National Academies Press: OpenBook

Air Quality in Transit Buses (2023)

Chapter: Day 1 Session 1

« Previous: Keynote Speakers
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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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Day 1 Panel Sessions

Session 1

Air Quality Inside Transit Vehicles

Nathan Edwards, MITRE (formerly), U.S. Partnership for Assured Electronics (USPAE) (current), Moderator

Presenters

Meghan Ramsey, Massachusetts Institute of Technology Lincoln Laboratory

William Lindsley, National Institute for Occupational Safety and Health; Centers for Disease Control and Prevention

Donald Milton, University of Maryland

William Lindsley and Meghan Ramsey laid the foundational knowledge of the science of disease transmission and air quality and the biological threats to the public.

Lindsley covered the basics on how airborne transmission diseases happen, starting with the definition of an aerosol: an aerosol is any kind of airborne solid or liquid material, that is, dust, smoke, clouds, or diesel soot. The terms “particles and “aerosol particles are also defined as solids or liquids or a mixture of anything airborne. Aerosols are airborne particles of less than about one-tenth of a millimeter, or less than 100 microns. Many respiratory infectious diseases can be spread by aerosols, and everyone is constantly expelling a cloud of tiny droplets. Droplets that are greater than 100 microns tend to fall through the air more quickly. Someone who has a viral respiratory infection has a virus that has entered the lung cells and is making copies of itself. The virus then bursts out of those cells and into mucus in the airways and uses that to spread to other cells. The viral load is a concentration of virus in a bodily fluid such as respiratory mucus. If mucus is transported outside of the respiratory tract, the virus spreads by being propelled from the mouth and nose along with the mucus. There is a lot of natural variability in aerosol production among people, but, generally, the larger droplets come from the larger airways such as the trachea, mouth, and nose. Being sick produces more mucus and aerosol generation through sneezing and coughing. Respiratory activity involving the vocal cords will shed a lot of aerosols versus quiet breathing; breathing and exercising and heavy breathing also increase aerosol generation.

Lindsley noted that once aerosols are generated, how long they stay in the air is determined by their size, shape, and density (i.e., how long it would take to fall from the mouth of a typical person down to the ground). The 100-micron aerosols will fall 5 feet

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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in about 6 seconds, which is very quick. Smaller aerosols, or those that measure about 10 microns, take about 8 minutes to go 5 feet. A 1-micron aerosol would take hours to reach the ground, so the smaller aerosols do not really settle; they just tend to float in the air until the ventilation system, or something similar, removes them. One example of small aerosols is cigarette smoke: If a smoker is in a room that is not well ventilated, others will smell the cigarette no matter where they are in the room, because those small aerosol particles can drift all over. Thus, the particles are not all going to settle to the ground, so everybody anywhere in the room is eventually going to be exposed.

Lindsley said that given how far aerosols can travel, where short ranges are equivalent to a few feet from another person, it is very easy for aerosols to travel from one person to another, especially if somebody coughs or sneezes. If someone is 6 feet away and they give a hard cough or a hard sneeze, those aerosols and droplets can easily travel across and land on people. As the distance between people increases, it is more likely those droplets and aerosols are going to fall to the ground before they can reach somebody. The smaller aerosols do not really settle and tend to stay airborne. Those will float around through a room or a bus until they are blown out by the ventilation system. The highest concentrations are closest to the source, and that concentration falls off farther away from the person. Air currents and ventilation systems have a big effect and can sweep aerosols toward people or away from people, and they can also take aerosols out of a confined space. A person’s breath tends to be warmer than the air around a person, and that creates thermal plumes, or hot air rising. That breath carries the aerosols with it, and that helps to carry them further within a confined space.

