## 3 The Nature of Malodorous Gases in Relation to Their Detection
& \\  & & & Limited selectivity—response to groups of compounds. Temperature, humidity and pressure affect sensitivity and stability. Ageing of biological elements. Calibration issues. Cross-sensitivity to other gases causing signal disturbances and misinterpretation. & \\ \hline  & & Low selectivity of individual sensors—sensors respond to groups of compounds. Temperature, humidity and pressure affect sensitivity and stability. & \\ Electronic nose & \multirow{2}{*}{ppm–\%} & \multirow{2}{*}{Effects—interference from other components causing signal disturbances and misinterpretation. Low misinterpretation. Low misinterpretation. To determine the value of the odour concentration, the number of so-called odour units is specified. The number of odour units indicates how often the sample is diluted with an inert gas (without substances that cause odour sensations) until the odour detection threshold is reached. The inert gas is usually clean air or nitrogen. Devices called olfactometers are used for this purpose [55; 56]. In a sample containing a type of odorous substance, the odour concentration can be calculated using quantitative chemical analysis based on the following Equation (1), which, however, requires knowledge of the previously determined odour detection threshold. \[S_{OC(\text{ou/m}^{3})}=\frac{S_{O(\text{ug/m}^{3})}}{S_{ODT(\text{ug/ou})}} \tag{1}\]  
where \(S_{OC(\text{ou/m}^{3})}\)--odour concentration [ou/m\({}^{3}\)]; \(S_{O(\text{ug/m}^{3})}\)--odorant concentration [ug/m\({}^{3}\)] and \(S_{ODT(\text{ug/ou})}\)--odour detection threshold [ug/ou]. Human odour receptors are stimulated by odour-producing substances in certain concentrations. Both too low and too high concentrations can lead to a lack of such experiences. If we only add an odour-intensive compound to odourless air (inert gas), we can determine the lowest concentration at which our body perceives its presence, the so-called odour detection threshold of this compound (\(S_{ODT}\) (ug/m\({}^{3}\))). The odour detection threshold is therefore the minimum concentration of a particular compound that triggers an odour sensation in a certain percentage of the population studied (usually 50% of a representative group of people) [57]. For gases that contain more than one odour-forming compound, the odour detection threshold is determined in a similar way. However, the sum of the detection thresholds of the individual components of the mixture is not equal to the odour detection threshold of the entire mixture (Equation (2)). \[S_{ODT_{mix}\ A,B,C,\ldots}\
eq\sum_{i=A}^{i=N}S_{ODT_{i}} \tag{2}\]  
The odour threshold of a particular mixture should be determined experimentally. The value of the odour concentration is expressed by the so-called odour unit. It is defined as the amount of odorous substance present in one cubic metre of gas that causes the odour threshold to be reached (1 ou/m\({}^{3}\)--odour unit). In Europe, n-butanol (123 ug/m\({}^{3}\) = 0.040 umol/mol) is usually used as the standard for determining the odour unit in environmental studies. This is indicated in the symbols by the corresponding index--ou\({}_{\text{E}}\).