## 5 Conclusions  
This study investigated the effect of the use of NG in kitchens on human exposure to NO\({}_{2}\) under different outdoor air quality levels. To achieve this goal, NO\({}_{2}\) concentrations associated with different cooking fuels were measured in kitchens placed in two different medium-sized European cities, Porto and Aveiro. In Porto, where electric cookers predominated, and Aveiro, where natural gas cookers were prevalent, significant differences in indoor NO\({}_{2}\) levels were observed. Homes with natural gas cookers consistently exhibited higher NO\({}_{2}\) concentrations and exposure compared to those with electric cookers, aligning with previous research highlighting the emissions from gas combustion as a substantial indoor pollutant source. The exposure results reveal that average daily NO\({}_{2}\) exposure is higher in homes in Aveiro, a less polluted city where natural gas is commonly used for cooking. In these homes, exposure tends to rise with the number of hours spent indoors. In contrast, Porto exhibits NO\({}_{2}\) levels that are approximately 35.8% lower, with a trend of decreasing daily exposure as indoor time increases. The results have significant policy implications, indicating that cities with better outdoor air quality, like Aveiro, may still have residents exposed to higher levels of pollution compared to those in more polluted cities, like Porto. However, it is important to note that these findings are preliminary due to the limited sample size of nine kitchens per stove type in each city. Expanding the sample size and including kitchens from both cities for each stove type could enhance the reliability of the comparison. In addition, several factors, such as cooking behavior--particularly the duration and frequency of gas stove use--as well as the type and level of ventilation, can significantly impact indoor NO\({}_{2}\) concentrations. These factors should be carefully monitored in future studies, and guidelines should be developed to improve indoor air quality. On the other hand, Passam samplers are designed to operate accurately over a wide range of temperatures (5 to 40 \({}^{\circ}\)C) and humidity (20 to 80%); we did not anticipate that these factors would significantly influence NO\({}_{2}\) concentration measurements made in this work. However, in order to maintain experimental control, in future sampling work, the indoor temperature and relative humidity of the study domain should be recorded. The findings underscore the importance of considering cooking fuel type in indoor air quality management. Despite the absence of indoor NO\({}_{2}\) standards in many regions, including the European Union and the U.S., countries like Canada have implemented stricter guidelines. This disparity underscores the need for broader adoption of indoor air quality regulations to mitigate health risks associated with indoor NO\({}_{2}\) exposure, particularly in homes relying on natural gas for cooking. Given the amount of time we spend in our homes (around 70% of the time), the results also suggest the need for a change in current human health protection policies, moving from a partial view focused only on outdoor air quality to a holistic view where indoor air quality must also urgently be considered. Conceptualization, methodology, and validation, All authors; literature review, T.F. ; formal analysis and investigation, All authors; writing--original draft preparation, T.F. ; writing--review and editing, All authors; supervision, project administration and funding acquisition, N.B. All authors have read and agreed to the published version of the manuscript. This research received financial support from Fundacao Fernando Pessoa. Not applicable. All the data used in this article are provided throughout the article. The authors thank all the participants who contributed to this research by allowing their kitchens to be used for the installation of diffusion tubes.