9

Occupant Responses to Indoor Particulate Matter

The workshop’s final session focused on how human behavior and public health considerations inform indoor particulate matter exposure mitigation strategies. The session featured three speakers. Stuart Batterman (University of Michigan School of Public Health) discussed human behavior as it relates to the use of portable indoor air cleaners, and Lindsay Graham (University of California, Berkeley) spoke about how building occupants interpret and respond to data from indoor air quality monitors. Sarah Coefield (Missoula City-County Health Department) then explained the role of public health in reducing community exposure to fine particulate matter (PM_2.5). Following the presentations, Rengie Chan and Seema Bhangar moderated the final open discussion of the workshop.

PORTABLE INDOOR AIR CLEANERS AND HUMAN BEHAVIOR

Stuart Batterman, who has conducted a number of field studies of indoor environmental exposures and health, began by providing the typical guidance he gives people who have children in Detroit who have asthma:

1. Do not smoke indoors or allow others to smoke indoors.
2. If you have a portable air filter, put it in your child’s room and use it all the time. People who turn it on and off tend to leave it off for long periods of time, which reduces its effectiveness.

3. Instead of rags, use a microfiber furniture duster when dusting. It removes dust better than ordinary rags, which tend to spread dust around rather than pick it up.
4. To clean floors, use a vacuum cleaner instead of a broom. If you need to use a broom, sweep gently. Vigorous sweeping can throw dust back into the air and under furniture and appliances.
5. If you have forced-air heat or central air conditioning, use a good furnace filter to reduce particulate matter. A standard furnace filter (costing $1–5, often colored blue) does not improve air quality. Look at the minimum efficiency reporting value (MERV) number, which is the efficiency of the filter for removing particles from the air; a filter with a MERV rating of 13 or greater is generally a good choice and costs $15–60. Change filters every 3 months.
6. Do not use air fresheners and minimize use of mothballs and deodorizers.

He also advises people to change certain behaviors to improve their environment, their health, and the health of their children. In the public health world, multilevel interventions or multilevel approaches to changing behavior are known to be most effective; individual-, community-, and national-level interventions work best when applied together. At the individual level, this would involve identifying individuals at risk, such as children with asthma. Community-based interventions aim to modify the environment and use peer influence, including the media, community-based screening, community-based organizations, and sometimes rules, restrictions, and taxes to motivate individual behaviors. Often, community health workers bridge the individual and the community to try to change behavior. At the national level, messaging, regulations, codes, and other actions are the levers to induce change.

These behavioral changes, said Batterman, apply to many types of decisions about things such as indoor and outdoor emissions and pollutant levels, exhaust fan operation, HVAC systems and ventilation rates, and the purchase and operation of filters. A systems approach should be taken for both buildings and health, bearing in mind that each indoor environment, household, and community is unique. “A major take-home here, though, is that we need to identify effective ways to change behavior,” he said.

Drawing from the literature on interventions, Batterman developed a conceptual diagram for thinking about behavior change to improve health by reducing particulate matter levels using filters (Figure 9-1) (Michie et al., 2018). Achieving that goal, he explained, depends on behaviors that can be triggered by an intervention and recognizing that the effectiveness of the intervention depends on the context, the population, the extent to which intervention information reaches different parties, and the extent

to which individuals engage in changing their behavior to meet that goal. The diagram, while simplified (it omits feedback loops, multiple behaviors, and the time dimension), does represent the fact that changing behavior is a multilevel indirect and complex approach—and that it often fails. Nonetheless, there have been successes, such as getting people to stop smoking indoors, which took many years to accomplish.

Portable Air Cleaners Have Their Uses

The preferred option regarding air cleaners is to have one installed in the central HVAC system, but portable air cleaners do have their uses, such as when a space does not have a filtration option, as in a home with radiators or baseboard heating, or when upgrading to in-duct filtration is difficult, impossible, or too costly. A third reason, said Batterman, is when additional cleaning is desired to reduce risk, such as when an installed forced-air HVAC system is operating less than 25 percent of the time, when there is inadequate airflow to a particular space, when there are local sources, or for susceptible individuals. Portable air clearers might also be useful for removing gaseous and biological contaminants that would not be removed by filters in a central HVAC system.

