## 4 Conclusions  
This study's aim was to determine the association between airborne PAHs, a PM component, and cough symptoms in patients with chronic coughs in two regions of Japan. Airborne BaA was associated with cough symptoms exacerbation in patients with chronic cough. Of note, this study has several limitations; first, the sample size and observational time are relatively limited compared to previous studies, and the background characteristics of participants were different between the two areas. Accordingly, it was not possible to perform a stratified analysis by age or body size. Second, indoor air pollutants were not measured. People tended to spend more time indoors and might be exposed to indoor air pollutants during the COVID-19 pandemic. Indoor air PAH sources include not only solid fuel combustion for cooking and heating as a source of indoor air PAHs but also secondhand smoke [33]. However, the indoor air pollutants' impact might be extremely small based on the environment surrounding the study subjects. Third, air pollutant concentrations in each region were regarded as the same as those measured at the central monitoring station of Kanazawa and Fukuoka City. Therefore, there is a possibility of misclassification of exposure, especially for people who live far from these monitoring sites. A higher number of sampling sites should be considered in future studies. Fourth, information on educational and socioeconomic status as risk factors for chronic cough [34] and prenatal exposure to PAH in children has not been collected. Taken together, further studies are needed to examine the association between specific PAHs and chronic cough on a larger scale and in longer periods in order to develop effective prevention measures in the future. This approach could lead to the optimized management of the PAHs sources and concentrations to reduce the onset and exacerbation of respiratory diseases associated with chronic cough. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122412505/s1](https://www.mdpi.com/article/10.3390/app122412505/s1)]([https://www.mdpi.com/article/10.3390/app122412505/s1](https://www.mdpi.com/article/10.3390/app122412505/s1)). Table S1: Daily air pollutant concentrations correlation matrix for study period 1 April-31 May 2020, Kanazawa city; Table S2: Daily air pollutant concentrations correlation matrix for study period 1 April-31 May 2020, Fukuoka city; Table S3: Adjusted estimates of cough symptoms in total PAH exposure in the GEE lag 3 model; Table S4: Adjusted estimates of cough symptoms in total PAH exposure in the GEE lag 4 model. Conceptualization, H.N. ; methodology, A.H. and H.N. ; formal analysis, A.H., K.-O.P. and K.S. ; resources, H.O., A.T., K.H. and H.N. ; data curation, A.H., H.O., M.F., T.T. and N.O. ; writing--original draft preparation, A.H.; writing--revising and editing, H.O., H.M., M.F., T.T., T.W., N.O., J.Z., K.-O.P., K.S., H.T., A.T., K.H. and H.N. ; supervision, K.H. and H.N. ; project administration, H.T., H.N. ; funding acquisition, H.N. All authors have read and agreed to the published version of the manuscript. This research was funded by the Environment Research and Technology Development Fund (JPMEERF20195051) of the Environmental Restoration and Conservation Agency of Japan. The study was conducted in accordance with the Declaration of Helsinki and approved by the Medical Ethics Committee of Kanazawa University (approval number: 1630). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. The data presented in this study are available on request from the corresponding author. The authors thank [PERSON] for her assistance with data management and [PERSON], a native English speaker, for his English editing of the manuscript. K.-O.P. currently belongs to Data Management Department, Asia Center for Air Pollution Research, Niigata, Japan. The authors declare no conflict of interest.