## 3 Scientometric Analysis: Results and Discussion
### Keyword Analysis
#### 3.3.2 Evolution of Themes  
The evolution of the topics over time in the research field was studied using an overlay graph, a thematic evolution map, and strategic diagrams, all generated by SciMAT. Figure 13a shows the overlay graph. It represents the stability of the keywords between two consecutive subperiods and measures the number of keywords shared by successive subperiods ([PERSON] and others, 2017). The analysis of the overlay graph shows a high number of keywords of the first year, as well as its increase in 2021, which has doubled. Taking into account the evolution during the first half of 2022, it is expected that the number of keywords in 2022 will reach 3000, a lower increase than that of the previous year, implying a tendency towards the stabilization of the research field. In addition, the analysis of the horizontal arrow, which represents the number of keywords shared by both periods, and the upper-incoming and -outcoming arrows of Periods 1 and 2, which represent the number of new  
Figure 12: Overlay visualization, in terms of average citations, of network co-occurrences of authors’ keywords with a minimum of 5 occurrences. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**A** & **B** & **C** & **D** & **E** & **F** & **G** \\ \hline
1 & Red & 9 & Single-use plastic & 62.55 & Plastic waste management & 1.68 \\
2 & Green & 8 & Medical waste & 22.00 & Environmental effect of medical waste & 1.00 \\
3 & Blue & 5 & Infection control & 21.80 & Infection control of pandemic & 0.89 \\
4 & Yellow & 5 & Waste & 29.25 & Waste management & 1.23 \\
5 & Purple & 5 & Coronavirus & 22.84 & Pollution of plastic waste & 1.77 \\
6 & Black & 2 & SARS Coronavirus & 32.40 & SARS Coronavirus and biosafety & 1.03 \\ \hline \hline \end{tabular} (A) Cluster number; (B) color of cluster in Figure 9a; (C) number of keywords in cluster; (D) most-influential keyword in cluster; (E) average normal citations of the most-influential keyword in cluster; (F) assigned theme to cluster; (G) average normalized citations of cluster.  
\end{table} 
Table 5: Most-significant characteristics of clusters of influential keywords in the field. keywords in each period, show the considerable renewal of terms related to the novelty of the field: the number of new keywords in 2021 was 1702, which represents 80% of those included in this year. In addition, the number of this year's keywords that will be replaced by 15 June 2022 reached 1592, a value that represents 75% of all keywords. Figure 13b shows the thematic evolution maps of the research field in terms of the average citations. Our analysis indicated the following:  
* In general terms, in each year, the themes are not the same, showing that new themes with their associated keywords appear, while others disappear, as was discussed concerning the overlay graph. Only the HUMAN theme appears in all three years. However, COVID appears in the three years, first as SARS-COV-2 in 2020 and then under the denomination of Coronavirus in 2021;  
Figure 13: Representation of the evolution of topics over time in the research field using overlay graph (**a**); thematic evolution map in terms of average citations (**b**). * Because the identified themes are connected with a line in a previous year, the research into the management of used COVID-19 PEE is very cohesive. In addition, the thickness of most of the edges is high, meaning that such an edge is a considerable thematic nexus [38]. In the case of solid lines, they link themes that shared the same name; this means that both themes are labeled with the same keywords or the label of one theme is part of the other, which is the case with themes such as infection prevention and health care personnel, plastic and human, or viral disease and Coronavirus and recycling.