Based on theory and experimentation, portable air cleaners can reduce particulate matter exposures substantially, and some studies suggest they

can reduce the frequency of asthma symptoms. In practice, though, they often do not live up to expectations, and Batterman listed some of the reasons why:

• A filter’s “clean air delivery rate” and particle removal capacity may be inadequate for the space involved.
• Air change rates and open windows can affect performance and are rarely measured.
• Performance evaluations have limitations, such as limited monitoring, no control over emission sources, uncontrolled seasonal variation, and small sample sizes.
• Placement, bypass, filter deterioration, and other factors that arise in real-life applications are not considered in performance evaluations.
• Filters are effective only if used and if they have high runtimes.

In a study that Batterman and his colleagues did in a low-income community in Detroit, they provided 89 families with HEPA filters fitted with a data logger to capture use history, cash to run the filters in the bedroom of a child in these households who had asthma, and community health worker home asthma education visits (Batterman et al., 2012, 2013; Du et al., 2011; Martenies et al., 2018). Another 37 families, serving as the control group, only received community health worker home asthma education visits. Batterman noted that many of the homes were not air conditioned in the summer, and 42 of the 89 families also received an air conditioner to control ventilation. Measurements of particulate matter in the houses with air filters showed that they significantly reduced particle number counts in the submicron and 1- to 5-micron size ranges, with reductions in the children’s bedroom of 50 to 80 percent during a one-week assessment at the beginning of the study. The HEPA filters were rated at 330 cfm and had four fan speeds, and they were large enough that, even running at the lowest, least noisy speed, they provided a good number of air changes in the spaces in these homes.

For the entire study, which lasted through all four seasons, the results were mixed (Figure 9-2). Compared to the control group, the intervention groups had 50 to 91 percent lower levels of particulate matter in the children’s bedrooms, which Batterman characterized as an effective reduction. When he and his collaborators monitored how the filters were actually used, they found that behavior was a substantial source of variability in the study. In particular, they found that the filters were used about 97 percent of the time at the start of the study, but that dropped to around 70 percent over the rest of the study, which he thought might be due to “a novelty

effect.” Moreover, some households used the filter 100 percent of the time, while others used it only 10 to 20 percent of the time.

One surprise was that filter use plummeted during the 3- to 5-month spans between monitoring periods, averaging only 28 percent of the time (Figure 9-3), and variability of use among the participants was dramatic. Batterman called this finding problematic and offered several possible explanations for the precipitous decline in use. One possibility is that the participants discounted the benefit of the filter, or that they only turned the filter on when they knew they were going to be visited and study staff expected them to be running the devices. For some households, the noise and draft associated with the filters were bothersome. There was an increase in electricity rates during the study, and though the study subsidized the cost of operating the filters, households may have cut back use when their electric bills rose. Batterman noted that operating a portable air cleaner can add over $200 to a household’s annual electric bill.

Overall, the findings from this study showed that portable air filters can work when they are used, but effectiveness depends on their use, and in particular, on the user’s behavior. In this case, Batterman characterized the filter use as “unexpectedly low.”

The other part of the study involved examining the children’s asthma symptoms and some biomarkers of inflammation. There was no significant effect on either, perhaps because of the low runtimes for the units. The lack of filter use, said Batterman, causes what is known as exposure misclassification, though he added that the indoor environment is just one of multiple exposure compartments.

Modeling Filter Performance

As a follow-up to this study, Batterman and his collaborators used indoor air quality models to estimate indoor exposures to ambient PM_2.5, determine an “equivalent” exposure concentration that considers time and activity patterns, estimate the health benefits of using filters in schools and homes, and calculate the marginal costs of increasing filter use (Batterman et al., 2021). They applied the model to classrooms and homes with forced-air systems with filters installed in a child’s bedroom and the living room. Overall, the model estimated that PM_2.5 exposure accounts for about 6.5 percent of asthma outcomes in children in the Detroit area. It also estimated that installing filters in classrooms would produce an 8 to 17 percent reduction in annual asthma burden, and in the homes of children with asthma would reduce the annual asthma burden by 11 to 16 percent. The marginal cost for a standalone filter unit was predicted to be nearly $500 a year, accounting for the initial cost, electricity, and filter replacements, and between $151 and $175 for a forced-air system in homes and $40 to $60 per classroom per year for schools.

