards of thermal comfort did the studies that analyzed the thermal conditions of bus cabins follow? The knowledge area of thermal comfort has international standards as guidelines, such as ASHRAE and ISO [35; 36; 37; 38; 39; 40]. Moreover, since the thermal environment in question is the bus, there are specific standards of the knowledge area directed at the vehicular environment, such as the ISO 14505 series [41; 42; 43]. RQ\({}_{5}\): What standard models of thermal comfort were employed to assess the bus cabins? A few attempts to generate models of thermal comfort can be found in the literature. However, the standard models for that purpose internationally are commonly PMV-PPD, SET* Adaptive, and [PERSON]'s [37; 40]. After defining the research questions, this paper adopted the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology, proposed by [PERSON] et al. [44], through the four stages: identification, triage, selection, and inclusion of works. It is worth mentioning that the software applications Mendeley, JabRef, Microsoft Excel, and R Project with Bibliometrix script [45] were employed as tools in the treatment and analysis of the works in order to generate the bibliometric and quantitative results of this investigation. The work identification stage was carried out in view of the two thematic axes previously mentioned (thermal comfort and bus), which were translated by the keywords of Table 1.  
In Table 1, it is possible to observe the columns with the keywords of the two mentioned thematic axes. The first refers to the thermal environment to be explored and the third concerns the expression and variants of the thematic axis thermal comfort. Besides the Boolean operator AND, employed to combine the keywords from the two columns in Table 1, this investigation utilized OR between the words from the third column. Thus, the final combination was denoted by the following expression: \"bus\" AND (\"PPD\" OR \"PMV\" OR \"Predicted Percentage Dissatisfied\" OR \"Predicted Mean Vote\" OR \"thermal comfort\" OR \"thermal discomfort\"). Afterward, a search was conducted in the databases Science Direct, Scopus, Springer Link, and Web of Science up to 17 October 2020, with no time restrictions.