## 3 Results and Discussion
### Indoor Air Pollutants Concentration in Offices  
Descriptive data of indoor VOCs, aldehydes, O\({}_{3}\), NO\({}_{2}\) and SO\({}_{2}\) concentrations are presented in Figures 9-11. Among the measured VOCs, the highest median concentrations were found for benzene 11.0 \(\upmu\) m\({}^{-3}\) (max: 32.5 \(\upmu\) m\({}^{-3}\)) and toluene 9.9 \(\upmu\) m\({}^{-3}\) (max: 32.9 \(\upmu\) m\({}^{-3}\)), while formaldehyde and acetaldehyde showed the highest indoor concentrations of the investigated aldehydes, with median values 21.9 \(\upmu\) m\({}^{-3}\) (max: 34.8 \(\upmu\) m\({}^{-3}\)) and 13.0 \(\upmu\) m\({}^{-3}\) (max: 40.3 \(\upmu\) m\({}^{-3}\)) respectively. Median concentration for O\({}_{3}\) was 2.3 \(\upmu\) m\({}^{-3}\) (max: 7.6 \(\upmu\) m\({}^{-3}\)), for NO\({}_{2}\) was 8.2 \(\upmu\) m\({}^{-3}\) (max: 25.1 \(\upmu\) m\({}^{-3}\)) and for SO\({}_{2}\) was 0.1 \(\upmu\) m\({}^{-3}\) (max: 0.9 \(\upmu\) m\({}^{-3}\)). Acrolein, crotonaldehyde, metharcoleni and m-tolualdehyde were not detected in any building. Finally, in the majority of the buildings, octane, d -limonene and SO\({}_{2}\) concentrations were below the limit of detection values. Figure 8: COVID-19 protocols applied on offices. Figure 9: Indoor concentrations (\(\upmu\) m\({}^{-3}\)) of VOCs. As a first evaluation, the computed median and mean values of pollutant concentrations of this study were compared with the relevant seasonal (summer) values reported in the OFFICAIR project [17], a European project in which a wide scale of indoor air pollutants measurements in 37 office buildings were performed [58]. Overall, among the VOCs, benzene, toluene, and d-limonene concentrations were higher in the investigated buildings compared to OFFICAIR corresponding values (1.4, 8.1 and 4.7 \(\mathrm{\SIUnitSymbolMicro  
acteristically for benzene: 1.1 (B3), 0.5 (B4), 0.7 (B6). However, VOCs are associated with indoor sources, as well. Specifically, toluene has been characteristically associated with environmental tobacco smoke [66]. Indeed, B1, B2, B3 present high levels of toluene likely due to smoking activity which was taking place either inside the office room (B1, I/O = 8) or in an adjacent outdoor smoking permitted area (B2, B3: I/O = 2), where smoke penetrated indoors. Another indoor source of toluene and xylenes (usually combined with the presence of aldehydes) is emissions from building materials, paints, adhesives etc. [18; 67; 68]. A typical case is the one of the recently renovated building B7 (toluene I/O = 2.1; m,p-xylene I/O = 22) as well as the offices of B1 (toluene I/O = 8) which is adjacent to a tiles and cement-based products unit. Last but not least, the measured concentrations inside a building are significantly influenced by the air renewal pattern of the building. Building 4, which is characterized by inadequate natural or mechanical ventilation conditions (B4 was rated as one of the lowest in perception for fresh air), presents comparatively higher concentrations of terpen (limonene, a-pinene) and xylenes, probably due to pollutants accumulation inside the building. Measured d-limonene and a-pinene median concentrations were considerably below the short term critical exposure limit (CEL) of 45 mg m\({}^{-3}\) and 90 mg m\({}^{-3}\) respectively, developed within the EPHECT (Emissions, exposure patterns and health effects of consumer products in the EU), project based on sensory irritation as the critical effect [69]. Carbonyls and aldehydes can originate from a wide variety of sources (building materials, equipment, adhesives, paints, smoking etc.,), the strength of which may significantly depend on the season i.e., differences in temperature, relative humidity and other environmental conditions [18]. Aldehydes also originate from O\({}_{3}\)-initiated reactions with many common and reactive VOCs that contain unsaturated carbon-carbon bonds, which react with O\({}_{3}\) much more rapidly than saturated organic compounds.