8

Indoor Particulate Matter Exposure Control and Mitigation

The final day of the workshop began with planning committee chair Richard Corsi posing three questions that would guide the day’s presentations and discussions:

1. How do we reduce exposure to and health effects associated with fine particulate matter (PM_2.5) in buildings?
2. How do the decisions of occupants in their homes, or teachers in their classrooms, affect exposure to PM_2.5?
3. To what extent can filters and mechanical systems or standalone air cleaners reduce exposure to PM_2.5 and its health effects indoors?

The day’s first session featured three speakers. Jeffrey Siegel (University of Toronto) discussed control of PM_2.5 levels in homes, and Elliott Gall (Portland State University) did the same for schools. Brett Singer (Lawrence Berkeley National Laboratory) then addressed how PM_2.5 exposure from cooking could be reduced. Following the three presentations, Rengie Chan and Seema Bhangar moderated an open discussion with the panelists.

PM_2.5 FILTRATION AND AIR CLEANING IN RESIDENTIAL ENVIRONMENTS

Indoor air filtration, said Jeffrey Siegel, involves more than just a device or filter. It comprises the totality of the filtration system and the context in

which that system exists, for it is that context that determines everything about the performance of the unit. Context, he explained, includes the virus, particle, droplet, or contaminant that has to reach the filter, a trajectory that is influenced by air flow; the ability of the filter to remove the virus, particle, droplet, or contaminant, which is determined by the system’s in situ efficiency; and removal that has to be big enough to compete with other removal processes in the environment. All of these determine the system’s effectiveness.

Most homes in North America, said Siegel, have recirculating heating, ventilation, and air conditioning (HVAC) systems, which means they do not add outside air but only recirculate air already inside the building. Measured recirculation rates—the number of house volumes that go through the system when it is operating—span a wide range (Figure 8-1), which means that there is going to be a big difference in the performance of any filter depending on air flow (Touchie and Siegel, 2018). Another important parameter regarding air flow is runtime, the fraction of time that the HVAC system operates, and that varies, too, depending on the weather and other

factors¹ (Figure 8-2). The median runtime is approximately 18 percent, which means that even with the best filter in the world, air will not be passing through it over 80 percent of the time.

Efficiency, measured by the percentage of particles that a filter removes in a single pass, can approach 100 percent for very small and very large particles (Hanley et al., 1994), which are removed by Brownian diffusion and inertial mechanisms, respectively, but it drops significantly for particles in the 0.1 to 0.3 micron range (Figure 8-3). Face velocity, the measured air speed at an inlet or outlet of an HVAC system, also affects efficiency, which drops as face velocity increases. “Right away, we see the beginnings of how operation might affect filtration performance,” said Siegel.

There are several rating standards for filters. The standard set by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE, Standard 52.2) is assessed as follows. In the laboratory,

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¹Touchie and Siegel (2018) note that “[r]untime is influenced by climate and season but also by building characteristics, equipment sizing, occupant preferences, and operations and maintenance” (p. 906).

a potassium chloride aerosol is run through a filter at fixed flow rate along with successive loadings of an artificial test dust. The test measures efficiency for three size ranges of micron—0.3 to 1.0, 1.0 to 3.0, and 3.0 to 10.0—and the results provide the minimum efficiency reporting value (MERV) for that filter.

Siegel noted three important points regarding this test. First, it is not relevant to many filters installed in residential buildings, but rather intended for filters used in commercial buildings; second, it is a laboratory standard, not an in situ standard; and third, it addresses only particles in the defined size ranges. In situ testing shows how various electret² and nonelectret filters perform in homes as opposed to in a laboratory setting (Figure 8-4).

