## 3 Results
### Mask Use
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & **Mean \({}^{\star}\)** & **SD** & **Median** & **Min** & **Max** & \(p\) **(Two Tail) \({}^{\star}\)** \\ \hline PM2.5 & 2020 & 15.037 & 5.844 & 13.595 & 3.996 & 45.547 & \(p=-0.2209\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 18.798 & 9.328 & 16.018 & 3.866 & 153.825 & \\ Cl\({}^{-}\) & 2020 & 0.052 & 0.053 & 0.02 & 0 & 0.356 & \(p=0.0037\)\({}^{\star}\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.075 & 0.066 & 0.041 & 0.003 & 0.614 & \\ NO\({}_{3}\)\({}^{-}\) & 2020 & 1.204 & 0.747 & 0.84 & 0.009 & 7.52 & \(p=0.1355\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 1.427 & 0.829 & 1.16 & 0.234 & 5.97 & \\ SO\({}_{4}\)\({}^{2-}\) & 2020 & 2.615 & 1.209 & 2.114 & 0.594 & 12.31 & \(p=0.5249\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 2.542 & 1.044 & 2.33 & 0.551 & 9.43 & \\ Na\({}^{+}\) & 2020 & 0.362 & 0.132 & 0.349 & 0.058 & 0.97 & \(p=0.4265\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.345 & 0.098 & 0.326 & 0.108 & 0.75 & \\ NH\({}_{4}\)\({}^{+}\) & 2020 & 0.74 & 0.54 & 0.516 & 0.038 & 5.888 & \(p=0.5178\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.698 & 0.491 & 0.484 & 0.027 & 4.13 & \\ K\({}^{+}\) & 2020 & 0.26 & 0.082 & 0.247 & 0.085 & 0.74 & \(p=0.5210\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.261 & 0.085 & 0.26 & 0.056 & 0.532 & \\ Ca\({}^{2+}\) & 2020 & 0.354 & 0.158 & 0.288 & 0.091 & 2.884 & \(p=3.0\times 10^{-7}\)\({}^{\star}\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.243 & 0.17 & 0.162 & 0.019 & 2.04 & \\ Mg\({}^{2+}\) & 2020 & 0.013 & 0.004 & 0.011 & 0.004 & 0.059 & \(p=9.6\times 10^{-8}\)\({}^{\star}\) \\ (\(\mu\)g/M\({}^{3}\)) & 2021 & 0.034 & 0.017 & 0.026 & 0.002 & 0.22 & \\ \hline \end{tabular} \({}^{\star}\)\(p<0.01\) (two-tailed, Mann–Whitney U test). Number of days: 90 days in 2020; 89 days in 2021.  
\end{table} 
Table 2: Comparison of concentrations of ionic components of PM2.5 in 2020 and 2021. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Cough** & **Runny Nose** & **Stuffy Nose** & **Sneezing** & **pbPEFR** & **pbPEFRm** & **pbPEFRe** & **2020\(\rightarrow\)2021 *** \\ \hline Cl\({}^{-}\) & 2020 & rs & 0.178 & 0.144 & 0.2165 \({}^{\star\star}\) & 0.0213 & 0.1708 & 0.0879 & 0.2064 & \(p=0.0037\) \\  & \(p\) & \(-0.0933\) & \(-0.1757\) & \(-0.0404\) & \(-0.8421\) & \(-0.1076\) & \(-0.  
a significant negative correlation with the symptom in 2020 and a significant positive correlation in 2021. Mg\({}^{2+}\) was negatively correlated with pbPEFR, pbPEFRm, and pbPEFRe in 2020 and positively correlated with all three variables in 2021, but without statistically significant correlations. Regarding the ions whose concentration did not change significantly from 2020 to 2021, NO\({}_{3}\) showed a significant correlation with runny nose and sneezing in 2020, but not in 2021. In addition, the ion was not significantly correlated with pbPEFR in 2020, but significantly positively correlated with it in 2021. Although no significant correlations were found between NO\({}_{3}\) and pbPEFRm or pbPEFRe, both correlation coefficients were negative in 2020 and positive in 2021. As for Na\({}^{+}\), it was significantly negatively correlated with coughing in 2020 and significantly positively correlated with a runny nose in 2021. In addition, we found a significant positive correlation with pbPEFR in 2021. SO\({}_{4}\)\({}^{-}\) and K\({}^{+}\) were not associated with symptoms or PEFR in 2020 or 2021. As a typical example, a scatter plot of the relationship between NO\({}_{3}\)\({}^{-}\) and a runny nose is shown in Figure 2 and that between Mg\({}^{2+}\) and pbPEFR is shown in Figure 3.  
In both cases, the correlation trend observed from February to April 2020, when the mask wearing rate was 31.3%, was no longer observed during the same period in 2021, when the mask wearing rate was 76.7%. It is highly likely that mask use is one of the factors behind this change, and further investigation is desired in the future.