## 4 Results and Discussion

Table 5: Statistics of NO\({}_{2}\) concentrations obtained during field campaigns (ug\(\cdot\)m\({}^{-3}\)). outdoor air levels, but also the indoor ones. Figure 3 outlines the daily exposure of the Aveiro and Porto population to NO\({}_{2}\) concentrations for different scenarios. The exposure results indicate that the average daily exposure to NO\({}_{2}\) levels is higher in homes located in the city of Aveiro, a less polluted city where people typically use natural gas for cooking. In these homes, exposure tends to increase as the number of hours spent indoors increases. Conversely, in Porto, the NO\({}_{2}\) concentration levels are approximately 35.8% lower, and the trend shows a decrease in daily exposure as the number of hours spent indoors increases. In fact, the population of small-/medium-sized European cities with no problems in terms of outdoor air quality, such as Aveiro, may be exposed to high levels of NO\({}_{2}\) due to the consumption of NG in their kitchens. On the other hand, the population of medium-sized cities, such as Porto, with air quality problems, particularly in terms of NO\({}_{2}\), may be less exposed to this pollutant if they do not use NG in their cooking. This is not to suggest that outdoor air quality is unimportant, quite the opposite. Outdoor air quality is crucial to ensuring good ventilation of indoor spaces. However, the findings indicate that the good air quality observed in Aveiro was insufficient to ensure acceptable NO\({}_{2}\) levels in its kitchens, where natural gas is used. Conversely, the poor outdoor air quality in Porto did not significantly degrade the NO\({}_{2}\) levels observed in its kitchens, which are generally powered by electricity. The results highlight that stricter air quality monitoring and enforcement are needed. Air is a fundamental resource for human health and well-being. The quality of the outdoor air, where we spend part of our time and which we use to ventilate our buildings, is important, but, above all, the quality of the indoor air, where we spend around 90% of our time (70% at home), is crucial for our health and well-being. Therefore, besides the restrictions on outdoor levels, policies should focus on improving indoor air quality through better ventilation systems and stricter regulations on indoor pollution sources to align with WHO guidelines, aiming for long-term health benefits. In Aveiro, the high NO\({}_{2}\) levels found indoors indicate a pressing need for public health interventions, including raising awareness about the health impacts of NO\({}_{2}\), and providing information on mitigation strategies such as the improving ventilation. Given the differences between Aveiro and Porto, policies should be tailored to regional conditions. Aveiro may require more aggressive measures to tackle indoor air quality issues compared to Porto, which currently shows better compliance with air quality standards. Therefore, in regions where air quality is poor, financial support or incentives may be needed for households to improve their ventilation systems and/or invest in new cookers with cleaner technologies. Figure 3: Integrated daily NO\({}_{2}\) exposure for different scenarios (\(\upmu\)g\(\cdot\)m\({}^{-3}-\)day).