# The Combined Effect of Indoor Air Quality and Socioeconomic Factors on Health in Northeast China
## 4 Discussion
### Link Between Socioeconomic Factors, Indoor Air Quality, and Health

Table 4: The model evaluation results (see Figure 4). in nearly 40.7% of the investigated houses. Only 7.4% of homes failed to reach TVOC concentration standard (0.6 mg/m\({}^{3}\), National Indoor Air Quality GB18883-2002). Furthermore, the formaldehyde in all the houses were below 100 ug/m\({}^{3}\) (National Indoor Air Quality GB18883-2002). As can be seen, TVOC and formaldehyde, as the indicators for indoor air pollution by building materials and fitting [29], are not major indoor air pollutants in the investigated houses. The primary reason is that about 82.8% of residential buildings were completed in 2010 or before (Table 3). Indoor air pollution from PM\({}_{2.5}\) and CO\({}_{2}\) (the indicator of contamination caused by persons) has presented a noteworthy challenge. Moreover, CO\({}_{2}\) concentration was related to the density of people indoors (as shown in Figure S3). About 42% of the variance in the indoor CO\({}_{2}\) concentration can be explained by the number of persons per floor area. Socioeconomic factors, including income level (0.595 \(\times\) 0.381 = 0.227), education (0.551 \(\times\) 0.381 = 0.209), and occupation prestige (0.508 \(\times\) 0.381 = 0.194), shared the similar contribution to indoor air quality. According to Table 5, PM\({}_{2.5}\) and CO\({}_{2}\) concentration both decreased steadily, with the growth of education level. For example, PM\({}_{2.5}\) and CO\({}_{2}\) concentration dropped by 8.4% and 10.4%, respectively, from middle school less to university or professional subgroup. Despite some minor fluctuations, TVOC and HCHO concentration showed the downward trend. For income level and occupation group, PM\({}_{2.5}\) and CO\({}_{2}\) concentration both followed a similar decline pattern. TVOC concentration generally still maintained the decrease. HCHO concentration fluctuated slightly. Furthermore, it can be found that there were significant differences for smoking status among the education level subgroups. A total of 45.0% of respondents reported smoking in the middle school or less subgroup, but the proportion dropped sharply to just 15.3% for the master, PhD, or specialization group. A large body of research has demonstrated that smoking has the great influence on pollutants indoors [52]. It was estimated that increases of PM\({}_{2.5}\) concentration in homes with smokers ranged from 25 to 45 ug/m\({}^{3}\)[53]. Smoking also strongly influenced the indoor concentration of VOC (\(p<\) 0.05) [54]. Moreover, smoking was known to have significant effects on health, such as carcinogenic, reproductive, and/or acute or chronic respiratory effects. For example, Canadian Tobacco, Alcohol and Drugs Survey revealed that around 52.5% of current smokers reported 'excellent' or'very good' health, compared to 58% of former smokers, and significantly lower than 69% of never smokers [55]. [PERSON] et al. estimated the effect of cigarette smoking on health among 26,504 American high school seniors, and found that respondents who were regular smokers were significantly more likely to report shortness of breath, coughing spells, productive cough, and wheezing or gasping [56]. There were no significant differences for household size among the income level subgroups, i.e., high income level (2.21), middle income level (2.12), and low income level (2.21). However, the average floor area in the high level income subgroup was 144.9 m\({}^{2}\), which was much higher than low level income subgroup (68.7 m\({}^{2}\)). Hence, crowding may be common in the low level subgroup, leading to high indoor air pollutant level.