Quantifying Indoor and Outdoor Particle Relationship
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, https://doi.org/10.1007/978-981-10-5155-5 (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.