###### Abstract  
Natural gas (NG) is commonly used in kitchens, powering stoves, ovens, and other appliances. While it is known for its efficiency and convenience, NG contributes to the release of nitrogen dioxide (NO\({}_{2}\)) and can have significant implications for human health. In this study, the importance of the use of NG in kitchens on human exposure to NO\({}_{2}\) was analyzed. An extensive literature review in the field was conducted, and the NO\({}_{2}\) levels were assessed in kitchens with NG cookers in Aveiro and electric cookers in Porto, both in Portugal. Higher levels of NO\({}_{2}\) were found in kitchens in Aveiro, where NO\({}_{2}\) levels outdoors are lower than in Porto. This pollutant can spread to other rooms, especially when ventilation is lacking, which is particularly concerning during colder seasons and at night. As around 70% of the time is spent at home, this can have a significant impact on human exposure to NO\({}_{2}\). Therefore, although Aveiro has low levels of NO\({}_{2}\) outdoors, its population may be exposed to much higher levels of this pollutant than the Porto population, a city with air quality issues, but predominantly using electric cookers. This finding emphasizes the need for the stricter regulation of NG use indoors to protect human health and also suggests a shift in human health protection policies from mere monitoring/control of outdoor air quality to a comprehensive assessment of human exposure, including exposure to indoor air quality. nitrogen dioxide; kitchens; indoor air quality; human exposure; natural gas Article Impact of Kitchen Natural Gas Use on Indoor Human Health: A Case Study in two European Cities. * 2024  
respiratory disease, cardiovascular disease, and diabetes [9]. Despite these significant health concerns and the increasing use of natural gas, research in this domain is scarce. According to the Web of Science database in May 2024, there were only 14 studies dedicated to examining the concentrations of NO\({}_{2}\) in kitchens resulting from the use of natural gas. The search was conducted directly on the topic, i.e., title, abstract, keyword plus, and author keywords, using the following query: (NO\({}_{2}\) & Kitchen & (\"natural gas\" or NO)). These studies compared different types of environments, fuels, ventilation conditions, and exposure periods. Among the studies analyzed, some measure the concentrations in the short term, typically less than 1 h [2], while others focus on long-term exposure, generally several days or weeks [10]. Few studies were found in the literature monitoring periods of less than 1 day and more than 1 week. Tables 1 and 2 summarize the main characteristics and findings of the studies in the field for short- and long-term periods, respectively. Several authors compared the combustion products generated in kitchens from different fuel types. The impact of using biomass fuels [11; 12; 13; 14], electricity [15; 16; 17; 18; 19], coal gas [20], liquefied petroleum gas [11; 12], propane gas [19; 21], and natural gas [10; 12; 14; 15; 16; 19; 20; 22; 23; 24] was analyzed by several authors. Some of the studies compared indoor concentration in kitchens with outdoor levels [3; 10; 14; 16; 18; 25; 26; 27; 28]. High concentrations of NO\({}_{2}\) were found in kitchens using natural gas cooking burners, compared to homes with electric cooking [15; 16; 17]. [PERSON] et al. [17] found that the average NO\({}_{2}\) levels in kitchens with a gas cooker were twice as high as those with an electric cooker, with no significant difference in the summer period, while [PERSON] et al. [16] find that the average hourly concentration of NO\({}_{2}\) in gas kitchens was more than seven times greater than that in electric kitchens. NO\({}_{2}\) concentrations are even higher in the case of biomass burning. [PERSON] et al. [13] found NO\({}_{2}\) concentrations from biomass burning ranging from between 0 and 978 \(\mu\)g\(\cdot\)m\({}^{-3}\). Indoor levels are also influenced by infiltration through ventilation and building materials.