In summary, Batterman said that providing portable filters should be considered an active intervention requiring behavioral change and multilevel interventions that address both the individual and community. His group’s experience indicates that filter use or runtime should be monitored in trials to reduce exposure misclassification and that low runtime may help explain some of the variation in results that other studies have seen.

Research Needs

Turning to research needs, Batterman cited a need to understand what makes for a successful behavioral intervention, not just for portable air cleaners but for range hoods, exhaust systems, and other aspects of particulate matter control. While the literature indicates that information and simple messaging are important, they are no guarantee that people will change their behavior. The exception to this seems to be when the message permeates so deeply that it cannot be ignored, in which case behavior will change. “We are not at that point yet with the understanding about particulate matter indoors,” said Batterman. But it is possible to facilitate behavior change by considering externalities, such as if an individual perceives that their behavior will help or hurt others or through peer pressure.

There are also questions about how to engineer portable air cleaners to encourage appropriate behaviors among users, make the cleaners more acceptable, and increase their use. This may require reducing their size, cost, noise, and the drafts they produce. Providing information and enhancing the public’s awareness regarding filters might encourage people to buy them, use them, and engage in good behaviors. Some of this might be done, said Batterman, by incorporating both use or runtime information and indoor air quality sensors in air cleaner units and making that information intelligible for the user.

A final research need is to improve the controls for portable air cleaners. Running them continuously is one option, but using automatic, smart, or timer-oriented approaches or incorporating a particle monitor and occupant sensor would provide a number of advantages compared to manual controls.

HOW BUILDING OCCUPANTS INTERPRET AND RESPOND TO INDOOR AIR QUALITY SENSOR DATA

The occupants of a home or building have the power to affect indoor air quality in their environment, said Lindsay Graham, so trying to understand how individuals think, feel, and behave in relation to the information they receive is important for being able to shift behavior changes in a positive way. She showed information from a large database her research group

has created from the Center for the Built Environment Occupant Survey,¹ a postoccupancy evaluation survey representing 90,000 respondents and nearly 1000 buildings worldwide (Figure 9-4). These data show that “occupants realize that air quality may be influencing them for better or for worse, but we are split on how those perceptions really fall out.”

When she and her team looked at why people were dissatisfied with indoor air quality, the biggest reasons were tied to odors, with food tending to be the biggest source of dissatisfaction, though other factors such as odors from furniture and cleaning products were also important reasons. Research has shown that occupants often include temperature in their assessment of air quality (Melikov and Kaczmarczyk, 2012; Schiavon et al., 2017). In fact, people often conflate the idea of positive air quality or “freshness” with air temperature, air speed, and humidity, which indicates that there are many factors that determine how people feel and think about their air.

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¹https://cbe.berkeley.edu/resources/occupant-survey/

“It also tells us this is something that is hard to detect” by an individual, said Graham—air quality is not something an occupant can see, touch, or feel without the aid of a device that provides a metric. That creates a problem for getting people to pay attention to their behaviors that affect the air quality in their environments. When communicating about air quality, much of what is known comes from how outdoor air quality is reported, and over the past few years there has been an increase in efforts to raise awareness about outdoor air quality.

As Batterman noted, addressing how to effectively influence behaviors with information is an emerging area for indoor air quality. Graham and her collaborators have begun to explore that at an individual level, attempting to identify what type of information occupants prefer when trying to interpret indoor air quality and what information will lead to easy and accurate interpretation of what the indoor air quality of a space might be. To do this, they identified common environmental quality data visualization tools, generated and tested three different types of data visualizations—“numeric” (data reported in a table), “scale” (presented as points on graphs), and “health” (via icons representing population groups and states of health)—and evaluated them in an online survey with almost 250 participants. Each of the three visualizations represented the same scenario depicting high PM_2.5 and elevated CO₂ levels that might be typical in a home during cooking or cleaning.

