# The Combined Effect of Indoor Air Quality and Socioeconomic Factors on Health in Northeast China
## 3 Results
### Description of the Indoor Air Pollution Level  
Figure 3 depicts the distribution of average values of each indoor air environment parameter during the measuring period for all the investigated houses. The median CO\({}_{2}\) level was 901.1 ppm (25 th percentile: 677.4 ppm; 75 th percentile: 1263.2 ppm). The lowest level was 309.61 ppm, and the highest level was 3239.31 ppm. The median concentration of TVOC was 0.28 mg/m\({}^{3}\) (25 th percentile: 0.14 mg/m\({}^{3}\); 75 th percentile: 0.29 mg/m\({}^{3}\)). The highest and lowest values were 0.04 mg/m\({}^{3}\) and 1.3 mg/m\({}^{3}\), respectively. Formaldehyde levels ranged from 3.1 ug/m\({}^{3}\) and 81.4 ug/m\({}^{3}\) (25 th percentile: 13.9 ug/m\({}^{3}\); 50 th percentile: 21.0 ug/m\({}^{3}\); 75 th percentile: 30.5 ug/m\({}^{3}\)). The median PM\({}_{2.5}\) concentration was 42.0 ug/m\({}^{3}\) (25 th percentile: 30.0 ug/m\({}^{3}\); 75 th percentile: 55.6 ug/m\({}^{3}\)) and ranged from 11.0 ug/m\({}^{3}\) to 132.6 ug/m\({}^{3}\).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Indicator**} & \multicolumn{4}{c}{**Category**} \\ \cline{2-5}  & **I** & **II** & **III** & **IV** \\ \hline CO\({}_{2}\) (ppm) & 700 & 700–850 & 850–1150 & \(>\)1150 \\ PM\({}_{2.5}\) (\(\mu\)g/m\({}^{3}\)) & 15 & 15–40 & 40–65 & \(>\)65 \\ HCHO (\(\mu\)g/m\({}^{3}\)) & \(<\)30 & 30–65 & 65–100 & \(>\)100 \\ TVOC (\(\mu\)g/m\({}^{3}\)) & \(<\)200 & 200–400 & 400–600 & \(>\)600 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameter limit values for the indoor environmental quality categories.  
According to Figure 3, there were high levels of average CO\({}_{2}\) and PM\({}_{2.5}\) concentration for the investigated houses. As can be seen from Figures S1 and S2, CO\({}_{2}\) concentration remained high, especially at night. There were CO\({}_{2}\) and PM\({}_{2.5}\) peak exposures during the period of time, such as cooking time. This was due to the fact that there was insufficient ventilation for most investigated houses. It was estimated that a large majority of residents opened windows for less than 10 min per day to avoid heat loss [42], and approximately 71% of homes had an air change rate lower than 0.5 h\({}^{-1}\) in winter [43]. Moreover, there were a lack of mechanical ventilation systems in most residential houses. The reason for this is that the majority of the houses had an air change rate lower than 0.5 h\({}^{-1}\) in winter [43].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Information** & **N (\%)** & **Information** & **N (\%)** \\ \hline Houses & & & Education level & \\ Construction date & & Master, PhD, or specialization & 26 (17.2\%) \\ Before 1990 & 9 (11.1\%) & University or Professional & 74 (49.0\%) \\
1990–2000 & 16 (19.8\%) & Middle school or less & 51 (33.8\%) \\
2000–2010 & 42 (51.9\%) & Length of residence & \\ After 2010 & 14 (17.3\%) & \(\leq\)5 years & 38 (25.2\%) \\ Number of people per floor area (person/m\({}^{2}\)) & & 5 \(\sim\) 10 years & 55 (36.4\%) \\ \(\leq\)0.02 & 15 (18.5\%) & \(\geq\)10 years & 58 (38.4\%) \\
0.02–0.03 & 22 (27.2\%) & Lifestyle & \\
0.03–0.04 & 21 (25.9\%) & Smoking status & \\ \(\geq\)0.04 & 23 (28.4\%) & Yes & 44 (29.2\%) \\ Respondents & & No & 107 (70.8\%) \\ Sex & & Alcohol consumption & \\ Men & 72 (47.7\%) & Yes & 96 (63.8\%) \\ Women & 79 (52.3\%) & No & 55 (36.2\%) \\ Age & & & \\ \(\leq\)30 & 22 (14.6\%) & & \\
30–50 & 101 (66.9\%) & & \\ \(\geq\)50 & 28 (18.5\%) & & \\ \hline \end{tabular}
\end{table}
Table 3: The information on the houses measured and respondents.  
Figure 3: Distribution of the average values of indoor environmental parameters. Boxplots show medians, 25 \(\sim\) 75 percentiles (box) and min-max (whiskers). The red dash lines indicate the standard value recommended by the National Indoor Air Quality Standard [30].  
buildings of Northeast China. Furthermore, burning coal was still the main source of central heating, leading to indoor and outdoor environmental pollution [44].