Lindsley talked about how long a virus lives. This is a complex question to study and takes specialized equipment, and different researchers tend to find different results because it depends on the conditions that are used. Generally, however, researchers found that respiratory viruses can survive anywhere from minutes to days, depending on a lot of different factors. The first factor is the species of the virus. Some viruses survive longer in the air than other virus species. Temperature also has a big effect, and viruses survive a lot better at cold temperatures than they do at warm temperatures. Humidity also has different effects on different viruses. Some viruses die quicker when there is low humidity, some die quicker when there is high humidity, while the survival of other viruses is not affected by humidity. Sunlight also has a big effect and can have a strong disinfecting effect on viruses contained in aerosols, which is one of the reasons why a virus does not spread as much outdoors as it does indoors. A recent laboratory experiment showed that it took 6.5 hours for 90% of the COVID-19 virus in an aerosol to die off at 20% relative humidity with no sunlight and at room temperature. When the relative humidity was increased to 70%, the time for 90% of the virus to die off was about 2.3 hours. When strong direct sunlight was added with 20% humidity and at room temperature, 90% of the virus died off in 10 minutes. At 70% humidity, 90% of the virus died off in 6 minutes. Ultimately, the study found that humidity had a modest effect and direct sunlight had a very strong effect.

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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Lindsley explained what happens when someone inhales an aerosol containing virus. Where those aerosol particles are going to be deposited in the respiratory tract depends on the size of the particles. Aerosols larger than about 20 microns are going to be deposited almost entirely in the mouth, the nose, and the pharynx, which is the back of the throat. As the aerosols get smaller, especially below about 10 microns, the aerosols can get deeper into the lungs. For example, if someone inhales a cloud of 1-micron aerosols, approximately 29% of those aerosols will be deposited in the upper (nasopharyngeal) airways, about 3% will be deposited in the trachea and larger lung airways, and about 12% will be deposited in the deepest parts of the lung. The remaining 57% will be exhaled. These are broad averages and depend a lot on person-to-person variation in the size of lung anatomy. Also, because the nose is a much better filter than the mouth, aerosols inhaled through the mouth tend to go deeper into the respiratory tract. For example, during exercise, taking deeper breaths tends to get deeper deposition of aerosols. Finally, the state of a person’s health can affect what happens when someone inhales aerosols. Some lung diseases can affect where aerosols are deposited.

Lindsley further discussed the chances of infection for a person who has breathed in an infectious dose of viral aerosols, or the amount of virus on average that is going to infect 50% of the people who are exposed to it. The infectious dose, however, is not one number and depends on several factors. The infectious dose depends on the species of the virus, because some viruses have a low infectious dose. COVID-19 seems to have a very low infectious dose. The infectious dose of other viruses can be much higher. The infectious dose also depends on where in the respiratory tract the virus deposits. Some viruses do a better job at infecting the nose or the mouth than they do deep in the lungs. For other viruses, it is the reverse. Obviously, the health of the person has a big effect on whether the person is going to become infected or not. If a person is young and healthy and has a robust immune system, he or she is much less likely to become infected than somebody who is immunocompromised. Additionally, immunity and vaccination have a big effect. If someone has been exposed to the virus before, or if he or she has been vaccinated, that person is much less likely to become infected.

Lindsley also discussed how to stop aerosol transmission, including all the actions that have been taken for the COVID-19 pandemic. Vaccination is a big factor because it helps keep people from getting infected, but there are also nonpharmaceutical behaviors and practices, which were especially important before vaccines became available. One such example is face masks. If an infected person is wearing a face mask and is coughing and exhaling, that face mask tends to block some of those aerosols and helps keep that virus from getting out into the environment and infecting other people. Also, if a person is uninfected, a good-quality face mask blocks some of that virus from being inhaled and, again, reduces the likelihood a person will be infected. Other helpful behaviors and practices include limiting the number of people indoors and physical distancing, especially with regard to large aerosols and droplets.

