Particle Sizes

Particle size plays a significant role in determining how particles behave indoors and their potential impact on human health. Fine particulate matter (PM2.5), with diameters of 2.5 micrometers or less, is a major concern due to its ability to penetrate deep into the respiratory system. An important subset of PM2.5 is ultrafine particles (UFPs), which have diameters less than 0.1 micrometers and are primarily emitted from combustion sources. These UFPs can even be transported to the brain via the olfactory nerve, potentially causing neurological effects. Particles in the 0.1 to 2.5 micrometer range can originate from indoor sources like soot or from the infiltration of outdoor particles. Coarse particles, larger than 2.5 micrometers, are mainly generated by mechanical processes like resuspending dust. These size distinctions are crucial because they influence a particle’s ability to penetrate indoors, remain airborne, and affect human health. For instance, the high surface area to mass ratio of UFPs makes them efficient at transporting harmful chemicals deep into the lungs. Understanding the sources and behavior of different particle sizes indoors is essential for developing effective strategies to mitigate their potential health risks.

PM2.5 Infiltration

The infiltration of outdoor PM2.5 into indoor spaces is a significant contributor to indoor air pollution. Outdoor PM2.5 can enter buildings through various pathways, including cracks, leaks, and openings in the building envelope. The rate of infiltration depends on factors like the age and construction of the building, ventilation systems, and occupant behavior like opening windows. Studies have shown that PM2.5 of both indoor and outdoor origin contribute almost equally to indoor concentrations when measured by mass. Notably, during events like wildfires, outdoor sources may dominate indoor PM2.5 concentrations. Building characteristics like airtightness can significantly impact infiltration. For instance, older homes with lower airtightness tend to have higher infiltration rates, leading to greater penetration of outdoor pollutants, including PM2.5. The presence and efficiency of filtration systems are also crucial in mitigating infiltration. While limiting infiltration can reduce exposure to outdoor PM2.5, it can also lead to an increase in the concentration of PM2.5 from indoor sources. This highlights the importance of considering both indoor and outdoor sources when developing mitigation strategies.

• Building characteristics: The age, construction, and maintenance of a building can all affect infiltration rates. Older buildings with more cracks and leaks will generally have higher infiltration rates.
• Ventilation systems: Mechanical ventilation systems can either increase or decrease infiltration rates depending on their design and operation (positive/negative pressure).
• Occupant behavior: Opening windows and doors can significantly increase infiltration rates.

Infiltration can be reduced by sealing cracks and leaks in the building envelope, properly maintaining ventilation/filtration systems, and limiting the opening of windows and doors.

PM2.5 Indoor/Outdoor Ratio (IO)

The ratio of indoor to outdoor PM2.5 concentrations (I/O ratio) is a metric used to evaluate the relationship between indoor and outdoor PM2.5 levels and assess the impact of indoor sources and outdoor air infiltration on indoor air quality. I/O ratios can vary significantly depending on factors such as building characteristics, ventilation systems, occupant behavior, and outdoor air quality.

For instance, studies have shown that I/O ratios tend to be higher during occupied hours due to increased human activity and ventilation rates. Moreover, the I/O ratio can be strongly influenced by occupant activities like window opening, making it a personal variable as much as a building-associated one. Research has revealed that I/O ratios for PM2.5 are generally lower than 1, indicating that indoor PM2.5 concentrations are typically lower than outdoor concentrations, except in cases where significant indoor sources are present. This suggests that the building envelope often provides some level of protection from outdoor PM2.5. However, it’s important to note that during periods of high outdoor PM2.5, such as during wildfires, infiltration factors tend to decrease, reducing potential exposures from outdoor particles but potentially increasing exposures to indoor-generated particles. While air cleaning systems can effectively remove PM2.5, they may be more efficient at removing larger particles than smaller ones, potentially leading to a skewed chemical composition of indoor PM2.5, with a higher proportion of harmful fine particles. This highlights the importance of considering both indoor and outdoor sources and implementing appropriate ventilation and filtration strategies to mitigate PM2.5 exposure in indoor environments.