The in situ testing results indicate that the MERV rating does not necessarily predict how well a filter will perform in a home, said Siegel. One reason for that is a phenomenon known as bypass, which can take several forms; it could be a large gap around a filter installed in an HVAC system, or a seating gap (space between the filter and the rack that holds it in place). In residential settings, it is common for HVAC systems to have a filter slot

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² Electret filters are those whose filter media has been modified by the manufacturer to initially have an electrostatic charge on the fibers.

that allows for easy filter changing, but that space also allows air to bypass the filter. Bypass always has a negative effect on efficiency, but how much of an effect depends on the filter and the bypass gap in the system (Chojnowski et al., 2009; VerShaw et al., 2009; Ward and Siegel, 2005).

Another issue is that filter performance changes with age, particularly for electric or charged media filters that account for about half of the US market (Lehtimäki and Heinonen, 1994). One set of experiments in a commercial building, for example, found that even after one week, efficiency began dropping for smaller particles, and after 36 weeks the filter’s performance had dropped significantly (Lehtimäki et al., 2002). Siegel has found similar results for charged or electrified filters in homes (Li and Siegel, 2020).

He noted that data from numerous studies show that filtration performance varies substantially and follows no particular pattern (Alavy and Siegel, 2019). He explained that there are many reasons for this, including runtimes, bypass, and the characteristics of the building. For example, in a leaky building where there is already removal of particles by ventilation, filtration will not have much of an effect (Zhang et al., 2020). “To have effective filtration you have to have a good filter, it has to be properly installed, it has to have a big enough flow rate, you have to have a long enough runtime, the filter has to be changed frequently, and of course the big picture is you have to effectively compete with what else is going on in the building,” said Siegel.

Noting that other speakers in the session would be talking more about portable air cleaners, Siegel did comment that the same issues apply to these cleaners. In addition, placement of a portable cleaner relative to particulate matter sources makes a big difference in effectiveness and performance (Novoselac and Siegel, 2009).

There are other air cleaning technologies available for consumers, including those marketed with terms such as photocatalytic oxidation or ionization. Based on the literature, Siegel said, there is no independent evidence of efficacy or effectiveness in real environments for many of these technologies, and in fact the parameters that have been evaluated point to poor performance by most of these devices. For some, such as those that use ozone or ion generation, there is the potential for harm related to byproduct formation.

Research Needs

In Siegel’s view, one of the biggest research needs, given contextual issues, is to develop a way of assessing in situ filtration performance in homes, potentially with low-cost monitors of the sort discussed during the second day of the workshop. The goal should be to have methods that can ensure

that filters perform as they should over time in real-life settings. He also would like to see how personal monitoring, rather than a central measure of performance, could assess how well a filter reduces exposure.

Research is needed as well on alternative control approaches that would maximize performance in the context of energy use and that would preserve filter longevity to reduce the cost of filtration. One idea would be to use a low-cost particle monitor as a thermostat-like control that would turn on the filter when particle levels reach a set value. Siegel also raised the idea of using artificial intelligence to enable predictive air cleaning. In fact, he and his collaborators have been working on just such an idea, and in early tests their model was able to predict high particle concentration events 2 hours ahead of time a little more than half the time, and changing the model to minimize false negatives enables it to predict high particle concentrations about 90 percent of the time.

Third on Siegel’s list of research needs would be to understand what exposures filters have avoided in the environments in which they have been deployed. In some 60 to 70 articles, investigators have used what he called filter forensics to quantitatively link contaminants extracted from HVAC dust filters to time-averaged integrated air concentrations (Haaland and Siegel, 2017). This work has produced detailed fingerprints of the concentrations of particle-bound contaminants, including the SARS-CoV-2 virus, that collect on filters. This type of research could provide a way of exploring indoor concentrations of various pollutants as well as the effectiveness of filtration and other control strategies.

PM_2.5 EXPOSURE CONTROL IN SCHOOLS

Schools are a critical environment for a susceptible population—children—and 15 percent of schools, with 6.4 million students, are less than 250 meters away from a major roadway, an important source of particulate matter (Figure 8-5) (Kingsley et al., 2014), said Elliott Gall. Schools with higher percentages of Hispanic, Black, and Asian students have disparate exposures to roadway-generated particulate matter (Grineski and Collins, 2018), making this an important environmental justice issue (Gaffron and Niemeier, 2015) as exposure to elevated levels of traffic-related air pollution can adversely affect student health and cognition.

