## 5 Target Gases  
In this section the most relevant gases in the areas of pollution monitoring, health issue and industries are listed and described. The continuous control of gas levels is necessary to reduce environmental pollution, to maintain safe working conditions, to prevent health disorders and to decrease industrial equipment failures. For each gas described below, there are various sensors studied and used for different applications. As examples, some of these sensors are quoted in the gas references. Hydrogen (\(H_{2}\))Is colorless, odorless and tasteless gas, highly flammable. It forms explosive mixtures with air. The most abundant element in the universe, otherwise it is quite rare in the Earth atmosphere. It is mostly a man-made product that is used as a reagent in chemical industry, as fuel, as a power generator, or as cooler in power plants. It is a marker in human breath related to metabolic diseases (e.g., lactose intolerance or malabsorption) [93; 94; 95]. Oxygen (\(O_{2}\))--Is the main gas necessary for the life and it is one of the three major constituents of Earth's atmosphere together with nitrogen and argon, and it is present at about 21% vol. In closed spaces, O\({}_{2}\) can decrease in concentration becoming seriously dangerous for the health. It is used in the treatments of medical diseases, in chemical and combustion processes and in automotive field, where suitably mixed to fuel allows best performances and reduces the fuel consumption [96]. Ozone (\(O_{3}\))--Is an atmospheric gas with a pungent characteristic odor, irritant and poisoning for human subjects. In the ozonosphere, it is in high concentrations and it filters the sun's ultra violet rays which are otherwise dangerous for living beings. In the lower atmosphere, it is an air pollutant that causes lung dysfunctions and worsens respiratory diseases [97; 98]. As chlorine, it is used in water treatment and purification. Water vapor (\(H_{2}\)O)--Is the Earth's primary greenhouse gas trapping more heat than carbon dioxide. The measurement and control of water vapor is important in many areas, such as meteorology, medicine, industry and agriculture [99]. Absolute humidity (AH) is the measure in g/m\({}^{3}\) of water vapor in the air not dependent of temperature, while the relative humidity (RH) is the ratio of moisture in the air to the saturation level of moisture (at same temperature and pressure). Generally, the developed humidity sensors are calibrated to measure RH [100; 101; 102], thereby the absolute humidity can be measured from RH and the ambient temperature. On the other hand, H\({}_{2}\)O is also crucial, being an interfering gas with respect to the most gases to be detected, pollutant or not. In this case, absolute humidity rather than relative humidity is the crucial parameter that determines the effects of humidity on the sensor's response [103]. Carbon monoxide (CO)Is an odorless, colorless and flammable gas created when the combustion of hydrocarbon fuels or in a forest fire takes place in shortage of oxygen or with an insufficient temperature. Due to the fact that its affinity to hemoglobin is higher than that of oxygen, when CO level in the environment is higher than 35 ppm, it binds with hemoglobin obstructing the O\({}_{2}\) transportation in the body of animals and human beings. The consequences are hypoxia, body tissue damaging, cardiovascular diseases until to death [104]. Carbon dioxide (CO\({}_{2}\))--Is an odorless and colorless gas. It is used as reagent for the photosynthesis by plants to synthesize oxygen and glucose and it is produced during the respiration process in humans and animals. It is also produced during the combustion processes. It acts as atmospheric pollutant and it is known as the major gas responsible for the greenhouse effect. When its concentration increases can cause suffocation and if it reaches levels above 3% [104] becomes lethal. Nitric oxide (NO)_--Is a colorless, toxic, irritant and corrosive gas.