In a study conducted in Beijing, the mean I/O ratio was found to be 0.36 when outdoor PM2.5 levels were high (above 150 μg/m3), suggesting that indoor concentrations were significantly lower than outdoor levels due to factors like filtration and reduced air exchange. Conversely, when outdoor PM2.5 levels were lower (below 100 μg/m3), the mean I/O ratio increased to 1.1, indicating that indoor sources and activities might have played a more dominant role in determining indoor PM2.5 levels. Research has also shown that the I/O ratio tends to be higher in residential homes compared to public buildings, potentially due to differences in ventilation practices and the presence of indoor sources. In a study examining the effects of energy-efficient refurbishment on indoor PM2.5 exposure in London, simulations predicted a decrease in the I/O ratio from 0.5 in the present day to 0.1 in 2050, attributing this reduction to improved building airtightness, mechanical ventilation with filtration, and anticipated reductions in outdoor PM2.5 levels. These findings highlight the dynamic nature of the I/O ratio and the need for comprehensive exposure assessments that consider both indoor and outdoor environments to accurately characterize PM2.5 exposure risks.

Understanding the factors that influence the IO ratio is important for developing strategies to reduce indoor PM2.5 exposure. For example, in areas with high outdoor PM2.5 concentrations, improving building airtightness and using ventilation systems with high-efficiency filters can help to minimize infiltration and protect indoor air quality.

References:

• Stamp, S., Burman, E., Chatzidiakou, L., Cooper, E., Wang, Y., & Mumovic, D. (2022). A critical evaluation of the dynamic nature of indoor-outdoor air quality ratios. Atmospheric Environment, 273, 118955. https://doi.org/10.1016/j.atmosenv.2022.118955
• Ferro, A. R., Zíková, N., Masiol, M., Satsangi, G. P., Twomey, T., Chalupa, D. C., Rich, D. Q., & Hopke, P. K. (2022). Residential Indoor and Outdoor PM Measured Using Low-cost Monitors during the Heating Season in Monroe County, NY. Aerosol and Air Quality Research, 22(9), 220210. https://doi.org/10.4209/aaqr.220210
• Deng, G., Li, Z., Wang, Z., Gao, J., Xu, Z., Li, J., & Wang, Z. (2015). Indoor/outdoor relationship of PM2.5 concentration in typical buildings with and without air cleaning in Beijing. Indoor and Built Environment, 26(1), 60–68. https://doi.org/10.1177/1420326×15604349
• Ścibor, M., Bokwa, A., & Balcerzak, B. (2020). Impact of wind speed and apartment ventilation on indoor concentrations of PM10 and PM2.5 in Kraków, Poland. Air Quality Atmosphere & Health, 13(5), 553–562. https://doi.org/10.1007/s11869-020-00816-8
• Madureira, J., Paciência, I., & De Oliveira Fernandes, E. (2012). Levels and Indoor–Outdoor relationships of Size-Specific particulate matter in naturally ventilated Portuguese schools. Journal of Toxicology and Environmental Health, 75(22–23), 1423–1436. https://doi.org/10.1080/15287394.2012.721177
• Lunderberg, D. M., Liang, Y., Singer, B. C., Apte, J. S., Nazaroff, W. W., & Goldstein, A. H. (2023). Assessing residential PM 2.5 concentrations and infiltration factors with high spatiotemporal resolution using crowdsourced sensors. Proceedings of the National Academy of Sciences, 120(50). https://doi.org/10.1073/pnas.2308832120
• Liu, Y., Ma, H., Zhang, N., & Li, Q. (2022). A systematic literature review on indoor PM2.5 concentrations and personal exposure in urban residential buildings. Heliyon, 8(8), e10174. https://doi.org/10.1016/j.heliyon.2022.e10174
• O’Dell, K., Ford, B., Burkhardt, J., Magzamen, S., Anenberg, S. C., Bayham, J., Fischer, E. V., & Pierce, J. R. (2022). Outside in: the relationship between indoor and outdoor particulate air quality during wildfire smoke events in western US cities. Environmental Research Health, 1(1), 015003. https://doi.org/10.1088/2752-5309/ac7d69