Health Risks of Indoor Exposure to Particulate Matter Workshop Summary (2016) / Chapter Skim
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2 Sources of Indoor Particulate Matter
2 Sources of Indoor Particulate Matter
Pages 7-24
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An open discussion moderated by William Nazaroff followed the three presentations. INDOOR EXPOSURES TO OUTDOOR PARTICULATE MATTER1 The documented adverse health effects of exposure to outdoor PM include stroke, heart disease, lung cancer, and chronic and acute respiratory diseases, including asthma, reduced lung function, and mortality (EPA, 2009)
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, so indoor exposure to PM is likely to be an important contributor to the adverse health effects caused by PM exposure. Indeed, Stephens said, outdoor PM enters into buildings at varying efficiencies, becoming indoor PM.
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FIGURE 2-2 Experimental data distribution of PM2.5 and PM10 infiltration factors (Finf) for homes in the United States and Europe.
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Stephens then addressed areas in which less is known about indoor exposures to outdoor PM, starting with how infiltration factor variability contributes to health effect estimates from epidemiology studies. One modeling study (Chen et al., 2012)
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Particles are removed from indoor air by a combination of air exchange and a variety of loss mechanisms which include deposition to surfaces, phase changes, and control by filters and air cleaners. For conditions in which air exchange only occurs through infiltration, the particle infiltration factor equals the product of the AER and the penetration factor divided by the sum of the AER plus and the other loss mechanisms:
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Little is known about how chemical transformations, such as evaporative losses, affect infiltration factors, and data are lacking concerning the spatial and temporal resolution of outdoor PM size distributions and outdoor size-resolved aerosol composition. Summarizing research needs in the area of outdoor PM transport to indoors, Stephens said that there is a need for more integration between epidemiologists and exposure scientists, building scientists, and indoor air scientists.
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. For example, Stephens and his colleagues recently measured the rate of PM and volatile organic compound emissions from desktop 3D printers and found UFP emission rates of between 108 and 1012 particles per minute (Azimi et al., 2016)
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She added that burning food can very quickly introduce large quantities of PM into the indoor environment. Natural gas stoves and ovens emit mainly UFPs, but how long they persist in indoor air is unclear, and their chemical composition is not well characterized (Minutolo et al., 2008)
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When indoor combustion is occurring under conditions of natural ventilation, PM will not disperse immediately throughout the indoor environment, she explained. The question is, How bad is it to be close to an active combustion source?
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What this size variation means as a practical matter, Hildemann explained, is that there will be a wide range of airborne residence times for indoor allergens, and there will be variability in terms of where these allergens deposit in human lungs. PM10, she said, clears from the lungs in hours, whereas PM2.5 and UFPs can take weeks to clear.
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Addressing areas that need further research, Hildemann said that little is known about the factors affecting the emission rates of bacteria and fungi from damp surfaces. Airflow, vibration, and material type are thought to play some role in determining the rate at which these organisms become airborne from damp surface, but the main challenge in learning more about these processes, she said, is that researchers are still not sure how to accurately estimate emission rates from damp surfaces.
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Resuspension rates and fractions can be determined by measuring the size-resolved concentration of particles in the air and on a surface as well as the frequency of movement. Resuspension is then linked to exposure through airborne particle transport processes and airflow patterns that create some concentration of PM in the breathing zone.
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Dust adhesion plays an important role in resuspension, yet most studies have been narrowly focused on spherical particles adhering to flat surfaces, and neither spherical particles nor flat surfaces reflect the indoor reality. "Indoors, we have non-spherical particles and complex surfaces such as fabric fibers, clothing, bedding material, and carpet fibers, and there are very few data on particle adhesion to different types of fabric fibers," Boor said.
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. As Boor had mentioned earlier, the effect of an infant crawling on the near-floor microenvironment is not well characterized, so in a recently completed study he and colleagues in Finland built a simplified mechanical crawling infant and used it to measure airborne particle concentrations as it scuttled across 12 area carpets borrowed from Helsinki residents.
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Boor and his colleagues in Finland used this technique to show that both crawling and walking triggered a burst of resuspended fluorescent particles from carpeting but that particle decay occurred more quickly in the infant breathing zone after crawling than in the adult breathing zone after walking. Boor mentioned some recent work using fluorescent and optical signatures to distinguish among bacteria, fungi, and pollen in real time (Hernandez et al., 2016)
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. What is needed to create a full, holistic picture of particle resuspension, Boor said in closing, are integrated measurements of particle resuspension to the breathing zone across all scales, from small-scale wind tunnel and chamber studies to full-scale controlled chamber studies and field measurements in offices and homes.
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More importantly, she said, these studies have shown that small fragments of microbes are also released, so it is important to look for biological components in the smaller size fractions of PM as well as in the large fractions. Howard Kipen from Rutgers University noted that while EPA regulates outdoor PM levels based on a substantial and sustained epidemiologic database linking outdoor PM levels with a wide range of adverse health effects, there needs to be work done to determine the health consequences of outdoor PM translated to indoor exposures, given that Americans spend 90 percent of their time indoors and that 50 percent of indoor PM comes from outdoor sources.
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William Fisk from the Lawrence Berkeley National Laboratory remarked that panelists had not addressed three categories of indoor PM sources that need further study: the outdoor air as a source of allergens and inflammatory agents; the wetted surfaces in HVAC systems; and episodic outdoors sources such as wood combustion and wildfires. An online participant asked the panelists if there were data on the contribution that cleaning product residues make to indoor PM and whether these residues alter the resuspension of other particles.
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