The results of this experiment showed that participants preferred the “health” visual, though they judged all three to be easy to interpret. “What this told us was that perhaps it is not about the complexity of the information given and one’s ability to interpret the information, but perhaps preference is driven by the content itself,” said Graham. To understand this in a deeper way, she and her team examined the qualitative data they collected and found three broad patterns: participants preferred information that was easy to understand and interpret, that provided visual or graphic information, and that provided actionable information. What the participants did not like in a visualization was complexity—graphics that were confusing, not clear, not intuitive, and not easy to understand. The also did not like when action items were not included or when there was little to no information on the cause and effect of pollutants. Graham inferred that “occupants are hungry for information to try to understand and educate themselves around why air quality may or may not be helpful to them in that moment.”

Regardless of the visualization type, participants who believed the indoor air quality to be poor were more likely to also believe that they needed to take action. However, the participants were more likely to believe the air quality was bad and the environment unhealthy when viewing the “health” visual. “This shows us that there is this intuitive linkage between

understanding that the environment is unhealthy and the need to try to move toward a healthier space, indicating that there may be some motive there that we could utilize when trying to depict information like this to occupants to nudge their behaviors,” said Graham.

One of the goals of this work, she explained, was to help create guidance on how to depict information for the general public regarding indoor air quality in a way that would motivate them to take action. She recommended providing occupants with clear, graphical, and actionable information relevant to their health and behavior, not just the air, and outlining steps to overcome poor indoor air quality in their spaces.

For research needs, Graham said it is important to keep working to understand individuals and what drives their behavior—to determine whether there are individual differences, such as personality type, gender values, motivations, and lifestyle that affect data interpretation. She would also like to see research on whether perceptions translate into occupant actions, and efforts to identify building or behavioral interventions that could be implemented to nudge people toward engaging in healthier behaviors. As a final comment, she emphasized that occupant education, engagement, and empowerment are paramount when seeking to foster actions to improve indoor air quality. “Without that educational piece, we do not give occupants the potential to help us craft healthy and effective spaces where we can thrive,” said Graham.

PUBLIC HEALTH RESPONSES TO REDUCE COMMUNITY EXPOSURE TO INDOOR PM_2.5

In the workshop’s final presentation, Sarah Coefield spoke about how public health air quality districts, emergency operations centers, and nonprofit organizations are working toward the shared goal of creating cleaner indoor air opportunities for people. In Montana, where she lives and works, wildfires are a significant contributor to indoor fine particulate matter (PM_2.5), but so too is biomass burning for heating and cooking. In fact, Missoula County has a wood stove changeout and removal program that provides a bounty for homeowners to remove them and install a better source for heating and cooking that would dramatically improve both indoor and ambient air quality. Montana also has an asthma control program that sends program staff to a client’s home to identify asthma triggers and provide a portable air cleaner with a HEPA filter to residents who have asthma triggers where they live.

The smoke generated during the extreme 2017 wildfire season drove home the inadequacy of standard wildfire smoke public health messaging, said Coefield, who referred specifically to the advice given at the time for people to stay inside their home. “If you have a smoke event that is a

couple hours or a day, yes, if you go inside your home, close your doors and windows, you might be okay. When you have the same hazardous air pollution day after day for days, weeks, and months, that is no longer helpful,” she said.

She noted that the role of public health as it relates to wildfire smoke has become much larger than it was a decade ago, and there are now three aspects to that role:

• communication: providing advice for creating clean air spaces and reducing exposure, providing health advisories and guidance documents, issuing smoke outlooks, and responding to public inquiries
• intervention: setting up cleaner-air shelters and providing portable air cleaners and N95 masks
• policy work: advocating for cleaner indoor air requirements, adopting updated ASHRAE guidance, canceling or postponing events, closing schools, and setting up safeguards for outdoor workers.

