## 5 Target Gases
It is an unavoidable by-product of fossil fuel combustion in presence of air and it is one of the primary air pollutants. It is used in the semiconductor industry and as vasodilator in medical treatments. It is a biomarker in human breath related to lung inflammation when the concentration is above 50 ppb [105]. Nitrogen dioxide (NO2)--Is a reddish-brown gas above 21.2 \({}^{\circ}\)C with a pungent, acid odor. NO2 is produced in combustion processes of fossil fuels and in the oxidation of nitrogen. It is present in heavy traffic areas, while indoors it is produced by heaters. It is a pollutant that contributes to acid rains, and it results toxic at low levels with a threshold contact level of 1 ppm. Its exposure causes respiratory diseases until possible death. [106; 107]. Ammonia (NH3)--Is an irritant, corrosive, flammable colorless gas, with a strong pungent odor. It is used in fertilizers for intensive agriculture, in hygienic products, in textile production, as refriger gas and in explosives [108; 109]. When its concentration exceeds the natural background that is down to few ppb, it could cause atmospheric, soil and water pollution, and severe damage for human health. In exhaled human breath, it is related to the liver failure [110] or disturbed urea balance (kidney disorder) [111], in both cases in concentration of few hundred of ppb. Hydrogen sulphide (H2S)--Is a colorless, flammable and corrosive gas with characteristic odor, resulting toxic even in low concentrations. It derives from volcanic activities, from decomposition of organic compounds and from the combustion of fossil fuel. Hydrogen sulfide is also the by-product of some industrial activities such as the food industry, water purification from sludge, coke production, leather tanning and oil refining [112]. The medical effects of H2S depend on its concentration and the duration of exposure: from 10 to 500 ppm can cause various respiratory diseases and temporary or permanent damages in the nervous, cardiovascular, renal, hepatic, and hematological systems, while in concentrations over the 500-1000 ppm H2S is immediately fatal [113]. Sulfur dioxide (SO2)--Is a colorless toxic gas with a stifling smell that is released during volcanic activity and produced in the burning of fossil fuels contaminated with sulfur compounds. It is one of the main atmospheric gaseous pollutants. Its reactivity with other substances in the atmosphere causes a wide variety of health and environmental negative effects, such as respiratory diseases, vision impairments, acid precipitations that damage buildings and plants, etc. Excessive exposure to SO2 causes problems to eyes, lungs and throat [114; 115]. Volatile organic compounds (VOCs)--Are a wide group constituted by carbon-based organic compounds (among them halogenated compounds, aldehydes, alcohols, ketones, aromatic compounds, and others), that easy evaporate in ambient condition [116]. VOCs are one of the major contributors to air pollution and their emissions from outdoor and indoor sources are growing due to rapid industrialization and urbanization. High concentrations of VOCs can cause health disorders or serious disease as cancer [117]. VOCs are also present in small concentration in human breath and they could be used as natural biomarkers to medical diagnosis [116]. Some of main VOCs are described below:  
Methane (CH4)--Is odorless colorless tasteless, no toxic but flammable. It results from decomposition of some organic compounds in the lack of oxygen. It is extracted from underground deposits, where it is often combined with other hydrocarbons. Primarily, it is used as fuel in home activities and in automotive field. It is a greenhouse gas, whose most important emission sources are the decomposition of landfill waste, the extraction from fossil fuels, and the digestive process in animals (livestock). In breath analysis it is a biomarker associated with lactose intolerance [95; 117]. Ethylene (C2H4)--Is a colorless gas with a slight sweetish smell and extremely flammable.