## 3 Results and Discussion
### Indoor Air Pollutants Concentration in Offices
In the present study, among the aldehydes, formaldehyde, acetaldehyde, and acetone showed a noticeable differentiation in the investigated buildings. For propionaldehyde, 2-butanone, benzaldehyde and hexaldehyde, values were almost similar among the office rooms. Remarkably, the indoor to outdoor ratio was significantly higher than unity in all buildings (I/O range for formaldehyde: 3.3-11.4, acetaldehyde: 2-20 and acetone: 2.5-11.3), indicating the prevalence of indoor sources inside the studied office buildings. Indeed, formaldehyde has been associated with emissions from electronic office equipment such as laser printers and photocopiers [70]. Formaldehyde and acetaldehyde are associated with emissions from wooden-pressed products and wall paints [71; 72; 73; 74; 75]. The median formaldehyde concentration (21.93 mg\({}^{-3}\)) was below the WHO Indoor Air Quality Guideline (IAQG) of 100 mg\({}^{-3}\) derived in 2010 based on sensory irritation as the critical effect [1] The campaign findings are in agreement with the literature, since similar materials and equipment are largely used in office rooms. Nitrogen dioxide originates mainly from anthropogenic outdoor sources i.e., combustion of fossil fuels (coal, gas and oil). In the present study, indoor NO\({}_{2}\) is observed to be higher inside buildings attached to high traffic roads (Buildings 3,4 and 6). The same findings were also revealed in a recent study in schools and offices where indoor exposure to NO\({}_{2}\) from the infiltration of ambient air can be significant in urban areas, and in areas with high traffic volumes [76]. However, in the cases of B2 and B4, the NO\({}_{2}\) I/O ratio was significantly higher than unity (3.8), implying the contribution of indoor sources and particularly combustion sources such as smoking, which is an activity that occasionally occurs inside the buildings. Regarding the O\({}_{3}\), I/O ratio, the values ranged from 0.01 to 0.13. Ozone is very reactive with high oxidizing potential and thus there are many possible scenarios for ozone decay indoors. Concerning O\({}_{3}\), there was an indication of strong sinks indoors as was also reported in office and school environments with similar values [77; 78]. Building materials in offices and various types of design decorations can be considerable sinks for indoor ozone due to the irreversible heterogeneous reactions between ozone and material surfaces [79; 80]. Concentrations of PM\({}_{1}\), PM\({}_{2.5}\) and PM\({}_{10}\) during the working hours are presented in Table 4 (available for B3, B4, B5, B6). The highest mean values of fine and coarse fractions (PM\({}_{2.5}\), PM\({}_{10}\)) were observed in B4 and B6 (though not important differences for PM\({}_{2.5}\)) possibly because of their location (close to traffic roads and industrial units). The mean values of PM\({}_{2.5}\) and PM\({}_{10}\) for all buildings were below the EU annual limits (25 ug/m\({}^{3}\)) and (40 ug/m\({}^{3}\)) respectively [81]. These results are similar to those of OFFICAIR project for Greece, where mean concentrations of PM\({}_{2.5}\) was 13.3 ug/m\({}^{3}\) and 14.3 ug/m\({}^{3}\) during summer and winter [82] respectively, while being higher compared to mechanical ventilated offices [83]. Maximum values of all fractions can be attributed to penetration events caused during building cleaning activities or instant high occupancy. The highest maximum values were noticed for B4 (the building with the poorest ventilation), indirectly implying pollutants accumulation.