## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **CO (ppm)** & **CO\({}_{2}\) (ppm)** & **NO\({}_{2}\) (ppb)** & **SO\({}_{2}\) (ppm)** & **O\({}_{3}\) (ppb)** & **PM\({}_{2.5}\) (\(\mu\)g/m\({}^{3}\))** \\ \hline Window opening & \multirow{2}{*}{0.32} & \multirow{2}{*}{84.84} & \multirow{2}{*}{\(-\)0.61} & \multirow{2}{*}{0.44} & \multirow{2}{*}{24.34 \({}^{\ddagger}\)} & \multirow{2}{*}{\(-\)0.021} \\ (Yes vs. No) & & & & & & \\ Occupants & 0.52 \({}^{\ddagger}\) & 51.62 \({}^{\ddagger}\) & 3.02 & \(-\)0.008 & \(-\)3.49 \({}^{\ddagger}\) & 0.004 \\ Cleaning & \multirow{2}{*}{4.73 \({}^{\ddagger}\)} & \multirow{2}{*}{\(-\)317.49} & \multirow{2}{*}{1.39} & \multirow{2}{*}{0.43} & \multirow{2}{*}{\(-\)6.24} & \multirow{2}{*}{0.047 \({}^{\ddagger}\)} \\ (Yes vs. No) & & & & & & \\ \multicolumn{1}{c}{Cooking (Yes vs. No)} & \multirow{2}{*}{228.02} & \multirow{2}{*}{\(-\)28.01} & \multirow{2}{*}{\(-\)0.21} & \multirow{2}{*}{1.79} & \multirow{2}{*}{0.065} \\ \multicolumn{1}{c}{Fan using} & \multirow{2}{*}{1.42} & \multirow{2}{*}{\(-\)32.97} & \multirow{2}{*}{10.58} & \multirow{2}{*}{\(-\)0.0003} & \multirow{2}{*}{\(-\)2.07} & \multirow{2}{*}{0.002} \\ (Yes vs. No) & & & & & & \\ Air- conditioning use & \multirow{2}{*}{\(-\)1.22} & \multirow{2}{*}{246.99 \({}^{\ddagger}\)} & \multirow{2}{*}{87.87} & \multirow{2}{*}{0.25} & \multirow{2}{*}{21.59} & \multirow{2}{*}{0.008} \\ (Yes vs. No) & & & & & & \\ Making tea & \multirow{2}{*}{37.04} & \multirow{2}{*}{-} & \multirow{2}{*}{\(-\)0.45} & \multirow{2}{*}{\(-\)0.13} & \multirow{2}{*}{14.21} & \multirow{2}{*}{\(-\)0.050} \\ (Yes vs. No) & & & & & & \\ Smoking & \multirow{2}{*}{17.21 \({}^{\ddagger}\)} & \multirow{2}{*}{1988.44 \({}^{\ddagger}\)} & \multirow{2}{*}{547.36 \({}^{\ddagger}\)} & \multirow{2}{*}{2.98 \({}^{\ddagger}\)} & \multirow{2}{*}{1.69} & \multirow{2}{*}{0.173} \\ (Yes vs. No) & & & & & & \\ Incense burning & \multirow{2}{*}{18.21 \({}^{\ddagger}\)} & \multirow{2}{*}{2927.87 \({}^{\ddagger}\)} & \multirow{2}{*}{193.11} & \multirow{2}{*}{3.66 \({}^{\ddagger}\)} & \(-\)108.9 \({}^{\ddagger}\) & 0.416 \({}^{\ddagger}\) \\ (Yes vs. No) & & & & & & \\ Essential oil use & \multirow{2}{*}{12.76} & \multirow{2}{*}{269.25} & \multirow{2}{*}{74.89} & \multirow{2}{*}{\(-\)0.66} & \(-\)89.29 \({}^{\ddagger}\) & \(-\)0.022 \\ (Yes vs. No) & & & & & & \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{6}{c}{Generalized estimating equations (GEE) \({}^{\ddagger}\)\(p<\) 0.05, \({}^{\ddagger}\)\(p<\) 0.01.} \\ \end{tabular}
\end{table} 
Table 5: Association between air pollutants concentrations (24-h average concentration of air pollutants in each house as dependent variable), and household characteristics: generalized estimating equations. lowest among the three areas, which was not consistent with the observations of previous studies that ambient NO\({}_{2}\) was related to industrial activities [24], and outdoor O\({}_{3}\) might be formed by the photochemical reaction of nitrogen oxides absorbing sunlight, and VOCs [25; 26]. We believed these may be related to Taiwan EPA's policies and efforts to control air pollution from stationary sources after that the \"Stationary Pollution Source Air Pollutant Emissions Standards\" was passed in 1992, and the \"Air Pollution Control Act Enforcement Rules\" was also implemented in 2003. In regard to the effects on the window opening, our study displayed that household NO\({}_{2}\) and PM\({}_{2.5}\) concentrations during window opening periods were significantly higher than that during reference periods. NO\({}_{\chi}\) and PM were related to traffic emissions [24; 32], and most of the houses in Taiwan were adjacent to roads, so window opening might increase indoor NO\({}_{2}\) and PM\({}_{2.5}\). For the influence of cooking, there were many simulated experiments exploring the air pollutant emissions of cooking-related fuel combustion [29; 33; 34; 35; 36], and they demonstrated that CO, CO\({}_{2}\), NO\({}_{\chi}\), and PM\({}_{2.5}\) would be emitted by the process of the experiments.