# The Combined Effect of Indoor Air Quality and Socioeconomic Factors on Health in Northeast China
## 2 Materials and Methods  
### Data Source  
Field measurement was conducted in six major cities (Harbin, Changchun, Shenyang, Jinzhou, Anshan, and Qiqihar) of Northeast China. Northeast China, as China's traditional industrial base, has a total population of about 100 million people, and is mainly located in the severe cold climate region. The cities were chosen on the basis of population distribution. Figure 1 presents the population density of Northeast China in accordance with China's Sixth National Population Census in 2010 [28]. The population densities of Harbin, Changchun, Shenyang, Jinzhou, and Anshan are all higher than 300 people per square kilometer, and Qiqihar is slightly lower (200-300 people per square kilometer). A total of 81 residential buildings were selected by the method of random digit dialing. All residential indoor environment were continuously monitored from 20 November 2014 to 24 February 2015 during winter central heating which is supplied by the government. The investigation of residential indoor environment consisted of two parts: Measurements of indoor air environment parameters and a self-administered questionnaire survey. CO\({}_{2}\), PM\({}_{2.5}\), TVOC, and formaldehyde concentration were selected as the criteria for indoor air quality. Because CO\({}_{2}\) concentration is a good indicator of contamination caused by persons indoors, while TVOC, and formaldehyde (HCHO) concentration are indicators for indoor air pollution by building materials and fittings [29]. PM\({}_{2.5}\) concentration represents the source of indoor air contamination caused by particulate matter. Table 1 shows the characteristics of measuring instruments. Indoor air parameters were monitored at the breathing zone height, i.e., 1.1 m above the floor, as recommended by the National Indoor Air Quality Standard (GB/T 18883-2002) [30]. Moreover, indoor environmental parameters were measured simultaneously in the living room, bedroom, kitchen to represent the conditions of the indoor environment accurately. The measurement data were recorded every five or ten minutes. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Indicator** & **Instrument** & **Range** & **Accuracy** \\ \hline CO\({}_{2}\) & MCH-383 SD & 0 \(\sim\) 4000 ppm & \(\pm\)5\% \\ PM\({}_{2.5}\) & SHINYEI PM SENSOR & 1 \(\sim\) 1000 \(\mu\)g/m\({}^{3}\) & \(\pm\)1\% \\ HCHO & SHINYEI IAQ SENSING & 0 \(\sim\) 5 mg/m\({}^{3}\) & \(\pm\)2\% \\ TVOC & GC112A & 0 \(\sim\) 1000 ppm & \(\pm\)2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of instruments for measuring the indoor environment parameters. Figure 1: The population density of Northeast China and the number of investigated residential buildings. The self-administered questionnaire included health variables, socioeconomic status, lifestyle and personal characteristics:  
Health variables: Health was measured by the widely used tool--the Short Form 8 Health Survey (SF-8) (as shown in Table S1). The reliability and validity of the Chinese version of the SF-8 Health Survey were confirmed [31]. The SF-8 Health Survey includes physical and mental components, and each component covers four sub-scales, the physical health: Physical functioning (PF), role physical (RP), bodily pain (BP) and general health (GH); the mental health: Vitality (VT), social functioning (SF), role emotional (RE) and mental health (MH). For example, role emotional refers to 'have you had problems with your work or other regular daily activities (such as walking or climbing stairs) as a result of any emotional problems?' Bodily pain indicates 'how much bodily pain have you had?' Each sub-scale was scored on a scale of 1 to 4 (1 represents the worst and 4 the best health status). Socioeconomic status: Socioeconomic factors are typically determined by the level of education, income, and occupation prestige [32].