Ambient air pollution is a topic of main interest due to its impact on human health. However, much less is known about indoor air, which may be even worse because pollutants are confined in a contained area that hinders their dispersion. Considering that people expend around the 90% of their time in indoor environments (US-EPA, 2009), the exposure to indoor air quality should be taken seriously.
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Fig.1. Ambient air intrues indoors mainly through windows. Source: own capture.

Outdoor air pollution gets into indoor environments mainly through doors and windows by natural ventilation (Fig 1.) but it can also pass through cracks in the building shell.
Contrary to what one might think, there are many sources of particles (PM, particulate matter) in any home or office. These include combustion sources (such as gas, wood, and also tobacco products); building materials and furniture that can emit volatile organic compounds (VOCs); asbestos-containing insulation; products for household cleaning and maintenance, personal care, or hobbies.
One of the most important and known source in indoor environments is, where smoking is allowed or in homes with smokers, tobacco emissions. It contributes substantially to indoor levels (Brunekreef et al., 2005; Slezakova et al., 2011).
But other human activities and simply indoor occupancy can also contribute to PM levels indoors (Kopperud et al., 2004). Our bodies lose skin flakes, hair and emit other organic compounds (Braniš and Šafránek, 2011). Nevertheless, probably the highest impact on PM levels is due to the resuspension of previously deposited PM on indoor surfaces (such as floor and furniture, Fig. 2) by many different activities like moving papers, dusting, vacuum, walking, etc., Karlsson et al., 1999; Long et al., 2000; Qian et al., 2014). Resuspension in indoor environments is a complex process that depends on particle size, the concentration of the particles in the surface, floor type, activity type and intensity, walking style, ventilation configuration and relative humidity (Ferro et al., 2004; Qian et al., 2014).
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Fig.2. Like dust does, particles with a diameter < 2.5 µm also settle on indoor surfaces and can be resuspended. Source: own capture.

However, other activities that take place indoors generate emissions of PM. Cooking (Fig. 3) can also be an important source of PM, especially in the range of ultrafine particles (those with an aerodynamic diameter <100 nm; Abdullahi et al., 2013; Kamens et al., 1991; Lanki et al., 2007). These cooking emissions are mainly organic carbon (OC; present in many diverse organic molecules) that originates from food and oils. In fact, cooking emissions are very influenced by the cooking method and the type of food that is being cooked. In fact, oil-based cooking increases PM2.5 (particles < 2.5 µm) more substantially than those cooking methods based on water (See and Balasubramanian, 2008). Furthermore, the higher the fat content in the ingredients is, the higher the organic emissions are (Rooge et al., 1991; Zhao et al., 2007).
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Fig. 3. Cooking is an important source of ultrafine particles. Source: own capture.

Many other indoor sources of PM are present in indoor environments (p.e., VOCs emissions from cleaning products) but the clue to keep a safe and clean indoor air quality is ventilation. Open your windows (ideally in traffic non rush hours if you live in a city, to avoid the entrance of outdoor air when the pollutant concentration is at maximum), and let the indoor air be replaced by fresh air.

REFERENCES
Abdullahi, K.L., Delgado-Saborit, J.M., Harrison, R.M., 2013. Emissions and indoor concentrations of particulate matter and its specific chemical components from cooking: A review. Atmos. Environ. 71, 260–294.
Braniš, M., Šafránek, J., 2011. Characterization of coarse particulate matter in school gyms. Environ. Res. 111, 485–91.
Ferro, A.R., Kopperud, R.J., Hildemann, L.M., 2004. Source strengths for indoor human activities that resuspend particulate matter. Environ. Sci. Technol. 38, 1759–64.
Kamens, R., Lee, C., Wiener, R., Leith, D., 1991. A study of characterize indoor particles in three non-smoking homes. Atmos. Environ. 25A, 939–948.
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Qian, J., Peccia, J., Ferro, A.R., 2014. Walking-induced Particle Resuspension in Indoor Environments. Atmos. Environ.
See, S.W., Balasubramanian, R., 2008. Chemical characteristics of fine particles emitted from different gas cooking methods. Atmos. Environ. 42, 8852–8862.
Slezakova, K., Pires, J.C.M., Martins, F.G., Pereira, M.C., Alvim-Ferraz, M.C., 2011. Identification of tobacco smoke components in indoor breathable particles by SEM–EDS. Atmos. Environ. 45, 863–872.
US-EPA, 2009. Highlights of the Child-Specific Exposure Factors Handbook, EPA/600/R- ed. Washington, D.C.
Zhao, Y., Hu, M., Slanina, S., Zhang, Y., 2007. The molecular distribution of fine particulate organic matter emitted from Western-style fast food cooking. Atmos. Environ. 41, 8163–8171.

Published by Ioar Rivas Lara

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