## 1 Introduction
### State-of-the-Art Indoor Air Quality Measurement Systems

Table 1: Acceptable levels of pollutant concentrations occurring in indoor air, according to World Health Organization (WHO) recommendations supported by the results of the EU research program HealthVent, values recommended by the standard EN 16798-1:2019 (replacing EN15251:2007), values recommended by the Chinese indoor air quality (IAQ) standard GB/T 18883-2002 and for acceptable levels of pollution for the certification of office buildings in Hong Kong. Theoretical work on a combined IAQ model allowing aggregation of the results of the assessment of components affecting humans [14] is not yet well recognized in the literature. However, studies on IAQ indicators, which aim to provide a quantitative description of indoor air pollution, have been conducted since the nineties. In 2003, a significant study by [PERSON] et al. [15] was published related to the standard indoor pollutant index (IPSI), the disease symptom index in the building symptom index (BSI), and to the often-cited works by [PERSON] and [PERSON] [16; 17] on the indoor environmental index (IEI), indoor air pollution index (IAPI), and the indoor pollutant standard index (IPSI). The IAPI characterizes air pollution in an office with a single number: the index. The index value ranges between zero (lowest pollution level, i.e., best indoor air quality) and 10 (highest pollution level i.e., worst indoor air quality). The IAPI is a composite index; sub-indices ed are aggregated using the arithmetic mean in conjunction with a tree-structured calculation scheme. This scheme gives rise to some reservations, because at the top of the tree-structured calculation scheme is the IEI (calculated as the arithmetic mean of the IAPI and the IDI (indoor air discomfort index), and the combination of IAQ sensation and thermal conditions does not appear until later. While considering the indicators for the quantitative description of pollution, the proposal of the IEA Working Group named \"Defining the Metrics of IAQ\" should also be mentioned. This group prepared, in 2017, the document entitled \"In the Search of Indices to Evaluate the Indoor Air Quality of Low-Energy Residential Buildings\" [18]. The group made the following assumption for the categorization of various indicators: there should be one index per individual pollutant and a dimensionless coefficient should be specified to evaluate the IAQ, provided that the current (observed) concentrations of a given pollutant \(c_{j}\) are related to the ELVs (exposure limit values) concentration \(c_{j,ELV}\). \[IAQ_{index}(j)=I_{j}=\frac{c_{j}}{c_{j,ELV}} \tag{1}\]  
The index is calculated for each individual pollutant [18], which is specific only for this exact pollutant. The report showed that aggregation can be performed by addition, by taking the maximum value or by other methods, in an attempt to define metrics that can be used to evaluate IAQ. The assumption was that the reference value usually refers to health risks (accounting for chronic or acute effects), but other metrics can also be used, (e.g., odor or irritation threshold). There are two important properties to be considered when aggregating sub-indices: ambiguity and eclipsing. As a result of the analysis, the authors concluded \"that there are problems with model aggregation methods. In the aggregation model \(I_{agg}=I_{1}+I_{2}\), ambiguity creates a false alarm and in the aggregation model \(I_{agg}=1/2(I_{1}+I_{2})\), eclipsing underestimates the effect\" [18]. Therefore, the discussion remains open [18]. The report also showed how there are large spreads of concentrations of individual pollutants (up to seven rows), even in the group of pollutants for which sub-indices were built. It determined the difficulties of building a weighted scheme based on the simplest percentage adjustment of the concentration shares and, thus, the share of the mass of pollutants to be removed by ventilation. The current state of knowledge does not provide information authorizing the omission of certain pollutants.