# Indoor Air Quality Assessment in Grocery Stores
## 3 Results and Discussions
### Carbon Dioxide  
Figure 4 presents the CO\({}_{2}\) levels measured in all the studied locations of the GSs, where the mean CO\({}_{2}\) level assessed was 486 \(\pm\) 90 ppm, ranging from 365 \(\pm\) 16 (GS 11, Location B) to 864 \(\pm\) 178 (GS 4, Location B). It is important to highlight that since CO\({}_{2}\) is only produced by occupants, this parameter is often used as an indicator of ventilation and occupancy [33]. Moreover, all the 13 studied GSs had only natural ventilation. Figure 3: Relative humidity levels assessed in all locations of the studied GSs. The green area represents the acceptable range (50–70%) for workplaces defined by Portuguese Legislation [31]. Portuguese legislation (Portaria _n.o_ 138-G/2021, de 1 de julho) [34] establishes a limit value of 1250 ppm and all GSs presented mean levels below that threshold, which was expected since the doors are mostly open during working hours, promoting good circulation of air. Studies focusing on working environments also found similar results. For instance, a study in a copy center in Aveiro, Portugal, found similar values to the present study, with CO\({}_{2}\) levels also below the LV, with a mean level of 552 \(\pm\) 65 ppm [35]. A study performed in 14 retail stores from Texas and Pennsylvania in the US also found that the average CO\({}_{2}\) levels varied between 381 ppm and 716 ppm in all retail stores [36]. Conversely, studies conducted in residential settings showed higher levels of CO\({}_{2}\), mainly due to the poor ventilation conditions of such spaces. For instance, a study carried out during the sleeping period of couples found levels of 1911 \(\pm\) 894 ppm, which are considered very high (and above the LV), and those values were due to the type of ventilation used in bedrooms [9]. Similarly, a study carried out in Elderly Care Centers in Lisbon also found higher values of CO\({}_{2}\) in bedrooms, with all of them exceeding the LV, with an average value of 1755 ppm [37]. The number of clients per day in each GS was found to have a significant impact on the mean CO\({}_{2}\) levels registered in the GSs (Kruskal-Wallis Test: \(\chi^{2}(2)\) = 9.902, \(p\)-value = 0.007), with the levels found in the GSs with fewer than 100 clients per day (412 \(\pm\) 13 ppm) being significantly lower than the levels found in GSs with more than 200 clients per day (636 \(\pm\) 80 ppm) (\(p\)-value = 0.009, by pair-wise comparison). The size of the GSs was also found to significantly influence the indoor levels of CO\({}_{2}\), with GSs with selling areas of below 110 m\({}^{2}\) having lower levels (421 \(\pm\) 14 ppm) than GSs with selling areas of more than 110 m\({}^{2}\) (546 \(\pm\) 40) (Mann-Whitney U Test: U = 127.000, \(p\)-value = 0.007). Since CO\({}_{2}\) is mainly produced by human breathing, it is expected that indoor environments with higher numbers of occupants may have higher CO\({}_{2}\) levels. Moreover, the size of the selling area of the GSs may also be an indicator of the number of clients (bigger areas may have more clients), which explains the impact of the selling area on CO\({}_{2}\) levels. Despite the CO\({}_{2}\) levels being below the LV in all cases, this fact also highlights that workers may be exposed to higher levels of CO\({}_{2}\) in GSs with larger selling areas and with higher numbers of clients. The statistical analysis also revealed that the cleaning procedure also influenced the CO\({}_{2}\) levels, GSs that did the cleaning daily at closing having lower mean levels of CO\({}_{2}\) (422 \(\pm\) 13 ppm) than GSs that did the cleaning daily at opening or more times per day (537 \(\pm\) 39 ppm) (Mann-Whitney U Test: U = 116.000, \(p\)-value = 0.033). The type of products used for cleaning also proved to have a significant impact on CO\({}_{2}\) levels (Kruskal-Wallis Test: \(\chi^{2}(2)\) = 6.846, \(p\)-value = 0.033), with the levels of CO\({}_{2}\) in GSs that used products of  
Figure 4: CO\({}_{2}\) levels assessed in all locations of the studied GSs.