## 3 Materials and Methods
### Site Selection  
Sampling was conducted in dwellings using natural gas stoves and dwellings using electrical stoves from two Portuguese cities, Porto and Aveiro. Porto is the largest city in the northern region with a population of approximately 237,000, while Aveiro, is a smaller city with around 78,000 residents. The reason why two cities were used instead of just one is to emphasize that the population of a city with no outdoor air quality problems, such as Aveiro, which mostly uses NG for cooking, is more exposed to NO2 than the population of the Porto region, a city with outdoor air quality problems but that mostly uses electric cookers for cooking.  
Aveiro has an extensive household natural gas network, while Porto predominantly relies on electricity. To the best of the authors' knowledge, there are no statistical data on  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Authority, Ref.**} & \multirow{2}{*}{**1 / O \({}^{a}\)**} & **Year to** & **Max.** & **Max.** & **Max.** & **Annual** \\  & & **Be Met** & **15 min** & **1 h** & **24 h** & **Concentration** \\  & & **Average** & **Average** & **Average** & **Average** & **Concentration** \\ \hline WHO [45] & Both & 1992 & - & 400 & 150 & - \\ WHO [46] & Both & 2000 & - & 200 & - & 40 \\ WHO [39] & Both & 2021 & - & 200 & 25 & 10 \\ \hline China [47] & I & 2022 & - & 200 & - & - \\ Canada \({}^{b}\)[48] & I & 2015 & - & 170 & - & 20 \\ Canada \({}^{b}\)[48] & I & 1987 & - & 480 & - & 100 \\ \hline Australia [42] & O & 2021 & - & 182 & - & 38.5 \\ Australia [43] & O & 2025 & - & 150 & - & 28.2 \\ EU [41] & O & 2010 & - & 200 (18) & - & 40 \\ Norway [44] & O & 2013 & 300 & 100 & - & 30 \\ US EPA [49] & O & 1971 & - & - & - & 100 \\ US EPA [49] & O & 2010 & - & 188 & - & - \\ South Africa [50] & O & 2002 & - & 200 (88) & - & 40 (0) \\ \hline \hline \end{tabular} \({}^{a}\) I: indoors, O: outdoors. \({}^{b}\) Values defined for the short term and long term.  
\end{table}
Table 3: Air quality NO2 (\(\upmu\)g\(\cdot\)m\({}^{-3}\)) guidelines and standards for several countries and organizations over time.  
the number of natural gas cookers per capita in either city. Despite this, there are figures for domestic natural gas consumption. In fact, in 2022, the city of Aveiro had a 363% higher per capita domestic consumption of natural gas when compared to the city of Porto ([[https://www.portata.pt/municipios/consumo+de+gas+natural+por+habitante-485](https://www.portata.pt/municipios/consumo+de+gas+natural+por+habitante-485)]([https://www.portata.pt/municipios/consumo+de+gas+natural+por+habitante-485](https://www.portata.pt/municipios/consumo+de+gas+natural+por+habitante-485)), accessed on 9 January 2024). Therefore, to represent the most common scenario, in our field study in Porto, all the kitchens were equipped with electric cookers, whereas, in Aveiro, all the kitchens had natural gas cookers.  
In this work, we define a \"cooktop\" as a flat surface with individual gas burners/electric coils, typically four or five per stove, and an occasional \"griddle\" element. The \"oven\" is an enclosed heated space, usually containing two burners: a \"bake\" burner below the bottom steel plate and a \"broil\" burner, typically exposed and affixed to the oven's roof. A \"stove\" is a freestanding unit that contains both a cooktop and an oven, typically both using gas, although some combine a gas cooktop with an electric oven. All cookers had an cooker hood.  
Aveiro and Porto, both coastal cities, are separated by approximately 60 km. Porto has a temperate maritime climate, with rainy and relatively mild winters, and moderately warm summers. Aveiro experiences a similar climate, but its closer proximity to the ocean leads to constant winds and higher humidity. Both cities see increased precipitation during autumn and winter. Although the two cities are relatively close and in the same climate zone, the meteorological variables of temperature, humidity, and wind speed were recorded during the campaigns.