Regarding policy work, she said that interventions might work in a small community, but when millions of people are exposed to huge amounts of particulate matter, the need is for policies and actions beyond what the local health department can do to safeguard health.

Messaging about Wildfire Smoke

Missoula County takes a multipronged approach that starts with crafting smoke-ready messaging before the wildfire season begins and establishing a system for issuing daily wildfire smoke updates during the season. This includes setting up listservs, email distribution lists, websites, and blogs; crafting news releases; establishing a social media presence; preparing guidance documents, posters, and pamphlets; holding public meetings; and targeting messages to vulnerable groups. The website (www.montanawildfiresmoke.org), hosted and run by Climate Smart Missoula, serves as the main source of information on the internet, providing information on how to create clean indoor spaces and what to know about wildfire smoke. Since there are people who do not rely on the internet for information, her office has taken the thousands of words she has written about wildfire smoke and condensed them into posters for rural community grocery and hardware stores, for example, and pamphlets geared to specific audiences, such as older adults. The health department has also hosted smoke-ready workshops for business owners and building operations people so they can learn how to create cleaner indoor air spaces for their customers and employees.

These workshops, said Coefield, were adapted from the Smokewise Ashland program in Oregon.

She noted that Washington State has a Smoke-Ready Week during which multiple government agencies coordinate an effort to flood the airwaves with information about preparing for fire season before it starts. The goal is to get people to have their filters ready, make sure their HVAC system is in good shape, buy portable air cleaners, and make a plan before the smoke arrives.

Communication efforts do not stop once smoke does arrive. Coefield, as an air quality specialist in the health department, issues daily updates providing information about current air quality conditions, associated health advisories, what the fires are doing, where the smoke is coming from, where it is going to go, and how it is going to behave throughout the day. The idea, she said, it to give people information so they can plan their day. “I cannot make the smoke go away, but I can at least tell them what is happening, and that can give them some sort of agency and get them through the smoke event,” said Coefield. She added that the updates also include mental health resources, because weeks and months of smoke can wear on people, as well as notices of areas with cleaner air in case people want to take a road trip to get some relief.

Interventions

Interventions during a smoke event include portable air cleaner distribution and instructions for creating a do-it-yourself air cleaner from a newer box fan and a good HVAC filter, which can be quite effective at removing particulate matter from indoor air. Coefield cautioned, however, that box fans built prior to 2011 had an unfortunate tendency to catch on fire, while newer fans have numerous safety features that prevent that from happening. She noted, too, that do-it-yourself air cleaners are louder than consumer-grade portable air cleaners. Interventions may also include evacuation (this happens more frequently in First Nation communities and in Canada).

In Missoula, her department has a cache of 125 portable air cleaners that she loans out to daycare centers and preschools to protect this vulnerable age group. She also keeps some in reserve for schools. When it comes to setting up an air cleaner distribution system, it is important to decide ahead of time how to select and triage recipients, whether the program will loan or give them away—she prefers giving them away—and if there will be follow-up to ensure that the recipients are using the cleaner correctly and replacing filters as needed. She noted that Ashland, Oregon, has a good system using text messages to keep track of the recipients, and she recommended the health department there as a good resource when setting up a

distribution system. There is also the matter of how to fund the acquisition of a cache of air cleaners, since grant funding for this purpose is rare and usually competitive.

In recent years, said Coefield, interest in setting up cleaner air centers has grown, though this is a resource-intensive intervention. The least resource-intensive step is to preidentify publicly accessible buildings in the community that have advanced filtration and a plan for wildfire season; but she acknowledged that the odds of finding a building that meets such requirements before fire season are not high. An approach that has a moderate resource demand is to designate respite sites where people could go during business hours to get a few hours of relief. Those places, however, have to provide child care, snacks, entertainment, sanitation, and security. The most resource-intensive plan is to create 24-hour shelters, with food, sleeping facilities, and staff available.

