# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 2 Background on Environmental Gases
**Sulfur hexafluoride (SF6)** is an odorless, colorless, chemically stable, non-flammable gas used as electrical isolator in the electrical power industry, due to its capability of extinguishes electrical arcs in high tension. It is a non-toxic greenhouse gas, and it is not a risk if inhaled in proportions under 20%. The detection of this gas leakage is essential to prevent damage to high-power electrical equipment, and with that, avoiding failures on power distribution; the detection can be done by the \(SF_{6}\) itself or their sub-products, generated by electrical discharges [45, 46, 47, 48, 49, 50, 51, 52, 53]. **Radon (Rn)** is a colorless, odorless, radioactive gas that can be emitted from soil and rocks like granite and its long-term exposition is related to lung cancer. It can be generated from the decay of radium (Ra) and uranium (U), and it has a half-life from approximately 3.8 days. Radon can be transported through water, or carrier gases such as CO\({}_{2}\), methane (CH\({}_{4}\)), Helium (He), and other gases. As this gas represent half of the radiation exposure to human beings, detection is crucial to avoid long term exposition, being a key factor in lung cancer prevention, mainly in underground miners, that suffer more contact with this radioactive gas [54, 55, 56, 57, 58, 59, 33]. **Ammonia (NH3)** is an irritant, corrosive, colorless gas, with a strong odor, employed in the production of fertilizers and explosives, as well as in the textile industry. It can be found as a refrigerant gas and in hygienic products. Its leakage can cause atmospheric, soil and water pollution, and severe damages to the eyes and respiratory system, causing even the death of people directly affected by the gas escape [60, 61, 62, 63, 20, 64]. **Nitric oxide (NO)** is a colorless gas that can be lethal if its concentrations reach certain ambient levels. In the atmosphere, it is one of the compounds responsible to the smog pollution, causing irritation to exposed people. It is employed in the semiconductor industry, as well as on medicine, as muscular relaxant and in the treatment of hypertension, due to its vasodilation property. This gas can be one of the sub-products of fossil fuel combustion, and it is also produced naturally, by the human body, being an important signal of inflammation if its concentration in human breath reaches levels above 50 parts per billion (ppb); it is used to monitor the inflammation conditions of asthma patients' lungs. The oxidation of NO results in nitrogen dioxide (NO\({}_{2}\)) [65, 66]. **Nitrogen dioxide (NO2)** is a result of the combustion process of fossil fuels, as well as the oxidation of nitrogen. It is one of the pollutants responsible for the formation of acid rain, and it is toxic at low levels, 1 part per million (ppm) is the maximum recommended contact volume; its exposure is related with respiratory diseases, pulmonary malfunction and death. NO\({}_{2}\) can be found in highway surroundings, as well as dense traffic areas; it can also be found indoors, as a result of the combustion of generators and heaters [67, 68, 39, 70]. **Hydrogen sulphide (H2S)** is a colorless, flammable, corrosive and toxic gas, which is poisonous even in low concentrations. Exposure to this gas can lead to damage to the human nervous system. Its generation can be either natural, from volcanic activities or from the decomposition of organic compounds, or due to the combustion of fossil fuels or from sewage. This gas can be found in coal mines, petroleum exploration and in diverse industrial processes [71, 72, 73]. **Chlorine (Cl2)** is broadly used in the chemical and pharmaceutical industries, water treatment and in domestic cleaning products. With a strong odor, in the gaseous state, it is extremely toxic; the exposure limit in workplaces are around 30 ppb. The inhalation of low concentration levels (50 ppm) of Cl\({}_{2}\) can cause severe damage to the respiratory system and levels of 1000 ppm are enough to be fatal to humans.