## 5 Target Gases
It is used by chemical industry as raw material to produce other VOCs and various plastic materials (e.g., polyethylene). Furthermore, it is involved in the ripening process of climacter fruits. Ethylene can be thought as a hormone that triggers the ripening process of fruits, and also as an indicator that fruit is ripening. Indeed, keeping suitable concentrations of ethylene during fruit storage it is possible to speed up or slow down ripening [4]. Isoprene (C5H6)--Is a colorless liquid and with a characteristic odor. In industry, it is used mainly to produce polymeric compounds. It is the major hydrocarbon found in human exhaled breath, ranging from 12.71 to 227 ppb in healthy human subjects. Its level increases naturally in human subjects with age and during physical activity. A high concentration is also correlated with chronic kidney disease (CKD) and other pathological states (hemodialysis, general anesthesia, liver disease, and cancer) [118, 119]. _Benzene_ (\(C_{6}H_{6}\)) and _Toluene_ (\(C_{7}H_{8}\))--Are colorless, toxic and carcinogenic liquids, with characteristic odor, that are naturally present in petroleum products and are subsequently released in the atmosphere during the incomplete combustion in road traffic. They are used in chemical industry and as solvents for paints, gums, adhesives, etc. These gases are responsible for the ozone layer reduction, they produce photochemical smog and they cause uneasiness at low-level of exposure, unconsciousness, dizziness, and even death at high level of exposure [116, 120]. _Formaldehyde_ (\(CH_{2}O\))--Is a colorless, corrosive, flammable toxic and carcinogenic gas. It is known as formalin in water solution. Formaldehyde is a powerful bactericide used to make disinfectants; it is used as a food preservative and in the industrial textile dyeing. However, it is mainly used in the production of polymers and other materials employed to build handwork, coatings and insulating foams that release over time molecules of formaldehyde in the environment. Formaldehyde is one of the most widespread indoor pollutants, with no effects on health up to 0.1 ppm; at higher concentrations it irritates mucous membranes and eyes, up to become potentially lethal. It is a potential breath marker for lung cancer [121]. _Acetone_ (\(C_{6}H_{3}O\))--Is a colorless flammable liquid, with an irritant characteristic odor. It is primarily used as a solvent, also at industrial level. It is a natural biomarker associated with some metabolic diseases, like the diabetes (few hundred of ppb in healthy people, more than 1 ppm in diabetic subjects) [83]. _Ethanol_ (\(C_{2}H_{5}OH\))--Colorless, alcoholic smell and taste, flammable, low toxicity liquid. Produced in nature by sugar fermentation, it is the most widespread alcohol, and the only one suitable for food use. It is used as alternative fuel, as disinfectant and as s solvent for resins and paints. Most swallowed ethanol is metabolized in the body, while a small amount is eliminated through the urine, the sweat and the breathed air. It represents a natural biomarker related to alcohol consumption [122, 123]. _Liquefied petroleum gas_ (\(LPG\))--Is a fossil fuel composed by a mixture of hydrocarbon gases, broadly employed in domestic environment and industry, to generate electricity, power heating systems, or cooking. It is also used as vehicular combustible. It is a highly flammable gas and dangerous because a leakage could result in ignition and explosion [124, 125]. _Sulfur hexafluoride_ (\(SF_{6}\))--Is a colorless, odorless, transparent and not flammable gas. It is not considered as toxic gas and it is used in industry as electrical insulator because of its capability of extinguishing electrical arcs in high tension [126]. However, it is one of the six gases responsible of greenhouse effects covered by the Kyoto Protocol. \(SF_{6}\) easily hydrolys into fluorinated compounds in water that are extremely toxic and corrosive. _Chlorine_ (\(Cl_{2}\))--In gaseous state has a strong odor and it is extremely toxic.