Creating a cleaner indoor air center will likely require retrofitting the HVAC system to provide cleaner air and installation of sensors to monitor indoor particulate matter levels. It is also important to be prepared for the fact that smoke events can last for days, weeks, and even months, as happened in Missoula in 2017, and to consider who will use the space and if there is another, less expensive, and less resource-intensive intervention available. As with the air cleaner cache, there is also the need to secure funds to create a cleaner indoor air center. It may be less expensive, for example, to provide air cleaners to people.

The Missoula health department partnered with EPA in a study called Advance Science Partnerships for Indoor Reductions of Smoke Exposures² to better understand indoor air quality in publicly accessible buildings that people might go to as a respite from the smoke (Hagler et al., 2020). Coefield and her collaborators put low-cost sensors inside and outside a variety of buildings and found that nearly every building had MERV-8 filters (buildings that were supposed to have MERV-13 filters because they are LEED Platinum certified³ did not. But the amount of particulate reduction in the buildings with MERV-8 filters varied significantly, as did different locations in the same building.

Coefield noted that the best reduction the study team found in the 2 years of field research was a 60 percent reduction in particulate matter—“not fantastic.” “If there are 200 μg/m³ outside, and you only have a 60

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² Additional information is available at https://www.epa.gov/air-research/wildfire-study-advance-science-partnerships-indoor-reductions-smoke-exposures.

³ LEED is a building certification program administered by the US Green Building Council that focuses on design, construction, and maintenance practices that promote environmental responsibility, indoor environmental quality, and energy conservation.

percent reduction, you still have unhealthy air quality inside the building,” she said; for “preschools, schools, and day cares, you want better than that.” She also reported that the project’s engineer found many troubling issues with HVAC maintenance that explained a great deal of the results they found in the buildings.

Fortunately, said Coefield, some states are enacting policies to implement some of the actions necessary to protect the citizenry from extreme wildfire smoke events. Montana, for example, now has a rule that schools must limit infiltration during air pollution events and inspect their HVAC systems annually. Washington state has issued school closing guidance, and California has allocated funds for a Wildfire Smoke Clean Air Centers for Vulnerable Populations Incentive Pilot Program and will provide money to upgrade filtration systems and purchase portable air cleaners. California has also established worker protection rules that require companies that do not filter their air to provide respiratory protection during wildfire smoke events.

In addition, ASHRAE has published the framework for a proposed guideline (Guideline 44P – Protecting Commercial Building Occupants During Wildfire Events) that would advise building owners on how to make buildings with HVAC systems smoke ready (Javins et al., 2021). This guidance, said Coefield, should have more weight than anything she could say to a building owner.

In summary, Coefield said there is a need to know more about the most effective means of communicating to trigger behavior changes and to know what the most effective interventions are that will provide the biggest return on investment. Monitoring is needed, too, to show how effective cleaning technologies are during an extreme wildfire smoke event. And more work is needed at the policy level to create cleaner air spaces using existing technology that works.

DISCUSSION

Moderator Rengie Chan initiated the discussion by asking Batterman to elaborate on the findings of the field research he described in his talk. He reiterated some of the major parameters of that work and the need to address a number of personal and community-level factors—such as air filtration unit size, noise, and drafts—to encourage filter use. His Detroit-based study used trained personnel from the neighborhood to provide engagement and education to the participants on an ongoing basis. However, “while providing information on fine particulate matter exposure hazards is helpful,” he said, “it’s not the same as behavioral change.” Study participants came from financially challenged households and spending

$400 to $500 a year for air filtration is not a viable option for people in those circumstances. At the same time, Batterman noted that many people with sufficient means still buy the cheapest filters available, so there are opportunities to improve labeling and merchandising practices to encourage different purchasing choices. He acknowledged that even he does not completely understand the current labeling system for filters he finds at the hardware store.

Graham responded to a question about the role of sociocultural factors and background in the selection of participants in her study by noting that they were all in the San Francisco Bay Area, where wildfire events are a regular occurrence and most individuals are instructed at some point to pay attention to the air quality around them. She acknowledged that culture can also influence perceptions and decision making, and said that it would be interesting to replicate her study in another location where air quality awareness was different to better examine such questions. She and her colleagues had asked participants whether they had lived in locations outside the Bay Area to begin to tease this out.