For example, research has shown that traffic-related air pollution exposure is associated with increased asthma diagnosis (HEI Panel on the Health Effects of Traffic-Related Air Pollution 2010) and with lower scores on measures of working memory and other cognitive markers (Sunyer et al., 2015). Gall said that some of the challenges in assessing and mitigating school-based exposures to elevated air pollution levels are that those levels

may vary in space and that weather conditions can be quite important in determining those levels both around and in schools.

Over the past several years, Gall and the members of his research group have been studying traffic-related air pollution constituents in a middle school in Portland, Oregon, that was renovated in 2018 to serve Portland’s historically Black community. The school is adjacent to Interstate 5, and measurements of black carbon, ultrafine particles, and PM_2.5 taken on the school’s rooftop prior to the renovation showed elevated levels of these three types of particulate matter, with the exact burdens changing with wind speed and direction. For example, air movement across the highway toward the school led to large masses of black carbon and ultrafine particles. Gall clarified, though, that while there was some association between PM_2.5 levels and wind speed and direction, the extent to which PM levels changed as a function of these parameters was less than was observed for

traffic-related air pollution constituents such as black carbon and ultrafine particulate matter.

This finding is important for two reasons, said Gall. Research suggests that, in general, the health effects associated with traffic-related air pollutants such as black carbon (Janssen et al., 2011) and ultrafine particles (Schraufnagel, 2020) are greater than those associated with PM_2.5. In addition, ventilation standards related to minimum acceptable indoor air quality, such as ASHRAE 62.1,³ trigger increased filtration efficiency requirements only if the National Ambient Air Quality Standards are exceeded. Given that ASHRAE 62.1 references PM_2.5 levels and that the air in the school’s region is in compliance for PM_2.5, there would be no increased filtration efficiency triggered by compliance with ASHRAE 62.1 unless an engineer or building designer identified a specific near-roadway source in a site survey.

Gall said that there has been a great deal of work on traffic-related air pollution in urban environments and that this work has shown that there are strong spatial gradients of traffic-related air pollutants in those environments. Within a zone of about 200 to 500 meters from a freeway, there are elevated levels of a variety of constituents associated with vehicle emissions (Karner et al., 2010). While that zone can expand to thousands of meters at night (Kozawa et al., 2009), that is not likely to be a major concern for schools. Some constituents of traffic-related air pollution, including ultrafine particles, black carbon, and volatile organic compounds, diminish rapidly within 150 meters of the highway, while others, including PM_2.5 and benzene, decay more gradually. “The opportunity here is that we can leverage this spatial gradient in traffic-related air pollutants in near-roadway schools to potentially reduce exposure,” said Gall.

Mitigation Approaches in Schools

One straightforward mitigation approach is to consider placement of the school’s fresh air intake as far as possible from the roadway. When monitors were placed on two faces of the school—one close to the freeway, the other 80 meters away and facing a residential street—the resulting data showed that relocating the school’s air intake to be further from the highway would have an effect equivalent to installing a MERV-8 filter, but without the energy cost of a filtration system (Laguerre et al., 2020).

Another mitigation strategy would leverage both diurnal trends in traffic-related air pollution and spatial trends to reduce student outdoor exposures during lunch and recess. Gall and his students did a spatial mapping of ultrafine particle concentrations in a nearby park, where students

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³ Additional information is available at https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2.

go during recess and lunch, and around the school site by walking around the area with a handheld condensation particle counter throughout the day. The data revealed that peak levels of traffic-related air pollution occurred during the morning rush hour and remained elevated until the midafternoon, suggesting that shifting outdoor activities until later in the day would reduce the students’ exposures.

