## 4 Discussion  
As the mask usage rate was significantly higher from February to April 2021 compared with the same period in 2020, we investigated whether the relationship between PM2.5 ionic components and respiratory symptoms and PEFR also differs between these two periods. The main finding of the study was that, on days when the concentration of ionic components of PM2.5 increased, the proportion of participants who showed allergic symptoms also increased. Furthermore, a higher proportion of participants showed a decrease in the pbPEFR value in 2020, but not in 2021. The results showed a trend towards less exacerbation of airway symptoms and less decline in pbPEFR in 2021 compared with 2020 (Table 3), especially in NO\({}_{3}\)\({}^{-}\) (Table 3), although the differences were not significant in many other ions. In 2020, Na+ had respiratory protective effects, including cough suppression (5), which had diminished in 2021. Therefore, it was presumed that the use of a mask might attenuate the effects of ionic components, both attenuating and enhancing the symptoms and pulmonary function. To confirm the differences between the same seasons in these two years, we first compared the concentration of air pollutants as an influencing factor. Among the examined ions, Cl\({}^{-}\) and Mg\({}^{2+}\) levels were significantly higher in 2021 than in 2020 and Ca\({}^{2+}\) levels were significantly lower, but otherwise, no significant differences were found. In 2018, when asked, \"What is an effective preventive measure when air pollution is severe?\", many people of all ages answered \"masks\" (data not shown). In Japan, there was a shortage of masks from the end of March to April, so the government decided to distribute cloth masks. In addition, the price of paper masks began to rise in December 2019 and reached its highest price on 20 June 2020; then, the price started to decrease, so the shortage is assumed to have ended in June 2020 in Japan. Once masks became available, Japanese people started using them quite regularly. One of the limitations of this study is the small number of subjects. To obtain more accurate results, we did not include participants for whom data on mask use were deemed unreliable. With this approach, we believe the data we obtain on mask use are pretty accurate, especially when it comes to 2021. Another limitation of the study is that there may be background factors other than masks that explain the differences between 2020 and 2021. For example, the ion concentra  
Figure 3: Relationship between daily Mg\({}^{2+}\) concentration and daily average of the ratio of peak expiratory flow rate to personal best peak expiratory flow rate on that day. tion in 2021 may have been lower than in 2020, so the concentrations of ions may have had less impact; however, that was not the case. On the other hand, even if the concentration of ions was higher in 2021 than in 2020, e.g., Cl\({}^{-}\), no associated effect was seen on symptoms and PEFR. In addition, a significant increase in Mg\({}^{2+}\) was seen in 2021 compared with 2020, and although the effect on nasal symptoms was attenuated, it still remained. As for Na+, it appeared to rather protect against nasal symptoms over a longer period [6]. In addition, it is necessary to conduct a detailed study of how to use the mask and the type of mask, but it is possible that a certain conclusion has been reached from the perspective of the possibility of the impact in actual daily life. Needless to say, further investigations are expected in the future. The strength of this study is that it was previously difficult to conduct research on the effects of mask use by the general public in actual clinical practice, but the spread of COVID-19 made such research possible. We believe that our findings will be useful in clinical practice in the future. Regarding PEFR, the PEFR metrics used in this study are slightly different from the commonly used metric %PEFR, and they have not been used often in the past.