## 1 Introduction  
Chronic cough is defined as a cough that lasts for 8 weeks or more, and its estimated global prevalence is 2-18% [1]. People with chronic cough report many physical and psychological effects, which can lead to quality of life impairment [1]. Chronic cough is often triggered by low thermal, mechanical, or environmental chemical exposure levels [1]. In addition to the effects of gaseous pollutants such as sulfur dioxide (SO\({}_{2}\)) and nitrogen dioxide (NO\({}_{2}\)) [2], ambient particulate matters (PM), especially those with diameters 2.5 \(\upmu\)m and smaller (PM\({}_{2.5}\)), have been reported to be involved in the development and exacerbation of respiratory diseases including bronchial asthma (BA) in both children and adults [2; 3] among these environmental factors.  
Polycyclic aromatic hydrocarbons (PAHs) are known to be important organic PM constituents for their toxic potential through their cytotoxicity, mutagenicity, and carcinogenicity [3]. PAHs are a group of hydrocarbons with two or more fused aromatic rings, and they are produced by the incomplete combustion of coal and various organic materials such as fossil fuels and cigarette smoke [3; 4]. Low-molecular-weight PAHs (two and three rings) occur in the atmosphere predominantly in the gas phase, whereas high-molecular-weight PAHs (five rings or more) are largely bound to PM. Intermediate-molecular-weight PAHs (four rings) are partitioned between the gas and particulate phases, depending on the atmospheric temperature [5]. The main sources of PAHs include plants, air, water, food and soil, and routes of exposure to humans include inhalation, ingestion or percutaneous penetration [4; 5]. Among these routes, inhalation has been considered more important for the respiratory effects of PAHs [3]. PAHs are now shown to be involved in the development and exacerbation of respiratory diseases, such as childhood and adult BA, chronic obstructive pulmonary disease, and others accompanied by cough symptoms, in addition to affecting respiratory function [3; 4; 6; 7]. However, a few epidemiological studies have demonstrated the possible association between airborne PAH exposure and respiratory symptoms, such as cough prevalence in people with chronic respiratory diseases, including asthma [6; 7].  
Heterogeneity exists in the association between PAH and cough symptoms or respiratory diseases, including asthma, in previous epidemiological studies, in addition to the small number of relevant studies. A US population-based cross-sectional study has reported an inverse association between urinary PAH metabolites and adult asthma, while the relationship between those and cough was not observed [8], contrary to PAH's positive association with respiratory diseases and symptoms. Another cross-sectional study in Mexican communities has shown no association between urinary metabolite 1-hydroxyrene concentrations and respiratory function [9]. A previous cross-sectional study has reported that there were no associations between ambient PAH and respiratory functions among asthmatic children [10] as these are adult population studies. These inconsistent results might be due to differences in race, age, site, underlying disease, and measured PAHs type, in addition to the sample size of each study. Actually, the specific composition and organic chemicals amount are highly dependent on the fuel burned and combustion technology worldwide [3].  
This study's purpose was to examine the airborne PAH exposure effects on cough symptoms in children and adults with respiratory disease accompanied by chronic cough in two Japanese cities, taking into account other pollutants of SO\({}_{2}\), NO\({}_{2}\), and PM\({}_{2.5}\) which may affect respiratory diseases based on these backgrounds.