Gall reported that this approach has been explored in four schools in Ottawa, Ontario, using a “smart” ventilation system that brings cleaner air into the schools from 5:30 to 6:00 AM and then changes to recirculation until the students arrive. With this approach, the two schools that started at 9:00 AM realized a significant reduction in ultrafine particles and VOCs in the school.

A third approach involves active air cleaning for particulate matter in occupied schools. A small body of research has shown that filtration can be effective, but the results obtained in real life are variable. The interventions studied included filtration, improvements to the efficiency of filtration and the HVAC system, or deployment of standalone air cleaning and mechanical filters in classrooms. While some interventions had removal efficiencies of 80 percent or higher, the average removal effectiveness was between 40 and 50 percent (Gao et al., 2019; Jhun et al., 2017; McCarthy et al., 2013; Park et al., 2020; Polidori et al., 2013; Scheepers et al., 2015; van der Zee et al., 2017). Gall attributed the variability to factors such as the presence of indoor sources of particulate matter, how leaky the environment was, and the location of the intervention in the case of standalone air cleaners.

In the case of the Portland middle school, the contractor installed a single air handler to serve the entire school. This air handler was outfitted with an advanced air cleaning system comprising a MERV-8 prefilter, a MERV-16 filter, and functionalized carbon that targeted gas-phase species that might be elevated in a near-roadway environment. The goal of installing this system, based on the monitoring work Gall and his students had performed prior to the renovation, was to reduce traffic-related air pollution in outdoor ventilation air provided to the school to be at least equal to that of a school located in the urban background with a standard filter (Laguerre et al., 2020).

For this school, prerenovation measurements showed that the site had approximately five times the black carbon levels of the urban background when the winds were blowing across the highway and toward the school. Calculations showed that the contractor’s design was likely to have a removal efficiency of at least 84 percent for black carbon, which would meet the targeted goal. In fact, a 1-week monitoring assessment found that the effectiveness of the installed system was approximately 85 percent and that it effectively decoupled indoor from outdoor air pollution levels.

As Gall noted, there are indoor sources of PM_2.5 matter that need to be addressed, especially when targeting PM_2.5 as opposed to the traffic-related air pollutants such as black carbon. Using a mass balance analysis, his group determined the contributions to particulate matter from the outdoor air fraction of supply air, the recirculation fraction of supply air from the building, and occupants and their activities. A large fraction of the PM_2.5 sources are occupants and their activities.

Chemistry in the School

Chemical reactions involving VOCs can be an important source of indoor particulate matter. In the Portland middle school, the carbon scrubber component of the air cleaning system appears to suppress levels of secondary organic aerosol precursors and thus formation of secondary organic aerosols. Monitoring showed that there is very little particle formation when the air handling system is on, but higher total particle counts in the indoor air than in outdoor air when the system is off. “In the middle of the night, when the school is unoccupied, we see higher total particle counts in indoor air than outdoor air, which we think is evidence of chemical reactions that are forming fine particulate matter,” said Gall.

In conclusion, Gall said there are opportunities for reducing PM_2.5 exposures in schools near roadways. They include increasing the distance of the HVAC system’s air intake from outdoor PM_2.5 sources, altering the timing of activities, installing air cleaning equipment, addressing indoor sources, and possibly managing indoor chemistry via air cleaning or source reduction.

Regarding research needs, he listed the utility of more data on the efficacy of installed interventions in school, lower energy and maintenance methods for ventilation and air cleaning, more information on the strength of particulate matter sources in schools, and studies on the health effects of exposures to particulate matter of indoor origin.

MITIGATION OF PM_2.5 EXPOSURES ASSOCIATED WITH COOKING

Brett Singer reminded participants that, as Marina Vance explained during the first day of the workshop, cooking generates particulate matter from the food being cooked and from some burners. When a gas burner is working well, it produces carbon dioxide, water, nitrogen oxides, some formaldehyde, and ultrafine particles. Electric burners also produce ultrafine particles, and cooking itself produces PM_2.5, a large amount of ultrafine particles, and irritant gases. Induction burners appear to emit many fewer

ultrafine particles and no nitrogen oxides, in part because there is no very hot surface involved (Less, 2012).