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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For the smaller aerosols, such as COVID-19, ventilation and air filtration are important to help remove the virus before people can breathe it in. Air disinfection, such as ultraviolet germicidal irradiation (UVGI), which is a disinfection method that uses short-wavelength ultraviolet light (ultraviolet C, or UV-C) to kill or inactivate microorganisms, can also help kill the virus. There are multiple nonpharmaceutical interventions that work best when used together in a layered approach. These are much better than using just one or two interventions to stop aerosol transmission.

Ramsey described her work in threat characterization and solutions analysis and understanding how biological and chemical threats would propagate in a real-world environment. This is done using safe simulants, meaning safe materials that mimic the relevant properties of a particular threat, to understand how these threats move and evaluate effective response strategies.

When the biodefense arena thinks about biothreats, they are traditionally Centers for Disease Control and Prevention (CDC) category agents (e.g., Bacillus anthracis, which causes anthrax, or Yersinia pestis, which causes plague) where small and observable quantities can have huge impacts. These threats are tricky to deal with because symptoms may be delayed by days after exposure, so there is no immediate indication that infection has happened and the disease can spread between infected individuals. Similar to the challenges and potential health and economic impacts that traditional biodefense or biothreat agents pose, naturally occurring diseases also pose an enormous threat. Both traditional biothreat agents and naturally occurring diseases are of increasing concern because of the reduced barriers to gaining access to these biothreat agents. Additionally, environmental destruction, disruption, and climate change contribute to predictions for an increased frequency of pandemics into the future.

Ramsey noted that threat characterization has focused on urban areas, in particular because of the high population density in these areas and the interconnectedness that transit networks bring about. Transit interconnects populations, but transit can also contribute at a macro scale to the dispersal of air and airborne particles through urban areas. The air that moves in front of trains can be a driver of air and airborne particles through an environment, and the air within the train cars will get captured and moved through the environment. Transit subway tunnels are not a closed system (i.e., there is also outdoor ventilation as a train moves through the tunnel), so there is movement of air into the outdoors. On a macro scale There are mechanisms of dispersion for the virus on a micro scale and also within transit vehicles; therefore, passenger proximity and density can provide opportunities for disease transmission.

Ramsey also discussed the smaller scale of biothreat dispersion within transit vehicles and what might be done to reduce this dispersion. Normal respiratory activities such as coughing, sneezing, and breathing generate liquid particles or aerosols in a range of sizes. For example, particles released during breathing are generally smaller than those released during coughing, and the size of the particles determines their dispersion

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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properties. Smaller particles will take much longer to settle over a comparable distance than a larger particle of 100 microns, which would fall out rather quickly.

For a respiratory disease such as COVID-19, the respiratory aerosols that are produced may contain virus, and studies have shown viral particles can be present in these smaller respiratory aerosols that get produced. A significant portion of these small particles can contain virus and, because they remain airborne for longer, are of particular concern. Once these aerosols are in the air, there are many factors that affect risk. Risk is a product of how prevalent the pathogen transmission is in the community at large at any given moment and how many pathogen-containing particles are shed by an infected individual. Knowing the size of these pathogen-containing particles is important because of how far they go before they settle out of the air and because their size will affect where they deposit within the lung. Knowing how long the pathogen retains infectivity in cells is dependent on lots of different factors, including the composition of the aerosol, the environment, the specific pathogen, a person’s health status, and the proximity of other individuals. Additionally, the number and type of mitigations in place might limit the dispersion of the virus. Generally, reducing the number of pathogen-containing particles in the environment will reduce risk.

Ramsey noted there are lots of different approaches in recent studies of COVID-19 transmission on transit vehicles. Some of these are case studies focused on what might have happened in reported cases of transmission by using a tracer, materials, or simulants to understand airflow on transit vehicles, actual sampling for COVID-19, and modeling studies. There is limited evidence across the literature overall for transit as a key driver of infections in urban areas. There is a range of human-generated respiratory aerosols that might contain pathogens such as bacteria or viruses, so these different air contaminants will settle onto surfaces based on size. They can be removed from the environment by filtration and by dilution with fresh air. A gas molecule (e.g., odors or exhaled carbon dioxide) is much smaller than any of these particles and does not settle onto surfaces. Gas molecules will not be removed by filtration, because of their small size; they can only be removed by dilution with fresh air. Mechanisms to reduce pathogen exposure include filtering the air to remove the particles, introducing fresh air to dilute the particles, ventilation, and using decontaminants to inactivate the pathogen. There is also a variety of individual actions that can be taken to reduce spreading, such as using masks and social distancing.

