## 4 Results and Discussion
The few studies that have measured both short-term and long-term NO\({}_{2}\) exposures confirm this relationship [2; 8; 10]. It is also important to note that larger dwellings in Aveiro city, with gas stoves, tend to have higher NO\({}_{2}\) values, when compared to smaller dwellings. In Porto, with electric stoves, this pattern, as expected, does not occur. In both cities, the NO\({}_{2}\) indoor and outdoor concentration values tend to decrease from the lowest to the highest floors, further away from road emission sources. This is particularly observed in the houses located in the city of Aveiro. Concerning the I/O ratio, the pattern is identical to that observed previously, with both cities behaving in a clearly different way. Aveiro has I/O values that are always greater than 1 and the Porto region has I/O values that are always less than 1 (Figure 2). Assuming that the values observed in both cities can be indicatively compared with the NO\({}_{2}\) air quality guideline level of the WHO annual average (10 ug\(\cdot\)m\({}^{-3}\)), it can be observed that, for both cities, both indoors and outdoors, this guideline is being exceeded. Looking at the NO\({}_{2}\) annual average limit value of EU directive 2008/50/EC (40 ug\(\cdot\)m\({}^{-3}\)), four of the nine dwellings in Aveiro exceed this value, unlike the observations made in the Porto region where there is no exceedance indoors. Finally, if we take the annual average limit value to be taken by the new air quality directive (20 ug\(\cdot\)m\({}^{-3}\)), all the observations made indoors in Aveiro exceed this value, while, in Porto, only two observations out of eight exceed this standard. As far as meteorology is concerned, no significant differences were found in the values of the meteorological variables observed in the two cities during the campaigns. The average absolute difference in temperature was around 0.6 \({}^{\circ}\)C, in relative humidity less than 4%, and in wind intensity less than 0.7 m.s\({}^{-1}\). We spend around 70% of our time in our homes, which makes residential air quality of crucial importance [52]. Based on this, it is important to consider for the assessment the \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & & **Indoors** & & & **Outdoors** & & \\
**City/Fuel Type** & **N** & **Min.** & **Max.** & **Avg. \(\pm\) SEM** & **Min.** & **Max.** & **Avg. \(\pm\) SEM** & **I/O** \\ \hline
**Fuel type** & & & & & & & \\ - NG (Aveiro) & 9 & 23.8 & 59.9 & 38.6 \(\pm\) 11.3 & 13.8 & 29.6 & 20.4 \(\pm\) 5.3 & 1.90 \\ - Electricity (Porto) & 9 & 10.8 & 26.1 & 16.7 \(\pm\) 5.5 & 23.1 & 34.2 & 28.8 \(\pm\) 3.6 & 0.58 \\
**House typology (flats)** & & & & & & & \\ - NG (Aveiro) & & & & & & & \\ - T2 & 1 & - & - & 30.0 & - & - & 17.7 & 1.69 \\ - T3 & 5 & 29.2 & 46.5 & 38.4 \(\pm\) 8.0 & 17.1 & 29.6 & 23.2 \(\pm\) 6.1 & 1.64 \\ - T4 & 2 & 43.8 & 59.9 & 51.9 \(\pm\) 11.4 & 13.8 & 10.8 & 16.8 \(\pm\) 4.2 & 3.09 \\ - Electricity (Porto) & & & & & & & \\ - T3 & 3 & 13.4 & 23.8 & 18.0 \(\pm\) 5.3 & 28.1 & 31.0 & 29.7 \(\pm\) 1.5 & 0.61 \\
**- T4** & 4 & 10.8 & 26.1 & 17.6 \(\pm\) 7.8 & 23.1 & 34.2 & 29.5 \(\pm\) 5.0 & 0.60 \\
**House type (flats)** & & & & & & & \\ - NG (Aveiro) & & & & & & & \\ - 1 st floor & 1 & - & - & 46.5 & - & - & 29.6 & 1.57 \\ - 2 nd floor & 3 & 43.8 & 59.9 & 49.8 \(\pm\) 8.8 & 13.8 & 28.8 & 20.8 \(\pm\) 7.5 & 2.39 \\ - 3 rd floor & 2 & 29.2 & 31.1 & 30.2 \(\pm\) 1.3 & 17.1 & 23.3 & 20.2 \(\pm\) 4.4 & 1.49 \\ - 5 th floor & 1 & - & - & 37.8 & - & - & 17.1 & 2.21 \\
**- Electricity (Porto)** & & & & & & & & \\ - 1 st floor & 1 & - & - & 10.8 & - & - & 23.1 & 0.47 \\ - 2 nd floor & 2 & 16.7 & 23.8 & 20.3 \(\pm\) 5.0 & 28.1 & 29.9 & 29.0 & 0.70 \\ - 3 rd floor & 1 & - & - & - & - & - & 34.2 & - \\ - 4 th floor & 1 & - & - & 13.4 & - & - & 31.0 & 0.43 \\ - 5 th floor & 1 & - & - & 26.1 & - & - & 32.7 & 0.80 \\ - 9 th floor & 1 & - & - & 16.0 & - & - & 28.0 & 0.57 \\ \hline \hline \end{tabular} N: n.\({}^{\circ}\) of samples, Min: minimum, Max: maximum, Avg: average, SEM: Standard Error of the Mean.  
\end{table}