*3.4. Concentrations of Selected Gaseous Pollutants in Jordanian Dwellings*

The indoor activities documented in the eight dwellings were associated with emissions of gaseous pollutants for which exceptionally high concentrations were observed (Figures S1–S8). For example, the shisha smoking and preceding preparation (i.e., charcoal combustion) were associated with CO concentrations that reached as high as 10 ppm in D1 and GFA3. The CO concentrations were further elevated in H1, with concentrations approaching 100 ppm. Emissions of SO2 were also recorded in D1 during charcoal combustion that accompanied shisha smoking. During shisha smoking, the CO concentrations exceeded the exposure level of 6 ppm due to smoking a single cigarette, as reported by Breland et al. [56], and 2.7 ppm as reported by Eissenberg and Shihadeh [52]. Previous studies have reported CO concentrations in the range of 24–32 ppm during shisha smoking events [51–53].

The eight dwellings exhibited variable concentrations of TVOCs, NO2, and HCHO. For instance, TVOC concentrations were in the range of 100–1000 ppm in A2 and H2, whereas they were in the range of 1000–10,000 ppm in all ground floor apartments (GFA1, GFA2, and GFA3). NO2 concentrations were in the range of 0.01–1 ppm in the duplex apartment (D1), ground floor apartments (GFA1, GFA2, and GFA3), and houses (H1 and H2). HCHO concentrations were in the range of 0.01–1 ppm in A2 and GFA1 and reached as high as 5 ppm in H2. O3 was not detected in any of the dwellings. It should be noted that the gaseous pollutant concentrations presented here are estimates and are likely uncertain due to technical limitations of the low-cost sensing module employed.

### *3.5. Indoor Versus Outdoor Particle Concentrations*

It is important to note that the indoor aerosol measurement periods at each dwelling were short during the winter campaign. Outdoor aerosol measurements were made on a few occasions at each dwelling; however, they were not of sufficient length to make meaningful conclusions about the aerosol indoor-to-outdoor relationship. However, comprehensive measurements of ambient aerosols have been made in the urban background in Amman [40,41,59–62], for which comparisons with the indoor measurements presented in this study can be made.

In the urban background atmosphere of Amman [62], outdoor PN concentrations were typically higher during the winter compared to the summer; the ratio can be 2–3 based on the daily means. Based on the hourly mean, the outdoor PN concentration had a clear diurnal and weekly pattern, with high concentrations during the workdays, especially during traffic rush hours. For example, the PN concentration diurnal pattern was characterized by two peaks: morning and afternoon. The afternoon peak (wintertime highest concentration range of 3 <sup>×</sup> 104–3.5 <sup>×</sup> 104 cm<sup>−</sup>3) was rather similar on all weekdays; however, the first peak was higher on workdays compared to weekends (wintertime highest concentration range of 4.5 <sup>×</sup> 104–6.5 <sup>×</sup> 104 cm−3). The lowest outdoor concentrations were typically observed between 3:00 to 6:00 in the morning, when they are as low as 1.8 <sup>×</sup> 104 cm−<sup>3</sup> during the wintertime.

When compared to the results reported in this study (Tables 4–7), the mean indoor PN concentrations were generally higher than those outdoors during the daytime, when indoor activities were taking place. For example, PN concentrations inside all dwellings were less than 1.5 <sup>×</sup> <sup>10</sup><sup>4</sup> cm−<sup>3</sup> between midnight and early morning; i.e., in the absence of indoor activities. However, the overall mean PN concentrations during the winter campaign inside the studied dwellings were in the range of 1.6 <sup>×</sup> 104–1.3 <sup>×</sup> 105 cm<sup>−</sup>3. Looking at the mean concentrations during the indoor activities, the PN concentrations were as high as 4.7 <sup>×</sup> 10<sup>4</sup> cm−<sup>3</sup> during non-combustion cooking activities. During cooking activities conducted on a natural gas stove, the PN concentrations were in the range of 4.6 × <sup>10</sup>4–3.8 <sup>×</sup> <sup>10</sup><sup>5</sup> cm<sup>−</sup>3. The combination of cooking activities and combustion processes (as the main source of heating) increased the PN concentrations to be in the range of 6.8 <sup>×</sup> 104–2.7 <sup>×</sup> 10<sup>5</sup> cm−3. Grilling sausages and burger indoors was associated with a substantial increase in mean PN concentrations, with levels reaching as high as 3.8 <sup>×</sup> 105 cm−<sup>3</sup> (PM2.5 = 378 <sup>μ</sup>g/m<sup>3</sup> and PM10 = 2094 <sup>μ</sup>g/m3). Both tobacco and shisha smoking were also associated with significant increases in PN concentrations, with levels reaching 9.1 <sup>×</sup> 104–4.0 <sup>×</sup> <sup>10</sup><sup>5</sup> cm<sup>−</sup>3.

It is very well documented in the literature that the temporal variation in indoor aerosol concentrations closely follows those outdoors in the absence of indoor activities [20,30,32,63–74]. As such, the aerosol indoor-to-outdoor relationship depends on the size-resolved particle penetration factor for the building envelope, the ventilation and infiltration rates, and the size-resolved deposition rate onto available indoor surfaces [20,30,64]. As can be seen here, and also reported in previous studies, indoor aerosol emission sources, which are closely connected to human activities indoors, produce aerosol concentrations that are usually several times higher than those found outdoors [17,75–77]. Indoor aerosol sources can thus have a significant adverse impact on human health given that people spend the majority of their time indoors [10,11,32].
