## 2 Background and Related Work
### Related Work
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Paper** & **Year** & **Monitored Air Component** & **Hardware** & **Real Time** & \
\begin{tabular}{c} **Kind of** \\ **Local/Remote** \\ **Computation** \\ \end{tabular} &
\begin{tabular}{c} **Level of** \\ **Intelligence/Automation** \\ \end{tabular} & **Application Scenarios** \\ \hline
[13] & 2020 & \begin{tabular}{c} Aerosol, VOC, CO, CO\({}_{2}\), \\ temperature, and humidity \\ \end{tabular} & \begin{tabular}{c} Pollutant detection sensors, \\ microcontroller, and an LTE modem \\ \end{tabular} & Yes & Remote & \
\begin{tabular}{c} Alert system and user \\ notifications \\ \end{tabular} &
\begin{tabular}{c} General indoor \\ environment \\ \end{tabular} \\ \hline
[14] & 2018 & \begin{tabular}{c} CO\({}_{2}\), CO, SO\({}_{2}\), NO\({}_{2}\), O\({}_{3}\), Cl\({}_{2}\) \\ temperature, and humidity \\ \end{tabular} & \begin{tabular}{c} Microcontroller for gas sensor \\ nodes, wireless sensor nodes, \\ gateways, and an IoT server \\ \end{tabular} & Yes & Local and remote & \
\begin{tabular}{c} Gateway data processing \\ \end{tabular} &
\begin{tabular}{c} Residential indoor \\ environments \\ \end{tabular} \\ \hline
[15] & 2018 & \begin{tabular}{c} Temperature, humidity, pressure, \\ particle material, TVOC, \\ and eCO\({}_{2}\) \\ \end{tabular} & \begin{tabular}{c} Sensor nodes, Raspberry Pi 3, and \\ Launchpad CC260 3 kt \\ \end{tabular} & Yes & Local and remote & \
\begin{tabular}{c} Data collection \\ and analysis \\ \end{tabular} &
\begin{tabular}{c} Public indoor \\ environments \\ \end{tabular} \\ \hline
[16] & 2021 & \begin{tabular}{c} Temperature, humidity, eCO\({}_{2}\), and TVOC \\ \end{tabular} & \begin{tabular}{c} Air quality sensors BME680 and \\ CS5811, along with ESP32 \\ microcontrollers \\ \end{tabular} & Yes & Remote & \
\begin{tabular}{c} Alert system and user \\ notifications \\ \end{tabular} &
\begin{tabular}{c} Residential indoor \\ environments \\ \end{tabular} \\ \hline
[17] & 2019 & \begin{tabular}{c} CO\({}_{2}\), temperature, humidity, CO, \\ formaldehyde, and VOCs \\ \end{tabular} & \begin{tabular}{c} ZigBee wireless sensor network \\ \end{tabular} & Yes & Remote & \
\begin{tabular}{c} Alert system \\ \end{tabular} &
\begin{tabular}{c} Healthcare indoor \\ environments \\ \end{tabular} \\ \hline
[18] & 2019 & \begin{tabular}{c} CO\({}_{2}\) \\ \end{tabular} & \begin{tabular}{c} ESP8266 Thing Dye (Sparkfun) \\ microcontroller and an MIZ-19 \\ CO\({}_{2}\) sensor \\ \end{tabular} & Yes & Remote & \
\begin{tabular}{c} Moderate level of \\ automation \\ \end{tabular} &
\begin{tabular}{c} Public and residential \\ indoor environments \\ \end{tabular} \\ \hline
[19] & 2020 & \begin{tabular}{c} Temperature, humidity, and \\ PM25 levels \\ \end{tabular} & \begin{tabular}{c} Dust sensor, Arduino Leonardo board \\ \end{tabular} & Yes & Remote & \
\begin{tabular}{c} alert system and user \\ notifications \\ \end{tabular} &
\begin{tabular}{c} Healthcare indoor \\ environments \\ \end{tabular} \\ \hline
[20] & 2019 & \begin{tabular}{c} Temperature, humidity, PM2.5, CO\({}_{2}\), \\ and formaldehyde \\ \end{tabular} & \
\begin{tabular}{c} STM32 MCU module, S8485, \\ and NB-IoT. \\ \end{tabular} & Yes & Local and remote & No automation &
\begin{tabular}{c} General indoor \\ environments \\ \end{tabular} \\ \hline
[21] & 2020 & \begin{tabular}{c} VOCs, CO\({}_{2}\), PM2.5, PM10, \\ temperature, humidity \\ \end{tabular} & \
\begin{tabular}{c} HUZZAH32 microcontroller board \\ \end{tabular} & Yes & Remote & Recommendation system &
\begin{tabular}{c} Public indoor and outdoor \\ environments \\ \end{tabular} \\ \hline
[22] & 2016 & CO and HCHO gases & \begin{tabular}{c} FC, Marvel board, sensors, and \\ mobile devices \\ \end{tabular} & Yes & Local and remote & \
\begin{tabular}{c} Alert system and user \\ notifications \\ \end{tabular} &
\begin{tabular}{c} General indoor and \\ outdoor environments \\ \end{tabular} \\ \hline
[23] & 2020 & \
\begin{tabular}{c} Toxic gases like NO\({}_{2}\), CO\({}_{2}\), benzene, \\ and smoke \\ \end{tabular} &
\begin{tabular}{c} Node-MCU ESP32, MQ-135, and \\ DHT-11 sensors \\ \end{tabular} & Yes & Remote & Alert system & Industrial areas \\ \hline \hline \end{tabular}
\end{table}