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Workshop Summary and Closing Reflections

To conclude the workshop, Planning Committee Chair Richard Corsi summarized the key points he heard during the 3 days of presentations and discussions. One point that came across was that great technology is worthless if it is not used, and poor technology is worthless even if it is used, highlighting the importance of both technology and human behavior and decision making. He then noted that indoor environments are a complex system, starting with the fact that indoor fine particulate matter (PM_2.5) of outdoor origin mixes with PM_2.5 generated indoors and chemical transformations occur indoors. Ventilation is an important defining factor, as is deposition of particles on surfaces and engineered controls. It is therefore important to understand the exposure of occupants, the inhalation dose, where particles go in the respiratory system, and what the health effects are.

Contributing to the complexity of the system are significant temporal variations for sources and indoor concentrations of PM_2.5, as well as significant spatial gradients, both outdoors and indoors. There are also differences in how occupants perceive indoor air quality and how they behave and respond, as well as different policies that affect how indoor PM_2.5 is handled. Superimposed on all of this are socioeconomic issues that further complicate these matters. “This underscores the fact that we need an interdisciplinary approach to deal with these problems,” said Corsi.

OUTDOOR PM_2.5

Starting with the presentations on outdoor sources of PM_2.5, Corsi listed the following highlights:

• There are significant temporal and spatial gradients with respect to outdoor sources of PM_2.5 that affect indoor PM_2.5, and there are significant environmental justice issues associated with the locations of underserved communities with respect to many of these sources.
• Outdoor-to-indoor transport of PM_2.5 across the building envelope results in an average of half of the outdoor PM_2.5 particles infiltrating to indoors, although there are many factors that affect that infiltration and large variation in how much of it occurs.
• Analysis of microenvironmental PM_2.5 exposures suggests that about 70 percent of PM_2.5 exposure occurs in the home and about 20 percent in other buildings, which means that less than 10 percent of an individual’s exposure occurs outdoors.
• Roughly half of the exposure indoors is to PM_2.5 of outdoor origin and half from PM_2.5 of indoor origin.
• Indoor sources of PM_2.5 include combustion (cooking, candles, and fuels), food being cooked, phase change as cooking oils heat and cool, mechanical (ultrasonic humidifiers and vacuuming), biological (respiratory aerosols and mold spores), and chemical reactions that generate secondary organic aerosols.
• Emissions may be high during cooking, with particle numbers dominated by the fuel source. Particulate emission levels and composition are highly variable, and there is little information about what this means for health.
• Transformations occur as outdoor PM_2.5 infiltrates indoors, where those particles encounter larger surface-to-volume ratios and fewer oxidants. Particles can shrink through evaporation and they can adsorb volatile and semivolatile organic compounds that are present at much higher concentrations indoors. There is also thermal partitioning that occurs indoors. The question is whether these transformations have relevance for human health.
• Relative humidity affects the water content of PM_2.5, which can change the size of the particles as well as the chemistry and microbiology in and on the particles. It is not known what the optimum environmental conditions are to reduce the health effects of PM_2.5.

HEALTH EFFECTS OF INDOOR PM_2.5

Corsi reviewed the following highlights of the presentations on the health effects of indoor PM_2.5:

• The vast burden of morbidity from air pollution is due to increased cardiovascular disease, largely because cardiovascular disease is highly prevalent in adults living in the developed world.
• Given that PM_2.5 of outdoor origin accounts for about 70 percent of the total dose, the epidemiologic findings on outdoor PM_2.5 apply to indoor exposures.
• Increases in indoor PM_2.5 are associated with increases in asthma and chronic obstructive pulmonary disease morbidity, but portable HEPA filtration can reduce symptoms.
• There is some evidence from animal studies that PM_2.5 of indoor origin produces more inflammation and cytotoxicity than PM_2.5 of outdoor origin.
• Assessing indoor PM_2.5 is complex, and personal measurements are best when possible, though they are cumbersome. Low-cost consumer instruments for PM_2.5 are making measurements more common, but protocols and reference standards are missing, and the accuracy and performance of these devices in real-life settings are not impressive.
• Multiple dimensions of complexity are associated with indoor PM_2.5, including their broad size range, chemical composition, high temporal variability for particle concentrations, and high spatial variability. Since no instrument can measure all of these factors, it is important to decide which measurements are important, where they should be made, and when to make them. The answers depend in part on whether the concern is short-term or long-term health effects.
• Inhalation dose and location of where particles deposit in the respiratory system are both important factors in determining possible health effects.
• Moving forward with the use of low-cost sensors will require understanding that individual perception of indoor air quality is important, and evaluating whether sensors can help with those perceptions.
• People want information that provides guidance on the actions they should take or not take. Clear, graphical, and actionable information is important to encourage desired occupant behavior.
• Improved filtration has been associated with subclinical cardiovascular benefits, increased birth weight, reduced asthma morbidity,

• and higher test scores in schools. Portable HEPA cleaners have been associated with substantial reductions in PM_2.5 exposure and an increase in symptom-free days for children with asthma. These studies are limited in number, and there is a need to do better at communicating the benefits of improved filtration and develop standards to help individuals realize those benefits.

MITIGATION OF INDOOR PM_2.5

Moving on to the presentations on approaches to mitigating indoor PM_2.5, Corsi listed the following highlights:

• Local exhaust of emissions from cooking, a major source of indoor PM_2.5, can be achieved with a range hood, downdraft exhaust system, ceiling or wall exhaust, or open window.
• Hood capture efficiency is important, and using back burners rather than front burners results in better removal of PM_2.5. Regarding hoods, recirculating systems fare poorly in mitigating exposure compared to those that exhaust to the outdoors. Communicating this to the public is important.
• Occupant behavior is key given that most people use their range hoods less than half the time when cooking, in part because they are noisy. Are there approaches such as sensors that either alert occupants to turn on their range hoods or automatic systems that can improve this situation?
• Filtration works only when there is a good filter that is properly installed to minimize bypass, when the filtration system is running and producing enough air flow, and when the filter is replaced regularly. Other air cleaning technologies are not well defined, lack independent evaluation, and may even be harmful.
• Given that some 15 percent of US schools are within 250 meters of a major roadway, it is important to mitigate indoor PM_2.5 levels in schools. Traffic-related air pollution affects not only respiratory health conditions such as asthma but also cognition. This is an environmental justice issue because schools with higher percentages of Hispanic, Black, and Asian students are more likely to be located near such outdoor PM sources.
• Significant reductions in traffic-related air pollution can be achieved with the proper filters and an efficiently operating HVAC system. Portable HEPA filtration systems installed in classrooms can be an effective and cost-effective exposure mitigation strategy for schools.

• Portable HEPA cleaners in the home can produce significant reductions in indoor PM_2.5 and may thus reduce the frequency of asthma symptoms. This benefit can only accrue, though, if the devices are used, which may not happen on a regular basis because units may produce drafts and noise, and the cost of operating these devices can be prohibitive for families of more limited means.
• Messaging is critical for reducing exposures to PM_2.5 during wildfires, as is preparation before an event occurs. Portable HEPA cleaners can be valuable mitigation tools during these and other high outdoor particulate matter events.
• There are substantial exposure and health disparities related to socioeconomic status, with underserved communities carrying a high burden due to proximity to outdoor sources, higher occupant density, older ventilation and exhaust systems, and economic challenges that affect the use of practical mitigation approaches. Other factors, like the amount of time spent in the home and cooking practices, also affect exposure.

Corsi concluded the session by acknowledging the US Environmental Protection Agency’s sponsorship of the workshop and thanking the presenters and the planning committee. The workshop was then adjourned.

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