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Quantifying Indoor and Outdoor Particle Relationship

2023-08-20T11:39:52Z

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Although a significant portion of people’s time is spent indoors, exposure to particles originating from outdoors is unavoidable. Outdoor particles can penetrate into indoor environments through building ventilation or infiltration. Buildings are typically ventilated using three mechanisms: 1) mechanical ventilation, 2) natural ventilation, and 3) infiltration. All of which allow the entry of outdoor particles into the indoor environment. Mechanical ventilation typically includes supplying fresh (outdoor) air that carries outdoor-originated particles. Depending on the filter present in mechanical ventilation system, particles may still escape and enter indoors. Natural ventilation occurs by moving wind and buoyancy-induced flow (or pressure difference) through opening doors and/or windows, transporting outdoor particles to indoors. Infiltration refers to the uncontrolled flow of air through gaps, cracks, and leaks in the building envelope.

There are several methods to investigate how indoor and outdoor particles are related. The following are the most common experimental methods:

'''Indoor/outdoor (I/O) ratio'''

I/O ratio directly displays the relationship between indoor and outdoor particle concentrations, which is very easy to understand and widely used. It provides a general impression of the relationship between indoor and outdoor particles using the expression:

I/O ratio = Cin/Cout

where Cin and Cout are the indoor and outdoor particle concentrations, respectively. Determining the I/O ratio is done by installing two particle samplers inside and outside the building.

The systematic review of indoor and outdoor relationships by Chen and Zhao (2011), also discussed in a chapter in the Handbook of Indoor Air Quality (2023), provided a summary of major studies (sampled more than 20 houses) showing an enormous range of I/O ratios for both PM2.5 and PM10, which is highly influenced due to indoor smoking. The lowest I/O ratio was attained when there are few indoor sources, such as houses with filtration and tightness of the building, e.g., 0.71 for PM2.5 in southern California (Clayton et al., 1993), 0.48 for PM10 during winter in a high-rise residential apartment that uses filtration system in South Korea (Jo and Lee, 2006). The difference between the I/O ratios of PM2.5 and PM10 may be because indoor sources emit more coarse than fine particles, while outdoor sources emit less coarse than fine particles.

In general, the I/O ratio is the easiest way to approximate the relationship between indoor and outdoor particle concentrations, but there are many influencing factors, especially the source of indoor particles, resulting in a wide range of measured values (from well below 1 to well above 1), which makes it difficult to deduce insight on indoor/outdoor particle relationships.

'''Infiltration factor'''

The infiltration factor represents the equilibrium fraction of ambient particles that penetrate indoors and remains suspended. The generalized definition of infiltration factor can be expressed as:

Cin = FinCout + Cis

where Fin is the infiltration factor, and Cis is the particle concentration that is contributed by indoor sources. After measuring the indoor and outdoor particle concentrations under various conditions, Fin and Cis can be solved from the regression of indoor concentration against the outdoor concentration. The slope of the regression represents Fin, and the intercepts characterize the Cis.

The infiltration factor is useful for qualifying the fraction of the total indoor particles coming from the outdoor environment. However, since it also includes the process of particle deposition indoors, the infiltration factor is difficult to reflect how outdoor particles enter indoors through buildings. Among other parameters, penetration, air exchange, and deposition rates can affect the infiltration factor. Principally, the infiltration factors of PM2.5 are higher than that of PM10 due to the weaker deposition strength than PM10. Results from experimental and simulated studies show that PM2.5 has approximately 0.5, which would be lower than ultrafine particles and higher than PM10 under the same conditions.

'''Penetration Factor'''

Penetration factor, ''P'', is a parameter that can define the fraction of particles that passes through the building shell. There are several experimental methods to determine penetration factors that can be conducted on real buildings or in a controlled laboratory environment.

One of the methods is by regression approach. This is done by measuring the indoor and outdoor particle concentration under different conditions, Cis, a linear expression can be written as:

Cout/(Cin-Cis) = (''K''/''P'')(1/''a'') + (1/''P'')

where ''K'' is the particle deposition rate, ''a'' is the air exchange rate. The deposition rate is a particle size-dependent parameter discussed in detail by Lai (2002), Thatcher et al. (2002), Hussein et al. (2005), and Hamdani et al. (2008).

