## 3 Methodology  
### Sources of Indoor Air Pollution  
Higher IAP levels could lead to certain health effects, while extremely high IAP levels could be fatal. Different IAP parameters may come from different sources and impose different health effects towards humans, as shown in Table 3. Table 3 shows that there are certainly a lot of activities and conditions which could trigger IAP, such as air fresheners, combustion appliances, water damage, cigarettes, fire and combustion equipment [10; 12; 13]. However, for the purpose of this study, the sources of IAP are limited to five conditions only, because they are commonly present in the indoor environment: ambient air, combustion activity, the presence of chemical products, the presence of food and beverages, and the presence of fragrances [17; 37; 38; 39]. The first condition of sources of indoor air pollution is the ambient air. Ambient air refers to the air that normally exists in the indoor environment without the presence of other sources of indoor air pollution. Ambient air could pollute the indoor air if it carries excessive dust from carpets and furniture or if it carries too much ozone from office machines [17]. The second condition of sources of IAP is combustion activity. Combustion activities such as smoking cigarettes and burning fire-wood release poisonous gases such as CO and CO\({}_{2}\), and PM at a higher concentration than ambient air does, which could harm human's health [37; 39].  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{2}{c}{**Node 1**} & \multicolumn{2}{c}{**Node 2**} & \multicolumn{2}{c}{**Node 3**} & \multicolumn{2}{c}{**Aeroqual**} \\ \cline{2-9}  & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** \\ \hline NO\({}_{2}\) (ppb) (0–80 ppb) & 35.9 & 2.0 & 34.9 & 1.7 & 34.6 & 1.7 & 35.5 & 2.2 \\ \hline Temperature (\({}^{\circ}\)C) (23–26 \({}^{\circ}\)C) & 25.6 & 0.3 & 26.5 & 0.1 & 25.7 & 0.1 & 25.7 & 0.1 \\ \hline Humidity (\%) 38–70\% & 39.7 & 0.8 & 39.6 & 0.6 & 39.3 & 0.6 & 40.2 & 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Means and standard deviations for three parameters.  
Figure 2: (a) NO\({}_{2}\) data; (b) Temperature data; (c) Humidity data.  
For the purpose of this study, cigarette smoking has been chosen as the proxy for combustion activities. The third condition of sources of indoor air pollution is the presence of chemical products or substances. Chemical products such as chemical cleaning agents which are usually used in homes and offices may release VOCs at a poisonous level. Excessive level of VOCs may lead to respiratory-related diseases, such as lung cancer [17]. Thus, for the purpose of this study, chemical cleaning products will be used as the proxy for the presence of chemicals. The fourth condition for sources of indoor air pollution is the presence of food and beverages. Cooking activity or certain food and beverages emit VOCs which could lead to an uncomfortable smell inside a building [38]. VOCs are known to have led to eye irritation, headache and nausea in certain people. Therefore, rotten cooked fish, which has a strong smell and a high level of VOCs, is chosen as the proxy to food and beverages.  
Finally, the presence of fragrances is the fifth condition for the sources of indoor air pollution. Fragrances such as air fresheners and perfumes usually deliver a pleasant smell. However, excessive use of perfumes may cause annoyance and headaches to certain people. In addition, air fresheners usually emit a high amount of VOCs, which may cause irritation and discomfort to certain people [17]. For this project, air freshener is used to substitute for the presence of fragrances. Table 4 summarizes the five conditions for the sources of indoor air pollution and their proxies which have been used in this study.