## 3 The Nature of Malodorous Gases in Relation to Their Detection  
In order to take appropriate measures to eliminate odorous air pollution, a reliable inventory of the sources of odour emissions must first be carried out, which is often a major challenge for the services [28]. This is due to the fact that the mechanism of human odour detection is still not fully understood [29]. The relationship between the odour of chemical substances and their structure as well as the relationship between the perceived odour and the composition of the gas mixture [30] has also not yet been clarified. These facts make the development of universal, reliable and generally accepted odour detectors difficult or even impossible. Currently, most of the methods used are based on the human sense of smell as the detector for odour-intensive air pollution. Unfortunately, this solution is often criticised and its credibility questioned due to the subjectivity of the human nose and sense of smell, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Name** & **Semi-Structural/** & **Odour** & **Odour** & **Ref.** \\  & **Empirical Formula** & **Threshold [ppb]** & **Description** & **Ref.** \\ \hline ethanethiol (ethyl mercaptan) & C\({}_{2}\)H\({}_{5}\)SH & 0.011 & leek, onion & [26] \\  & /C\({}_{2}\)H\({}_{6}\)S & & & \\ \hline indole & C\({}_{8}\)H\({}_{7}\)N & 0.0014 & repulsive, faeces & [26; 27] \\ \hline
1-4 dimethylphenol (p-cresol) & C\({}_{7}\)H\({}_{8}\)O & 0.0018 & - & [26] \\ \hline pentanoic acid (valeric acid) & C\({}_{5}\)H\({}_{10}\)O\({}_{2}\) & 0.005 & unpleasant, sweet, honey-like & [26] \\  & /CH\({}_{3}\)(CH\({}_{2}\))\({}_{3}\)COOH & 0.006 & faeces & [27] \\ \hline skatole & C\({}_{9}\)H\({}_{9}\)N & 0.000565 & faeces & [27] \\ \hline methanethiol (methyl mercaptan) & CH\({}_{3}\)SH & 0.07 & rotten cabbage & [26; 27] \\  & /CH\({}_{4}\)S & 0.001 & rotten vegetables, garlic & [26; 27] \\ \hline
2–thiaethane (dimethyl sulphide) & (CH\({}_{3}\))\({}_{2}\)S & 0.21 & rotten vegetables, garlic & [26; 27] \\  & /C\({}_{2}\)H\({}_{6}\)S & 0.004 & garlic & [26; 27] \\ \hline hydrogen sulphide & H\({}_{2}\)S & 0.47 & rotten eggs & [26; 27] \\ \hline trimethylamine & (CH\({}_{3}\))\({}_{3}\)N & 4 & fish & [26; 27] \\  & /C\({}_{3}\)H\({}_{9}\)N & 1.7 & sharp, irritating & [26; 27] \\ \hline ammonia & NH\({}_{3}\) & 10 & sharp, irritating & [26; 27] \\  & /NH\({}_{3}\) & 5.75 & sharp, irritating & [26; 27] \\ \hline sulphur dioxide & SO\({}_{2}\) & 10 & sharp, garlic & [26; 27] \\ \hline xylene & C\({}_{6}\)H\({}_{4}\)(CH\({}_{3}\))\({}_{2}\) & 38 & plastic-like & [27] \\  & /C\({}_{8}\)H\({}_{10}\) & & & \\ \hline toluene & C\({}_{6}\)H\({}_{5}\)CH\({}_{3}\) & & & \\  & /C\({}_{7}\)H\({}_{8}\) & 46 & fruity, tart, & [27] \\ \hline benzene & C\({}_{6}\)H\({}_{6}\) & 270 & paint-thinner & [27] \\ \hline dimethylamine & (CH\({}_{3}\))\({}_{2}\)NH & 340 & fish & [27] \\  & /C\({}_{2}\)H\({}_{7}\)N & 0.02 & fish & [26; 27] \\ \hline acetic acid & C\({}_{2}\)H\({}_{4}\)O\({}_{2}\) & 1000 & vinegar & [27] \\  & /CH\({}_{3}\)COOH & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Selected chemical compounds responsible for odour nuisance in the air and their detection thresholds. as well as a number of other objective difficulties [31]. It is therefore necessary to search for, test and optimise alternative techniques for monitoring and measuring malodorous compounds, taking into account the diagnosed limitations of the methods used so far. In the nomenclature of environmental engineering, odours are all substances that have the ability to stimulate the nerve cells of the olfactory epithelium, both those that are generally perceived as unpleasant and those that evoke pleasant experiences [32]. Odour nuisance arises from exposure to compounds that are classified as unpleasant and disturbing odours, but also from constant exposure to compounds that may evoke pleasant sensations or from their excessive concentration [33].