## 3 Results
### Publications  
The first publication about the topic occurred in 1967 (Figure 2). The study analysis showed that there was an increase in publications on this subject beginning in 2009; in the last decade, 83 studies were published, and these corresponded to 78.3% of publications. The year with the highest number of publications was 2013 (13 studies). All the studies included in the present study's sample were published in the English language, except one published in Portuguese. The quantitative analysis of the number of title words, keywords, tables, figures, pages, and references can also be carried out through bibliometric studies [24; 25], though this depends on the potential of these results to allow analyses of the scope and depth. The number of words in the title ranged from six (6) to 34 (median of 14). Relevant keywords started to appear in the studies only from 1994 onward, and in the last few years, the number of keywords was between four and eight. The number of pages ranged from four to 204 (median of 9). The total number of citations to the published articles was 2,391, with an average of 22.55 (min.0; max.418) citations per document (Table 1). The usage count is a measure provided by WoS; it measures interest in a platform-specific item. This information reflects the number of times an article has met the user's information needs, and it is updated daily. Figure 1: PRISMA flow diagram illustrating the literature research and selection process. Figure 2: Distribution of the number of publications per decade. Among the 106 publications, the most cited was the study by [PERSON] et al. published in 2002 in the Lancet and presenting 462 citations in the main collection of WoS; this publication also presented the highest average citations per year (24.3) in WoS [26]. This study aimed to evaluate the association between participating in sports teams and the subsequent development of asthma in children. In the study, concentrations of PM\({}_{10}\), PM \({}_{2.5}\), O\({}_{3}\), and NO\({}_{2}\) were monitored. The ten selected studies with the highest amount of citations in WoS are presented in Table 2.  
The studies used 411 keywords in total. Most of the studies cited five keywords (min.3; max.11). Figure 3 shows the keywords that were used in at least three studies. The larger size of the source indicates that more studies cited the specific keyword, and the smallest source indicates that the smallest number of studies cited the specific keyword. The most cited keywords were air pollution (20 studies), physical activity (15), and particulate matter (11). The categorization of the keywords into clusters was performed in accordance with the work of [PERSON] et al. [22]. Most of the keywords were related to pollutants (85), human health (58), and exercise/sports (50). Figure 4 shows the keyword categories, number of related keywords, and examples of words. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mean** & **Median** & **Minimum** & **Maximum** \\ \hline Length of title & 15.12 & 14 & 6 & 34 \\ Number of keywords & 5.13 & 5 & 0 & 11 \\ Number of tables & 3.07 & 3 & 0 & 9 \\ Number of figures & 3.77 & 3 & 0 & 30 \\ Number of References & 41.39 & 39 & 5 & 149 \\ Length (pages) & 11.942 & 9 & 4 & 204 \\ Number of citations WoS & 22.70 & 7 & 0 & 418 \\ Number of citations WoS by year & 2.68 & 1.33 & 0 & 24.59 \\ Citations in all databases & 24.17 & 7 & 0 & 427 \\ Usage Count & 28.57 & 19 & 0 & 243 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Length of title, keywords, and number of tables, figures, and pages of studies about outdoor air quality of environments used for exercise and sports practice. Figure 3: Keywords used in published studies.