Ramsey stated that the specifics of the air filtration and air exchange systems are going to vary a lot across vehicle type and model, and even between different models of subways or buses. Therefore, the options that are available for improving air quality or reducing pathogen transmission are going to vary. At a high level, doors and windows provide an opportunity for increasing ventilation. Train cars generally have a roof-mounted ventilation system unit on either end; this unit provides filtration of the air. Air is pulled from inside the train car up through the vents and then filtered and conditioned

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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and returned along the length of the car. In this process, fresh air from the outside is also mixed with this recycled or filtered air, providing another opportunity for improved insulation or introduction of fresh air.

Ramsey noted that modifying any of these parameters or changing anything often comes with a trade-off. The efficiency of the filter that is put in the HVAC system might be increased, but there might be a limit because more efficient filters could place an unacceptable burden on the system. To dilute the particles, one can increase the intake of fresh air through the HVAC system or through open windows; however, it may not be possible to introduce fresh air through the HVAC system or through open doors and windows. Depending on the environment, extreme temperatures reduce the ability for climate control. Decontamination through spraying or wiping of surfaces or treating the air can be performed, but the duration of viral viability can pose logistical challenges to implementing effective decontamination in the real world. To reduce shedding and exposure, wearing masks, conducting surveillance testing, and maintaining distance are mitigation options, but there are policy challenges and compliance issues associated with many of these options.

Ramsey further discussed filtration efficiency and explained that filter efficacy is rated on the basis of the minimum efficiency reporting value (MERV) rating. In general, filters with higher MERV ratings are more efficient at removing smaller particles such as the very small aerosols that might contain COVID-19 or other threats. A MERV 13 filter has a higher efficiency and will remove more of the smaller particles than a MERV 8 filter. In general, a higher MERV rating will lead to a higher pressure drop, which will place a higher burden on the HVAC system and will reduce airflow through it. While there might be higher filtration efficiency, the air is moving more slowly, and there are also potential mechanical burdens being placed on the system. There are different mechanisms for achieving the MERV rating, and one of these is physical pore size or the physical components of the filters. Smaller pore sizes trap small particles more efficiently, but there are also filters that have surface treatments such as electrostatic coatings, so that the impedance on the airflow does not change, but they are more efficient at collecting particles from the air. It is important to know that surface treatments may interact with the environment and change the filter’s longevity over time.

Ramsey went on to discuss how tracer materials can be used to characterize airflow and mitigation effectiveness. Air changes per hour (ACH) is a quantitative measure of air exchange and is simply the number of times per hour that the air and space are replaced with outdoor air. A higher ACH, or a higher number of ACH, means more air exchange and generally lower risk for breathing in infectious aerosols that might be in the environment. One way of measuring ACH for a particular environment is by using a tracer gas such as sulfur hexafluoride to get a quantitative measure of the impact of changing operating conditions. A study done by the Massachusetts Institute of Technology Lincoln Laboratory released a small amount of sulfur hexafluoride gas—

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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which is colorless, odorless, and nontoxic—mixed evenly through a shuttle bus. If one starts with a consistent concentration of this gas in the vehicle when implementing operating conditions, the concentration of gas can be measured over time. For example, the bus being driven with the windows closed produced a slow decay in the concentration of sulfur hexafluoride gas over time, likely due to leakage around the doors and around the windows. When windows were open on the bus when it was driven around, a very rapid reduction in the gas concentrations occurred. These measured data are described by the equation

C(t) = C0e−λt

where

λ = air changes per hour (ACH) and

t = time.