The penetration factor of UFPs (0.59-0.78), PM2.5 (0.72-1), and PM10 (0.69-0.86) are systematically reviewed by Chen and Zhao (2022). They concluded that for accumulation mode particles (0.1-1mm particle diameter), the penetration factors are 0.78 ± 0.17. The value decreases for smaller particles due to Brownian diffusion, or due to strong gravitational setting and impaction for larger particles. Overall, penetration factor is affected by particle size, pressure difference, geometry, surface roughness of cracks, etc.

References:

Chen, C., Zhao, B., (2022) Impact of Outdoor Particles in Indoor Air, Chapter 10, Handbook of Indoor Air Quality, <nowiki>https://doi.org/10.1007/978-981-10-5155-5</nowiki> (and references therein)

Chen, C. and Zhao, B. (2011) ‘Review of relationship between indoor and outdoor particles: I/O ratio, infiltration factor and penetration factor’, Atmospheric Environment, 45(2), pp. 275–288. doi:10.1016/j.atmosenv.2010.09.048.

Clayton, C.A., Perritt, R.L., Pellizzari, E.D., Thomas, K.W., Whitmore, R.W., Wallace, L.A., Ozkaynak, H., Spengler, J.D., 1993. Particle total exposure assessment methodology (PTEAM) study: distribution of aerosol and elemental concentrations in personal, indoor and outdoor air samples in a southern California community. J. Expo. Analysis Environ. Epidemiol. 3, 227-250.

Hamdani, S.E., Limam, K., Abadie, M.O., Bendou, A., 2008. Deposition of fine particles on building internal surfaces. Atmos. Environ. 42, 8893-8901.

Hussein, T., Hameri, K., Heikkinen, M.S.A., Kulmala, M., 2005. Indoor and outdoor particle size characterization at a family house in Espoo-Finland. Atmos.Environ. 39, 3697-3709.

Jo, W.K., Lee, J.Y., 2006. Indoor and outdoor levels of respirable particulates (PM10) and carbon monoxide (CO) in high-rise apartment buildings. Atmos. Environ. 40, 6067-6076.

Lai, A.C.K., 2002. Particle deposition indoors: a review. Indoor Air 12, 211-214.

Thatcher, T.L., Lai, A.C.K., Moreno-Jackson, R., Sextro, R.G., Nazaroff, W.W., 2002. Effects of room furnishings and air speed on particle deposition rates indoors.Atmos. Environ. 36, 1811-1819.

[[Category:Factors affecting indoor air quality]]

Quantifying Indoor and Outdoor Particle Relationship

2023-08-20T11:39:08Z

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Particles in Indoor Air

2023-08-20T11:03:52Z

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Indoor air particles play a critical role in determining indoor air quality, which directly affects human health and well-being. These particles range in various sizes – from large visible dust particles to small stable gatherings of molecules. Indoor air quality refers to the condition of the air inside buildings, that includes homes, offices, schools, and other enclosed spaces. The study of modern indoor air science started in the 1970s and has since raised concern about indoor air pollution's potential health effects. Health-related studies have investigated the composition, sources, and behavior of indoor particulates and how it is connected to respiratory diseases, allergies, and other cardiovascular problems. Understanding the sources of indoor air particles is crucial to developing effective strategies to maintain a healthy indoor environment and improve the overall quality of life.

Provided below are the primary and secondary sources of particles that people are exposed to indoors.

'''Primary Source'''

'''Cooking'''