The operational concept for mitigating particle exposure from cooking is ventilation, which may involve a venting range hood, vented over-the-range microwave oven, downdraft exhaust system, wall exhaust fan, and/or open windows. The idea, explained Singer, is to remove particles generated during cooking before they can mix into the rest of the air in the home. Vents that run air through a filter and then back into the kitchen produce only small reductions in particulate matter.

There are some standards and codes that require kitchen ventilation, of which ASHRAE 62.2 is the most prominent; others include those promulgated by the California Building Code, the Home Ventilating Institute, Energy Star, and the International Residential Code. The basic requirement of ASHRAE 62.2 is to have a range hood above the stove that moves air more than 100 cubic feet per minute (cfm) and has a sound level of less than or equal to 3 sones, which is a moderately annoying level, said Singer, noting that the standard was set several decades ago based on the equipment available at the time; ASHRAE is now considering whether the requirements should be updated to mandate quieter hoods. He described the International Residential Code, which was created to enable local jurisdictions to set local requirements, as “basically worthless” for kitchen ventilation, with the result that US homes are still being built without kitchen ventilation installed or with ducting to enable the homeowner to have it installed later. “That is probably the single biggest problem here, because all the other problems I am going to talk about with the quality of the range hood and users actually activating the range hood, none of that is possible unless you have the device installed in your house, and that is very expensive if you do not have ducting installed for it,” said Singer.

The effectiveness of a range hood is measured by its capture efficiency, which is the fraction of pollutants released at the cooktop or oven that the hood removes before it mixes with other air in the home. Singer explained that capture efficiency is calculated using carbon dioxide released from gas burners (or alternatively, a tracer gas), but a different approach is needed for particulate matter. His group has tried to quantify capture efficiency in the lab and shown that it increases with airflow and is much better for back burners (Lunden et al., 2015; Singer et al., 2012). In fact, for front burners, a range hood operating at 100 cfm captures only about 30 percent of the emitted particles. Over-the-stove microwave range hoods can be installed as either venting or recirculating, and when installed they need to be set to vent. If they are, tests by Singer’s group found that they work as well as comparable range hoods. In his opinion, they are a good option if they are used.

Initial work on range hood capture efficiency focused on gases, which begs the question of whether the capture efficiency for them is the same as for particles. Singer’s group attempted to answer that question and found that test results varied by a factor of two even when using consistent food sources and preparation time. Nonetheless, the results showed that range hoods effectively removed particles when cooking on the back burner and were less efficient at capturing particles released when cooking on the front burners. “When capture efficiency is high for gases, it is high for particles,” he said. Data from his group showed, not surprisingly, that more particle capture occurs as airflow rates increase (Sun et al., 2018).

In one set of experiments, Singer and his colleagues measured gas and particle levels in the kitchens and bedrooms of nine homes, six of which had range hoods of various types (Singer et al., 2017b). The installed range hoods provided varied levels of exposure reduction, with the best performance seen with a hood that extended over the entire stove. However, the presence of a good range hood does not guarantee that the occupants of the home will use it, and a Web-based survey or more than 2000 people living in Southern California homes built between 2003 and 2010 found that about three-quarters of those in homes with a kitchen range hood that exhausts outdoors used their hood sometimes, most of the time, or always, whereas hood use was less in homes where the hood vented back into the kitchen.

When asked why they did not use their range hoods, the majority of people said they did not think it was necessary. A surprisingly large number said their range hoods did not work. In a subsequent study, Singer and his colleagues looked at range hood use in California houses and low-income apartments and found that occupants were more likely to use the hood when cooking for longer periods, but that, overall, most homes used a hood less than half the time they used their stoves or ovens. Research by a Canadian team found similar results (Sun and Wallace, 2021).

