## 4 Results and Discussion  
Descriptive statistics of NO\({}_{2}\) concentrations, categorized by fuel type, house typology, and house type, are outlined in Table 5. Additionally, the concentrations obtained at each point are shown in Figure 1, and indoor-to-outdoor ratio levels are presented in Figure 2.  
Different patterns of NO\({}_{2}\) levels are found in the two cities. In Aveiro, the highest levels are observed indoors (59.9 ug\(\cdot\)m\({}^{-3}\)) and the lowest outdoors (13.8 ug\(\cdot\)m\({}^{-3}\)). In the Porto region, the pattern is the opposite, with lower values indoors (10.8 ug\(\cdot\)m\({}^{-3}\)) and higher values outdoors (34.2 ug\(\cdot\)m\({}^{-3}\)). The average NO\({}_{2}\) concentration observed in kitchens in the city of Aveiro is 38.6 ug\(\cdot\)m\({}^{-3}\), as opposed to the average value outdoors, which is 20.4 ug\(\cdot\)m\({}^{-3}\), corresponding to an average I/O ratio value of 1.90. On the other hand, in the kitchens of homes in the Porto region, the average value of NO\({}_{2}\) concentrations observed is 16.7 ug\(\cdot\)m\({}^{-3}\), which is 56.7% lower than the concentration observed in Aveiro. Furthermore, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**ID** & **City** & **House Characteristics** & & \\  & & **Type** & **Typology** & **a** & **Total n.a** of Inhab.** & **Cooking** \\  & & & & **(n.a of Children b)** & \\ \hline P1 & Aveiro & House & T5 & 3 (0) & NG \\ P2 & Aveiro & Flat (2 nd floor) & T4 & 4 (0) & NG \\ P3 & Aveiro & Flat (3 rd floor) & T3 & 3 (0) & NG \\ P4 & Aveiro & Flat (5 th floor) & T3 & 4 (0) & NG \\ P5 & Aveiro & Flat (2 nd floor) & T3 & 4 (0) & NG \\ P6 & Aveiro & Flat (G floor) & T2 & 3 (0) & NG \\ P7 & Aveiro & Flat (1 st floor) & T3 & 2 (0) & NG \\ P8 & Aveiro & Flat (3 rd floor) & T3 & 2 (0) & NG \\ P9 & Aveiro & Flat (2 nd floor) & T4 & 2 (0) & NG \\ P10 & Porto & House & T4 & 3 (0) & Electric \\ P11 & Porto & House & T4 & 3 (0) & Electric \\ P12 & Porto & Flat (3 rd floor) & T4 & 5 (0) & Electric \\ P13 & Porto & Flat (1 st floor) & T4 & 4 (0) & Electric \\ P14 & Porto & Flat (2 nd floor) & T3 & 4 (0) & Electric \\ P15 & Porto & Flat (5 th floor) & T4 & 3 (0) & Electric \\ P16 & Porto & Flat (9 th floor) & T4 & 4 (0) & Electric \\ P17 & Porto & Flat (4 th floor) & T3 & 4 (0) & Electric \\ P18 & Porto & Flat (2 nd floor) & T3 & 4 (2) & Electric \\ \hline \hline \end{tabular}  
* Ti: one bedroom, dining room, kitchen, and bathroom (at least one). b Number of inhabitants under 18 years old.  
\end{table} 
Table 4: Main characteristics of samples collected. in the Porto region, the average value observed outdoors is 28.8 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\), which is 41.1% more than the concentration observed in Aveiro. These concentrations corresponded to an average I/O ratio of 0.58 in Porto, which is 69.3% lower than the I/O observed in Aveiro. Regarding the standard error of indoor NO2 data, we observed a higher value for Aveiro (11.3 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)) compared to Porto (5.3 \(\mathrm{\SIUnitSymbolMicro m^{-3}}\)). This discrepancy is due to the presence of indoor NO2 sources, particularly the use of natural gas appliances in Aveiro kitchens. By applying the Shapiro-Wilk test, we observed that the indoor NO2 concentration levels follow the normal distribution in both cities, Aveiro (\(p\) = 0.843) and Porto (\(p\) = 0.216). Therefore, despite the small sample size, values obtained are compatible with a normal distribution. Significantly higher levels of NO2 were found in homes with gas appliances (Aveiro) compared with homes without gas appliances (Porto) (\(p\) < 0.001, \(t\)-test). If we look at the results presented in detail in Figure 1, we can see that the results are consistent in both cities, both indoors and outdoors, following the described pattern. In Aveiro, indoor NO2 concentrations are consistently higher than outdoor levels, whereas in Porto, the opposite trend is observed. magnitude, especially near the source.