**4. Conclusions**

Indoor air quality has been given very little attention in the Middle East. Residential indoor environments in Jordan have unique characteristics with respect to size, ventilation modes, occupancy, activities, cooking styles, and heating processes. These factors vary between the winter and summer. In this study, we reported the results of one of the first comprehensive indoor aerosol measurement campaigns conducted in Jordanian indoor environments. Our methodology was based on the use of portable aerosol instruments covering different particle diameter ranges, from which we could investigate particle number and mass size distributions during different indoor activities. We focused on standard particle size fractions (submicron versus micron, fine versus coarse). The study provides valuable information regarding exposure levels to a wide range of pollutant sources that are commonly found in Jordanian dwellings.

In the absence of indoor activities, indoor PN concentrations varied among the dwellings and were in the range of 6 <sup>×</sup> 103–1.5 <sup>×</sup> 104 cm−<sup>3</sup> (corresponding PM2.5 of 5–12 <sup>μ</sup>g/m3). The coarse PN concentrations were in the range of 0.4–5.2 cm−<sup>3</sup> (corresponding PMcoarse of 0.9–39.9 μg/m3). Indoor activities significantly impacted indoor air quality by increasing exposure to particle concentrations that exceeded what could be observed outdoors. Non-combustion cooking activities (microwave, brewing coffee, and toasting bread) had the smallest impact on indoor aerosol concentrations. During such activities, the PN concentrations were in the range of 1.1 <sup>×</sup> 104–4.7 <sup>×</sup> 10<sup>4</sup> cm<sup>−</sup>3, PM2.5 concentrations were in the range of 7–25 μg/m3, micron PN concentrations were in the range of 1–9 cm−3, and PM10 concentrations were in the range of 44–181 μg/m3. Cooking on a natural gas stove had a more pronounced impact on indoor aerosol concentrations compared to non-combustion cooking, with measured PN concentrations in the range of 4.6 <sup>×</sup> 104–2.1 <sup>×</sup> 105 cm−3, PM2.5 concentrations in the range of 16–88 μg/m3, micron PN concentrations in the range of 1–14 cm−3, and PM10 concentrations in the range of 42–201 μg/m3.

The combination of cooking activities (varying in type and intensity) with heating via combustion of natural gas or kerosene had a significant impact on indoor air quality. PN concentrations were in the range of 6.8 <sup>×</sup> 104–2.7 <sup>×</sup> 105 cm−3, PM2.5 concentrations were in the range of 9–130 <sup>μ</sup>g/m3, micron PN concentrations were in the range of 1–27 cm<sup>−</sup>3, and PM10 concentrations were in the range of 16–458 μg/m3. Grilling sausages and burgers indoors was identified as an extreme event, with mean PN concentration reaching 3.8 <sup>×</sup> 105 cm<sup>−</sup>3, PM2.5 concentrations reaching 378 <sup>μ</sup>g/m3, micron PN concentrations reaching 131 cm<sup>−</sup>3, and PM10 concentrations reaching 2094 μg/m3.

Both tobacco and shisha smoking adversely impacted indoor air quality in Jordanian dwellings, with the latter being more severe. During tobacco smoking, the PN concentrations were in the range of 9.1 <sup>×</sup> 104–1.5 <sup>×</sup> 105 cm−3, PM2.5 concentrations were in the range of 40–98 <sup>μ</sup>g/m3, micron PN concentrations were in the range of 6–8 cm−3, and PM10 concentrations were in the range of 158–189 <sup>μ</sup>g/m3. During shisha smoking, the PN concentrations were in the range of 1.2 <sup>×</sup> 105–4.0 <sup>×</sup> 105 cm<sup>−</sup>3, PM2.5 concentrations were in the range of 61–173 μg/m3, micron PN concentrations were in the range of 2–36 cm<sup>−</sup>3, and PM10 concentrations were in the range of 92–424 μg/m3.

The above-mentioned concentration ranges were reported during the winter campaign, when the houses were tightly closed for heating purposes. Indoor aerosol concentrations during the summer campaign were generally lower. The overall mean PN concentrations during the summer campaign were less than 2 <sup>×</sup> 104 cm−<sup>3</sup> and PM2.5 concentrations were less than 50 <sup>μ</sup>g/m3. Some of the reported indoor activities were accompanied with high concentrations of gaseous pollutants. TVOC concentrations exceeded 100 ppm. NO2 concentrations were in the range of 0.01–1 ppm. HCHO concentrations were in the range of 0.01–5 ppm. During shisha smoking and preceding preparation (e.g., charcoal combustion), the mean CO concentrations reached as high as 100 ppm.