Chan asked Coefield about challenges in helping schools to interpret the air quality data they get from sensors installed in their buildings. Currently, said Coefield, there is no requirement for schools to monitor indoor particulate matter levels or to provide cleaner indoor air for their students. “We need to get to a place where we actually require cleaner indoor air so we can see the types of changes that we need, because if everything is voluntary, folks find other uses for their money,” she said.

Asked about the value of adding ultraviolet, ionization, and photocatalytic oxidation systems to existing HVAC systems, both to remove indoor particulate matter and to address COVID-19 concerns, Siegel called this the “kitchen sink” approach to air cleaners and said that none of those devices have been demonstrated to add any value. In some cases, there may even be negative consequences. “That does not mean that those technologies maybe do not have a place, just that we lack evidence of efficacy, and I think because of the potential harm of some of the devices and some of the technologies out there,…we have to be careful,” said Siegel. Batterman reported that some city and state governments believe that plasma air purifier units work to limit exposures, and he is finding it challenging to convince them otherwise.

Singer reiterated an earlier message that basic filtration works well as long as the system is moving air through the filter. One of the nice things about portable air filters, he said, is that many come with a certified Association of Home Appliance Manufacturers rating that tells the consumer that if they operate it at the set speed, they will know what they are getting and not have to worry about bypass—and many of them are more efficient to operate than a central HVAC system.

Richard Corsi remarked that a good, portable HEPA filtration device installed in a typical classroom of 25 students costs about $10 per student per year, compared to the $15,000 annual cost of educating that student. He would like discussion of the health benefits of limiting indoor particulate matter exposures to focus on that cost per student and believes it would get more attention. Singer said that was an excellent point, particularly given that classroom HEPA filtration units would be one of the easier solutions for a school district, which could do a mass procurement and not need to upgrade an HVAC system that might be 30 to 50 years old. “It’s quick, it’s affordable, and the maintenance is very simple and we all need to keep hammering away that sometimes the simple solution is the best solution.”

A participant asked if higher-rated MERV filters can create problems in older HVAC systems, and Siegel replied that low air flow can be a problem in older central, forced-air systems. He countered, though, that the filter is rarely the problem; instead, often there is not enough airflow through the negative pressure return side of the system. The real answer, he said, is to fix the fundamental problem with the system and then put in a good filter. He also noted that the common assumption that a better, more efficient filter produces a higher pressure drop is not borne out by the evidence. He recommended replacing the filter slot to address bypass with a gasketed filter slot that can accommodate a 2-inch or 4-inch filter, which will have a lower pressure drop and provide the needed performance. The key is to educate consumers and installers. Batterman added that many buildings still don’t have air conditioning: it is difficult to provide adequate filtration when windows are open.

Chan asked the speakers to comment on the idea of focusing more on reducing infiltration or achieving tighter building envelopes as a way to mitigate indoor exposure to outdoor particulate matter. Batterman replied that his group has looked at dozens of schools and found chronic problems with lack of ventilation or inadequate ventilation for the capacity of the space, which has to be addressed. Coefield added that tightening the building envelope can be a good idea for homeowners in areas with poor outdoor air quality, but the trade-off is that indoor-generated particles are trapped inside and it’s necessary to mitigate that exposure. Graham noted the importance of educating the public so that people understand how their behaviors and their daily actions influence their indoor air quality. “That is central to all of these issues that we are talking about because that is going to dictate the success or not of how we continue to engage within those spaces regardless of the building intervention,” said Graham.

A question was then raised about the effectiveness of filtration on particulate matter with different chemical compositions. Gall replied that

the primary determinant of removal efficiency was particle diameter, which means that a filter can be effective for particulates originating from multiple sources in the same size range. Singer qualified, though, that organic compounds can accumulate on filters during events like wildfires and lead to both odors and subsequent exposures. Filter replacement isn’t cheap and there is a need to refine strategies for determining when replacement is appropriate and whether there are other strategies for mitigating this problem.

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