## 4 Discussion
Although CO also was produced by cooking, it was revealed that combustion of high-grade fuels (such as natural gas, and LPG which contained propane, butane, etc. ), the main fuel-burning stoves use in Taiwan households usually produce much less CO than combustion of low-grade fuels [29; 33]. In the previous study, [PERSON] et al. revealed the residential cooking exhaust hoods could not completely capture the pollutants and their efficiency was highly variable [37]. Our results showed that indoor CO\({}_{2}\), NO\({}_{2}\), and PM\({}_{2.5}\) levels during cooking periods were significantly higher than during reference periods, but the indoor CO level during cooking periods was lower than during reference periods, possibly indicating that the emission rate of CO\({}_{2}\), NO\({}_{2}\), and PM\({}_{2.5}\) might be higher than the capture rate of the exhaust hood and the emission rate of CO might be lower than the pollutants capture rate of the exhaust hood. In terms of influence factors, we found there were significantly positive correlations between the number of occupants and CO and CO\({}_{2}\) concentrations. Our study was consistent with the observations of the previous study that CO\({}_{2}\) was produced by human respiration [24; 31]. In addition to the combustion, the indoor CO also was related to the status of residents; the previous studies revealed either a smoking person or person with inflammatory diseases exhaled higher CO levels than control group [38; 39]. We also found smoking was significantly positively associated with household CO in our study. According to previous studies, smoking, incense burning, and mosquito coil burning were significantly positively associated with CO, CO\({}_{2}\), SO\({}_{2}\), NO\({}_{\chi}\), and PM [40; 41; 42], and these results were consistent with our observation. The cleaning behavior would increase indoor PM\({}_{2.5}\) and CO levels; it was consistent with the previous study that indoor PM\({}_{2.5}\) and PM\({}_{5}\) levels could be elevated by the cleaning behavior of dry dust, and vacuuming [43]. In addition, commercial cleansers and disinfectants contain VOCs [44], and [PERSON] et al. found VOCs concentration was positively correlated with CO concentration [45]. We also revealed that air-conditioning use was positively associated with indoor CO\({}_{2}\) concentrations with a statistical significance, which was consistent with a previous observation that CO\({}_{2}\) levels were higher in mechanically ventilated buildings than in naturally ventilated buildings [46]. There was a significantly negative association between essential oil use and O\({}_{3}\) concentration. The commercially available essential oils contain many VOCs (e.g., D-limonene, \(\alpha\)-pinene, etc.) [47], in addition, a study displayed that indoor VOCs level had increased significantly after burning essential oils [48]. O\({}_{3}\) was one of the indoor oxidants [49; 50], and [PERSON] et al. demonstrated that 68% of all O\({}_{3}\) reactions were with D-limonene, and 26% of all O\({}_{3}\) reactions are with \(\alpha\)-pinene [50]. This might be the reason why the essential oil use could decrease the O\({}_{3}\) level. Finally, by questionnaire, it was found that window opening was significantly correlated with increased O\({}_{3}\) concentration, which was not consistent with the results from the time-microenvironment-activity-diary that only NO\({}_{2}\) and PM\({}_{2.5}\) levels during the window opening periods were significantly higher than that of reference periods. We believed O\({}_{3}\) was a major component of photochemical pollution, so it is more relevant to outdoor sources than indoor sources. Thus, compared with the households which closed the windows, the households which opened the windows had a significantly higher 24-hour average concentration of O\({}_{3}\).