# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 2 Background on Environmental Gases  
Some gases are the key to ensure the functionality of systems and entire industries, as well as the presence of other gases being a problem in other fields, causing the loss of entire production lines, as in the food industry, or even cause the loss of lives and explosions. In this section, the most important gases in terms of pollution monitoring and control, health issues, and accident prevention are listed with their main characteristics. **Oxygen (O2)** is the most important gas for life, and is crucial in numerous fields. Patients under anesthesia, or recovering from surgery and from certain diseases need controlled O2 doses to keep them alive and fully recovered. The decrease of oxygen levels in enclosed spaces can be related with other gases leakage, which would lead people inside these spaces to asphyxiate, leading to an unconscious state, or even to death [29]. Numerous industrial processes rely on the correct concentration of this gas to achieve the best results, mainly chemical and combustion, which without the correct percentages will not grant the best performance of systems. Engine control systems depend on the correct mixture to achieve the expected performances, whether it is lower fuel consumption or power and speed [29, 30, 31, 32]. **Carbon dioxide (CO2)** is a colorless, odorless gas, generated by the oxidation and combustion of hydrocarbon, as well as by living beings in the respiration process. It is a key gas for the greenhouse effect, and the increase of its levels, in the presence of other gases, is related with atmospheric pollution. It is also the key gas on the oxygen production by the photosynthesis process. The accumulation of this gas in enclosed spaces can be responsible to suffocation; it can be deathly when the concentration of CO2 reaches levels above 3% [33, 34, 35]. **Carbon monoxide (CO)** is a result of the incomplete combustion of hydrocarbon fuels, due to the lack of oxygen or insufficient temperature. It is an odorless, colorless gas, mainly originating in enclosed or semi-enclosed spaces, such as closed parking garages, home heaters and fireplaces. It is well known that this gas has around 200 times more affinity to hemoglobin than oxygen, making the protein unable to carry the second gas to the body cells, leading to hypoxia, causing damage to body tissue. The poisoning symptoms are easily mistaken with fatigue. Depending on the exposition time and concentrations of this gas, it can be lethal or reduce lifetime, causing cardiovascular diseases and brain damage [19, 20, 35, 36, 37, 38]. **Volatile organic compounds (VOCs)** are carbon-based organic compounds, in a vapor state at room temperature, generated by the combustion of fossil fuels, or natural emissions, where some compounds are toxic, affecting human health by causing irritation of the respiratory system and eyes, diseases as cardiovascular and respiratory malfunctions, or even cancer [39, 40]. **Liquefied petroleum gas (LPG)** is a fossil fuel composed of a mixture of hydrocarbon gases, used in domestic situations and industry, to generate electricity, power heating systems, vehicular combustible and cooking. It is a highly flammable gas, capable of severe damage if a leakage is followed by an ignition; major explosions and fire incidents were reported in the literature, due to its gas leakage, in numerous countries, such as the 2011 Karakopru incident, where an entity plant was destroyed due to an explosion. The leakage of this gas can lead to great expense and the loss of many lives [17, 18]. **Ozone (O3)** is present at the atmosphere, in high altitudes, where it is fundamental to maintaining life on Earth, acting as a natural filter to ultra violet (UV) light emitted by the Sun, avoiding skin cancer on humans and allowing the agriculture, by filtering the UVc light. The increase of this gas in lower atmospheric layers is an indicator of air pollution and bad air quality, being one of the causes of lung dysfunctions, worsen respiratory diseases [41, 42, 43, 44].