# A Systematic Literature Review of Non-Invasive Indoor Thermal Discomfort Detection
## 3 Results  
After the fulfillment of the first step of PRISMA, 306 results for the PIOC search and 2086 results for the PIO search were organized in Excel tables (see Table 3) with the following columns: Authors, Title, Year, Source Title, Document Type, Source. This step provides an overview of the filtering process, specifically, if combined with \"sort _Title_ by alphabet\" function which provides the opportunity to quickly filter out repeated material. Once the third exclusion criterion was performed, 83 articles were allocated in a more detailed table. An ID number was assigned to each unit in order to keep track over publications. The final table had the following columns: ID, Type (Journal article, Conference paper), Year, Keywords, Goal, Measuring Equipment/Used Equipment, Experiment Duration, Facility Type, Fixed Values or Conditions, Number of Males, Number of females, Age, Sensors position, Data collected, Data volume, Target values, Algorithms (used), Algorithms (developed), Results, Notes. Figure 1: PIOC for given research \begin{table}
\begin{tabular}{|c|c|c|c|c|c|c|c|} \hline  & \multicolumn{3}{c|}{**PIOC**} & \multicolumn{3}{c|}{**PIO**} & \multicolumn{2}{c|}{**Step**} \\ \cline{2-9}  & \multicolumn{1}{c|}{**Web of Science**} & \multicolumn{1}{c|}{**Sogog**} & \multicolumn{1}{c|}{**Engineering Village**} & \multicolumn{1}{c|}{**Web of Science**} & \multicolumn{1}{c|}{**Sogog**} & \multicolumn{1}{c|}{**Engineering Village**} & \\ \hline  & 114 & 212 & 250 & 529 & 996 & 675 & All literature \\ \cline{2-9}  & 114 & 191 & 239 & 523 & 936 & 627 & 1 st exclusion criterion \\ \cline{2-9}  & \multicolumn{1}{c|}{544} & & \multicolumn{1}{c|}{2086} & \multicolumn{1}{c|}{All together} \\ \cline{2-9}  & \multicolumn{1}{c|}{306} & & \multicolumn{1}{c|}{1175} & \multicolumn{1}{c|}{Duplicates filtered} \\ \hline  & \multicolumn{3}{c|}{PIOC + PIO} & & & & \\ \cline{2-9}  & \multicolumn{3}{c|}{1481} & & \multicolumn{1}{c|}{All} \\ \cline{2-9}  & \multicolumn{3}{c|}{1154} & & \multicolumn{1}{c|}{Duplicates} \\ \cline{2-9}  & \multicolumn{3}{c|}{207} & & \multicolumn{1}{c|}{2 nd exclusion criterion} \\ \cline{2-9}  & \multicolumn{3}{c|}{83} & & \multicolumn{1}{c|}{3 rd exclusion criterion} \\ \hline  & \multicolumn{3}{c|}{35} & & \multicolumn{1}{c|}{4 th exclusion criterion} \\ \hline \end{tabular}
\end{table} 
Table 3: PIOC and PIO PRISMA results. Only 25 articles out of total 35 contained keywords which were repeated at least once among papers. The most popular keyword was \"thermal comfort\", used in 12 articles (see Figure 2). Some less popular but still repeated keywords were \"infrared thermography\" (four times), \"machine learning\" (three times), \"skin temperature\" (three times) and \"thermal sensation\" (three times). According to the analysis, the first article that investigated possibility of determining thermal comfort by non-invasive means was published in 1997 by [PERSON] and [PERSON]. They used a chamber to control the experimental environment and an infrared camera to determine the skin temperature of the person [29]. Further studies showed interest in using an office environment or regular room in addition to a laboratory or thermal chamber (see Figure 3). Each study had its own special set of predictive variables, but all of them may be divided into the following categories: indoor and outdoor parameters (such as temperature, humidity), system parameters (such as HVAC control) and bio-metrical data (such as heart rate, skin temperature). Bio-metrical data were collected in only 26 studies out of 35 filtered for review (see Table 4). Skin temperature was the most frequent parameter and was detected mainly on the wrist, hand, fingertip or head. Some studies [30; 31; 32] combined different body parts for skin temperature extraction. Facial skin temperature was measured in five studies [29; 33; 34; 35; 36], which performed facial mapping for zonal temperature extraction, and in few of them, the mean value was calculated in order to have only one input for a given type of variable.