One of the big problems Singer said he has observed with range hoods is that the actual airflows in practice are much less than their product certification test result. He blamed this on the higher static pressure and more airflow resistance in homes compared to test chambers.

In conclusion, Singer noted that there are resources and guidance for increasing range hood efficacy (Phillips, 2019). His simplest advice was to use low-resistance ducting,⁴ use a range hood that can move 250 cfm, make sure the range hood is quiet at reasonable air flows, and cook on the back burner.

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⁴Phillips (2019) notes that considerations also include whether range hood ductwork is installed properly and whether the diameter of the ductwork is sufficient to allow the unit to operate efficiently without generating unacceptable noise levels.

DISCUSSION

Moderator Rengie Chan invited questions from the planning committee and workshop participants. Singer led off by asking Siegel if the key to using an electret filter is to replace it more frequently, and if so, how frequently. Siegel replied that in principle, replacing filters more frequently is a great idea. “In fact, if I could get people to change their filters we would be a long way toward being in a better place,” he said. The problem with electret filters, he continued, is that performance degradation can happen within a few hours depending on the loading, and many of the homes in his study had less than 100 hours of runtime. What he would rather see than having people replace their filters twice as frequently is to invest in a sealed filter rack that eliminates bypass.

Noting that bypass is common in both commercial and residential systems and a problem that is well known to both researchers and practitioners, Chan asked Siegel to comment on what is needed to move to better-quality installations. Siegel replied that he has addressed bypass in every home in which he has lived and has been surprised at how inexpensive a problem it is to address. Consumer and installer education is what is needed, he said. Singer commented on the cost of a decent MERV-13 filter, and added that getting consumers to actually change their filters is still an issue. Gall noted that the main concern of the Portland school districts he studied was the cost and recurring maintenance associated with the installed HVAC system.

Chan then asked Siegel if the guidance on using MERV-13 or higher filters as a mitigation measure for the SARS-CoV-2 virus is a useful strategy. He replied that since the virus is much smaller than many of the particles he studies, in theory filtration should be an effective mitigation strategy. However, if the virus is part of a larger droplet, then the answer depends on how efficient a MERV-13 filter is at capturing those larger aerosols. His understanding is that most virus transmission involves 1- to 3-micron particles and a MERV-13 filter would have an 85 percent single-pass efficiency for that size, which suggests that filters can be an effective part of a mitigation strategy. He added that published results show that filters capture RNA viruses and sometimes DNA viruses. The problem is that if the HVAC system is running only 10 percent of the time, then 90 percent of the time it does not matter what the MERV rating is.

An audience member asked Gall if there is a level of airborne black carbon that is high enough that schools should not allow outdoor activities. The short answer, he replied, is no, there is not a specific number at which you might confidently say, ‘Do not let children outside to play in the park adjacent to the school.’ He noted that there are no federal standards for ambient black carbon levels, and, while Oregon does have a benchmark, it

is based on the risk of one additional cancer in a population of one million people and does not address the question of the incremental risk of being outside when black carbon levels are elevated. He did point out that there are data showing that short-term exposures to traffic-related air pollution, even on the order of minutes, have been associated with certain cardiovascular metrics such as increased heart rate. But, he added, “it becomes quickly a very political process to say where students can go, when can they go there, what is outdoor recess going to look like at a near-roadway school.”

Asked how the Portland school fared during Oregon’s bad wildfire season in 2020, Gall said that he did not have intensive monitoring then, but there were low-cost particle monitors in the school that indicated that the HVAC system did a good job of keeping PM_2.5 levels low inside the school during the wildfire event. In fact, there were discussions with the school district and local county health department about the possibility of using the school as a clean air shelter for those who might seek to reduce their exposure during a wildfire. The COVID-19 pandemic derailed that idea, but it might be reconsidered in the future.