There are a number of limitations of the present study: (1) the measurement periods were short at each dwelling during the winter campaign, (2) the sample population was small (eight dwellings), and (3) outdoor measurements were only conducted on a few occasions for short periods. These limitations can be addressed in future indoor–outdoor measurement campaigns in Jordan. However, indoor aerosol concentrations were compared to long-term outdoor PN measurements conducted in past studies in Jordan.

The results of this study can offer several practical recommendations for improving indoor air quality in Jordanian indoor environments: source control by prohibiting the smoking of tobacco and shisha indoors, improved ventilation during the use of fossil fuel combustion for heating, and cooking with a natural gas stove under a kitchen hood.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4433/11/1/41/s1. Table S1: Average particle mass and number concentrations (mean ± stdev) during selected indoor activities. Figure S1: Aerosol concentrations inside apartment A1 during the winter campaign (23–25 December 2018). Figure S2: Aerosol concentrations inside ground floor apartment GFA1 during the winter campaign (25–27 December 2018). Figure S3: Aerosol concentrations inside duplex apartment D1 during the winter campaign (28–30 December 2018). Figure S4: Aerosol concentrations inside ground floor apartment GFA3 during the winter campaign (31 December 2018–2 January 2019). Figure S5: Aerosol concentrations inside house H1 during the winter campaign (2–4 January 2019). Figure S6: Aerosol concentrations inside apartment A2 during the winter campaign (4–5 January 2019). Figure S7: Aerosol concentrations inside house H2 during the winter campaign (6–9 January 2019). Figure S8: Aerosol concentrations inside ground floor apartment GFA2 during the winter campaign (9–12 January

2019). Figure S9: Mean particle number size distributions and corresponding particle mass size distributions in the absence of indoor activities during the winter campaign at each study site. Figure S10: Mean particle number size distributions and particle mass size distributions during selected activities reported inside Apartment A1 during the winter campaign (23–25 December2018). Figure S11: Mean particle number size distributions and particle mass size distributions during selected activities reported inside ground floor apartment GFA1 during the winter campaign (25–27 December 2018). Figure S12: Mean particle number size distributions and particle mass size distributions during selected activities reported inside duplex D1 during the winter campaign (28–30 December 2018). Figure S13: Mean particle number size distributions and particle mass size distributions during selected activities reported inside ground floor apartment GFA3 during the winter campaign (31 December 2018–2 January 2019). Figure S14: Mean particle number size distributions and particle mass size distributions during selected activities reported inside house H1 during the winter campaign (2–4 January 2019). Figure S15: Mean particle number size distributions and particle mass size distributions during selected activities reported inside apartment A2 during the winter campaign (4–5 January 2019). Figure S16: Mean particle number size distributions and particle mass size distributions during selected activities reported inside house H2 during the winter campaign (6–9 January 2019). Figure S17: Mean particle number size distributions and particle mass size distributions during selected activities reported inside ground floor apartment GFA2 during the winter campaign (9–12 January 2019).

**Author Contributions:** Conceptualization, T.H., M.M., A.A.-H., and O.A.; methodology, T.H., O.J., K.A., A.A., and O.A.; validation, T.H.; formal analysis, T.H., O.J., and A.A.; investigation, T.H.; resources, T.H. and M.M.; data curation, T.H., O.J., K.A., and A.A.; writing—original draft preparation, T.H. and A.A.-H.; writing—review and editing, T.H., B.E.B., A.J.K., J.L., M.M., and A.A.-H.; visualization, T.H.; supervision, T.H. and A.A.-H.; project administration, T.H. and A.A.-H.; funding acquisition, T.H. and M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the World Health Organization regional office in Amman. The research infrastructure utilized in this project was partly funded by the Deanship of Academic Research (DAR, project number 1516) at the University of Jordan and the Scientific Research Support Fund (SRF, project number BAS-1-2-2015) at the Jordanian Ministry of Higher Education. This research was part of a close collaboration between the University of Jordan and the Institute for Atmospheric and Earth System Research (INAR/Physics, University of Helsinki) via the Academy of Finland Center of Excellence (project No. 272041 and 1307537).

**Acknowledgments:** The first author would like to thank the occupants for allowing the indoor measurement campaigns to be conducted in their dwellings. Some of them also helped in follow-up aerosol measurements and reporting of indoor activities. This manuscript was written and completed during the sabbatical leave of the first author (T.H.) that was spent at the University of Helsinki and supported by the University of Helsinki during 2019. Open access funding was provided by the University of Helsinki.

**Conflicts of Interest:** The authors declare no conflict of interest.
