## 4 Discussion
### Geochemical Indices and So-Called Contamination Classes
For example, according to As, both dust of Klaipeda (Table S5: mean As is 5.26 \(\mu\)g \({}^{-1}\), so _Igeo_'N = 0.04) and dust of Guilin (_Igeo_'N = 0.13) [86] were attributed to the same \(SCC\) \"_uncontaminated to moderately contaminated_\". However, soil related \(Bn\) value of As used for calculation of this index in Guilin was 20.5 \(\mu\)g g\({}^{-1}\), while \(Bn\) used in Klaipeda was 2.9 \(\mu\)g g\({}^{-1}\). Taking into account the formula (2), we found that the mean value of As in dust of Guilin was 33.6 \(\mu\)g g\({}^{-1}\). Let us suppose that such an As content was determined in Klaipeda dust. In this case (Table S2: _Bn_ is 2.9 \(\mu\)g g\({}^{-1}\)), the \(SCC\) for As in Klaipeda became \"moderately/strongly contaminated\" (_Igeo_ would be 2.95), i.e., formally, dust would be considered as much more contaminated than in Guilin. But does this mean that the residents of Guilin inhale much cleaner ambient air than in Klaipeda, when the detected real content of As in dust of Guilin (33.6 \(\mu\)g g\({}^{-1}\)) is even 6.4 higher than in Klaipeda (5.26 \(\mu\)g g\({}^{-1}\))? Maybe due to various aforementioned reasons, there are a lack of manuscripts where the researchers provide responsible comparison of \(SCC\) distinguished by them using gl of dust with data presented in other publications. After attempts to compare their results with data published by others, [PERSON] et al. [44] politely but aptly notice: \"This shows the difficulty of directly comparing results obtained for various cities. The main challenge here is the method of determining the geochemical background that plays a key role in the case of some pollution indices\". Taking into account the material of this subsection we dared to propose to refuse from using the term \"contamination classes\", if they are distinguished according to geochemical indices of dust. Classification into such classes was firstly applied not for dust (Section 2.5). The attempts to adapt gl for dust without changes of terminology often cause confusion, since the assignment to certain SCC is understood in the society as the index of ambient air contamination. However, this assignment is related to subjective factor, i.e., when different values are taken in the denominator (Equations (1)-(4)) during calculation of gl, so manipulations are possible. Even small difference in selected \(Cv\) values (the study of [PERSON] and [PERSON] [32] contains data of all eleven compilers of _Clarke values_) or \(Bn\) values the problems of selection of which have been described by us [25,87], can determine attribution of sample to different \(SCC\). However, whichever unique \(Cv\) or \(Bn\) value is chosen, the level of analyte content in ambient air is the same. In general, we do not object to using some of geochemical indices, like _Igeo_ or _aE (Kk)_, for evaluation of dust relative contamination level using the terminology \"less\", \"more\", or \"most\" contaminated only within the dataset which is studied. However, the terminology should be selected responsibly and purposefully as much as possible. Having in mind that either _soil_- or _upper continental crust_-related values are in denominators of _Igeo_' formulas, when using them, the term \"_enrichment_\" can be proposed instead of the terms \"contamination\" or \"pollution\". Following [PERSON] [39], we tried to apply namely such terms (\"_enrichment_\") for _aE'N_ (Section 2.5). For example, in the case of _Igeo_'N, the term \"_(top)soil-enrichment Igeo index_\", while in case of _Igeo_'C--_\"Clarke enrichment Igeo index_\", or similar alternatives can be used. In our opinion, _normalized enrichment factors_ cannot be used for naming \"_contamination classes_\". Their purpose is different: help in attempts to find out the origin of presumable contaminant in dust, probably also the emission source of dust and hotspots formed by it. Namely for this aim they are most commonly used, although there is warning about their indiscriminate usage [88,89].