Singer brought up the issue of equity and the challenge of air cleaning in places that do not have mechanical cooling, which is still the case in many schools around the country. Gall replied that this is a significant issue in Portland, which historically has a mild climate and so every school does not have a mechanical cooling system installed. In fact, he said, at least one published study reported that an intervention was less effective than designed because people were opening windows to provide ventilation and cooling during hot weather, which substantially reduced the effectiveness of the standalone air cleaning system being tested. At a Portland middle school that was the subject of an intervention, the windows facing the highway were modified to no longer be openable to reduce infiltration from the freeway. He noted, though, that this school was the focus of a years-long effort and a substantial investment to improve the air quality in that one school. “This is a major challenge that school districts are going to face,” said Gall. Singer added that the Environmental Protection Agency’s Tools for Schools program is a good source of information for schools that are concerned about their indoor air quality.

Kim Prather commented that she has been helping many schools reopen after the COVID-19 pandemic forced them to close and that the most common mitigation procedure has been to install plexiglass barriers to prevent viral transmission. Siegel responded that while plexiglass barriers protect against very large droplets, those droplets are not responsible for most infections, so plexiglass barriers do not address the big risk. He affirmed the importance of physical distancing and people management.

Stuart Batterman (University of Michigan School of Public Health) noted that schools often have HVAC systems with filters that are hard to get to and change, and he wondered if dedicated systems could provide better filtration for outside air. Gall said that the mechanical contractor for the Portland school he studied did look at such a system, since it allows for a dedicated outdoor air supply with air cleaning that would have addressed the issue that school was facing. He was not privy to the discussions of why the school decided on a single air handler, though it does offer the advantage of cleaning human aerosols involved in disease transmission. Siegel remarked that there is a need to develop systems that require less maintenance given the maintenance gap that already exists in many schools.

Elizabeth Matsui worried that disparities will be exacerbated without a solution to address the indoor air quality of schools that are in poor communities and disproportionately in communities of color. Singer agreed and thought that an important step in the right direction would be for school districts to use money in the American Rescue Plan Act of 2021 (Public Law 117-2) designed to support school facility repairs and upgrades. In fact, he said, schools recognize the importance of providing ventilation and this is one challenge that can be addressed at a relatively low cost by improving the filter compartment, using a good filter, and maintaining the system properly. “This is a relatively small lift to potentially greatly reduce exposure for some of the most vulnerable people in our society,” said Singer. Gall added that it took a confluence of community interests to put potential solutions forward for the Portland middle school and force the school board to consider them and act. “There certainly needs to be that ecosystem of awareness and action to bring these issues to attention and to have them acted upon,” said Gall.

Turning to the subject of range hoods, Chan asked Singer to comment on the idea of phasing out gas stoves as a solution to the indoor particulate matter issue. Singer replied that getting rid of gas stoves would eliminate one important source of ultrafine particles and nitrogen oxides, though it does not remove the need for good ventilation. He commented that research that he and others have done has shown that gas stoves, especially in smaller homes, can produce high exposures to nitrogen oxides. He also noted that when it comes to making quieter range hoods, manufacturers are going to continue to charge more for quieter models until they are required to produce them and stop making noisier models.

When asked to talk about the type of low-cost particle monitors they are using, Siegel replied that he uses several different models, and he encouraged the audience to review Dusan Licina’s discussion of various issues with these monitors. “It is important to understand the monitor you are using and its response, and I do not think that low-cost monitors eliminate the

need for using more sophisticated instrumentation, but I certainly think that they have a huge role in helping us improve the performance of filtration,” said Siegel. Gall agreed and noted that the low-cost monitors in the Portland middle school during the wildfire event provided some assurance that the system was working and could convince people that the air was better in that environment. Singer added that a test method for low-cost particulate matter monitors is making its way through the ASTM⁵ process and his hope is that when it is produced it will allow a consistent standard to be applied to these monitors. While this standard will be imperfect—as all standards are—it will serve as a general reference and help consumers distinguish between products that work fairly well from those that do not work at all.

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⁵ ASTM is an international testing and standards organization.

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