Particulate emissions from cooking come from two sources, 1) the heat source (e.g., stove, oven) and 2) the cooking process itself. The type of appliance and ventilation to the outside air influences the input of particles in the cooking area which may affect the overall indoor particle concentration. Heat from direct combustion, such as natural gas, propane, liquid petroleum gas (LPG), kerosene, and solid fuels (i.e., wood, coal, etc.) emit different ultrafine particle sizes. A properly tuned natural gas combustion produces particles with a geometric mean (GM) mobility diameter of 19.5 ± 1.4 nm, propane flame has a GM of 26.5 ± 1.3 nm, LPG creates 52 ± 1.2 nm particles, and solid fuels are reported to emit 1.23 ± 1.8 nm, 48 ± 1.9 nm, and 152 ± 1.9 nm particles for firewood, coal, and dung cake, respectively. Electrical cooking surfaces can also produce ultrafine particles from organic constituents that condense into the heating elements, which get volatilized and then nucleate to form less than 10 nm particles. Cooking food makes additional source of particles and often represents a primary short-term source of particle emissions. During frying in oil or melted solid fats above their smoke temperature, the water from the food vaporizes to form bubbles which then burst and eject tiny oil droplets into the air. Other cooking activities, such as boiling and broiling, also cause emissions of particles of different sizes.

'''Heating'''

For many houses, stoves are also used to heat the room. Thus, similar sizes of particles may be emitted into the indoor air space.

'''Cleaning'''

Paradoxically, cleaning aims to reduce the level of contamination in indoor spaces, but it could also pose as a source of particles in several ways. Brooming may cause resuspension of particles greater than 1mm. While, vacuuming may also increases particulate matter resuspension, which depend on the efficiency of dust bags (cloth bag vs. HEPA filter).

'''Lifestyle'''

Human daily activities, such as smoking, vaping, candle or incense burning, and use of spray products, can also introduce particles indoors. Electronic cigarettes (e-cigs), commonly called vaping, have increased in popularity recently as an alternative to tobacco cigarettes (t-cigs). The use of e-cigs produces similar high levels of fine and ultrafine particles (UFPs) as to t-cigs. Combustion of candles and incense can also produce high concentrations of particles. Candles with added scents elevate particle emissions by releasing volatile and semi-volatile organic compounds. Different types of wax and wick also affect the particle size distribution of emitted particles, usually in the range of 5.4 – 7.1 nm. Incense has lower combustion temperature which produces much larger particles with particle diameter size of around 136 nm. Other consumer products, such as air fresheners, spray cleaners, and other aerosolized liquids or solids, provide a source of particles indoors. In products with active solutions, particles may be created after the solvent has evaporated.

'''Infiltration of Ambient Aerosol'''

Outdoor particles transporting into indoor space is a major long-term source. The exchange can occur directly from ventilation or infiltration through gaps in the building envelope. Buildings can be ventilated in three ways: mechanical ventilation, natural ventilation, and infiltration. Mechanical ventilation includes the use of mechanical equipment such as fans. Opening the windows or doors offer natural ventilation. Infiltration of outdoor particles into the indoor environment occurs through unintentional openings in the building such as cracks and gaps.

'''Secondary Source'''

Indoor chemistry has a significant influence on the composition of indoor particles. Indoor air particles may arise from various chemical reactions and transformations that occur within confined spaces. These reactions can result in the emission of particles of varying sizes and compositions, leading to complex mixtures that may negatively impact human health. Some of the prominent secondary sources of indoor air include photochemical and chemical reactions. Photochemical reactions in indoor spaces are usually fueled by sunlight entering the building although light intensities are usually insufficient to drive the same types of photochemical reactions observed outdoors. There can be photochemical reactions where indoor pollutants, such as volatile organic compounds (VOCs) and nitrogen oxides (NOx), are exposed to light, they can absorb energy and become excited, which lead to the breaking of chemical bonds and formation of reactive intermediates or radicals that further reacts with molecules in the air forming new compounds or particles. Another pathway in creating particles in indoor environments is the presence of surfaces on which heterogeneous reactions can occur.

Reference: “Handbook of Indoor Air Quality, Volumes 1. <nowiki>https://doi.org/10.1007/978-981-10-5155-5</nowiki> (and references therein)”

[[Category:Factors affecting indoor air quality]]

Particles in Indoor Air

2023-08-20T11:02:00Z

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Particles in Indoor Air

2023-08-20T11:01:13Z

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Particles in Indoor Air

2023-08-20T10:59:12Z

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Quantifying Indoor and Outdoor Particle Relationship

2023-08-17T21:46:20Z

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Quantifying Indoor and Outdoor Particle Relationship

2023-08-17T21:44:58Z

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Particles in Indoor Air

2023-08-17T21:32:44Z

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