## 2 Overview of Developed IAQMS System  
Basically, the developed system consists of the sensor module cloud (SMC), base station and service-oriented client, as shown in Figure 1. The sensor module cloud (SMC) contains collections of sensor modules that measure the air quality data and transmit the captured data to the base station through a wireless network. Each sensor module includes an integrated sensor array that can measure IAQ parameters. There are various IAQ parameters involved in measuring the IAQ level. These parameters are divided into four categories: physical condition, chemical contaminants, biological contaminants, and other common IAQ parameters. However, for the purpose of this project, only nine parameters have been chosen, which are nitrogen dioxide (NO\({}_{2}\)), carbon dioxide (CO\({}_{2}\)), ozone (O\({}_{3}\)), carbon monoxide (CO), oxygen (O\({}_{2}\)), VOCs and particulate matter (PM\({}_{10}\)) along with temperature and humidity. This study chooses the parameters for its IAQMS based on the indoor air parameters highlighted by the Occupational Safety and Health Administration (OSHA), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the United States Environmental Protection Agency (US EPA) and Malaysian regulations on indoor air as stipulated by Department of Occupational Safety and Health (DOSH) [12; 17; 31; 32]. The sensor can be divided into three types of sensor: gas sensor, particle sensor and thermal sensor. Lists of sensors used in the proposed system along with their operational range are presented in Table 1 below. The sensors are chosen based on their detection rate, which comply with the range required by IAQ [10; 12]. Each sensor generates a voltage signal based on the current environment. These sampling voltage levels are read by the microcontroller periodically. Selecting a proper gas sensor is a relatively \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No** & **Sensor Model** & **Sensor Type** & **Target Parameter** & **Typical Detection Range** & **Required Range in IAQ** \\ \hline
1 & CDM 4161 & MOX & CO\({}_{2}\) & 400–2000 ppm & 400–1000 ppm \\
2 & TG5 5342 & Electrochemical & CO & 0–100 ppm & 0–10 ppm \\
3 & TGS 2602 & MOX & VOCs & 0–30 ppm & 0–3 ppm \\
4 & MICS-2610 & MOX & O\({}_{3}\) & 0–1 ppm & 0–0.05 ppm \\
5 & MICS-2710 & MOX & NO\({}_{2}\) & 0.05–5 ppm & 0–0.08 ppm \\
6 & KE-25 & Electrochemical & O\({}_{2}\) & 0–100\% & 19.5–23\% \\
7 & HSM20G & Thermal & Humidity & 20–95\% RH & 38–70\% RH \\
8 & GP2Y1010 AU6F & Thermal & Temperature & 0–50 \({}^{\circ}\)C & 23–26 \({}^{\circ}\)C \\ \hline
8 & GP2Y1010 AU6F & Optical & PM\({}_{10}\) & 0–0.5 mg/m\({}^{3}\) & 0–0.15 mg/m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of sensors used in the system. Figure 1: System architecture for real-time indoor air quality (IAQ) monitoring. complicated issue, as many factors need to be taken into consideration. For this study, most of the gas sensors are metal oxide (MOX)-based, while the rest are electrochemical-based. The MOX gas sensor is composed of a sensor cap, sensing element and sensor base. Basically, the gas sensing element is coated with a metal oxide--tin dioxide (SnO2)--material that responds to the gas molecules, which are typically volatile compounds [33]. It consists of two major parts; namely, the heater and sensor substrate. The substrate has two terminals, and its resistance is measured as a representation of the amount of gas concentration in the environment, while the heater provides the stabilized temperature needed for the measurement [34]. Due to its long lifetime, high sensitivity response and low cost, this type of sensor is commonly used in many indoor applications such as homes, offices and factories appliances. The second type of gas sensor used in this study is the electrochemical-based sensor. This type of sensor has high sensitivity to environmental change and it does not need power to operate.