The exponent λ in this equation denotes the ventilation rate expressed as the ACH number, which, for example, is greater when windows are open compared to when windows are closed.

Ramsey reiterated that all these mitigations around changing operational parameters come with trade-offs and that gas tracing is a way of determining how big the impact might be. For example, with windows closed on a shuttle bus, the ACH number was approximately 2, whereas opening the windows on the shuttle bus increased the ACH number to 35. The challenge with tracer gases is that they do not capture all the complexity of respiratory aerosols because they will not settle onto surfaces as particles do and they cannot be removed by filtration.

Ramsey explained that more complex simulant materials can be designed to mimic the properties of respiratory aerosols to help researchers understand how those aerosols move through vehicles. Researchers can customize the size of the simulant to see how it is released and in what velocities and quantities. Researchers can add specific constituents to these particles, such as fluorescence or molecular barcodes, so that in a complex environment that already has a lot of aerosols, researchers can track the specific aerosol through the environment. Then researchers can measure concentration and its decay over time and do a similar sort of experiment as discussed for sulfur hexafluoride to determine equivalent ACH. Filtration, and not just the introduction of fresh air, reduces the particle concentration. Ramsey reiterated that safe simulants offer a powerful approach to mimicking pathogens of interest and will generate quantitative actionable information.

During the question-and-answer session, Lindsley agreed that measuring the contaminants in the air was an art as well as a science. There is equipment available to

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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measure airborne particles, but measuring how much virus is in the particles is more complicated, especially in real time. Usually, samples are collected and then taken back to the laboratory. Ramsey added that the real-world environment contains a number of contaminants, and not just the one contaminant that the researcher is looking for. These other contaminants, both biological and nonbiological, can interfere with measuring the presence and amount of the one contaminant in the environment being tested for by the researcher.

Donald Milton added that because of the way aerosols are formed in the respiratory tract through little bubbles, the aerosol has a lot more virus in the smaller particles than in the larger particles. Viruses and bacteria are slightly hydrophobic, and therefore try to escape the liquid and get into the bubble films and concentrate in the aerosol. Research on both influenza and COVID-19 shows there is more virus in the smaller particles, even though the volume of the particles is smaller. The particles of less than 5 microns have more virus than the particles larger than 5 microns. Milton further described research that cultured the COVID-19 virus from both particles of less than 5 microns and larger than 5 microns; the fine particles penetrated filters the most and traveled the farthest. Milton also noted that on a crowded bus, direct UV air sanitation with 222-nanometer UV is a useful technology for sanitizing air at that short range, but how much UV is needed and how to best design a deployment to make it the most effective are challenging issues. Upper-room UV is much cheaper, but it is not going to work in the low head space associated with transportation. However, some in the airline industry are starting to install far UV; they have shown that it does not cause problems with the deterioration of materials on aircraft, so there is promise.

In response to a question about viral load and quantification of air quality considerations, Milton further explained that human noses capture a majority of everything, even down to the particle size at which filters no longer work, as the nose has a lot of defense mechanisms in place. For example, if the influenza virus is squirted into the nose, it takes 100,000 times more viral particles to cause an infection than it does if it is in a fine particle that gets past the nose and into the lungs. The lungs are much more vulnerable, and that is the concern with COVID-19 and the small particles that can penetrate past the nose and into the lungs. Other infections, such as tuberculosis (TB), cannot get started in the nose and must be in the alveoli, the tiny air sacs of the lungs, to find the target cells that they can infect. COVID-19 and influenza can infect cells that are throughout the airways.

Milton described upcoming studies in which researchers will be exposing susceptible people to cases of influenza naturally acquired in the community. In that context, researchers may be able to answer some of these questions about how much virus was in the air, that is, was it really by inhalation exposure or was it touch or spray exposure?

Another audience member asked whether there were statistics on the effects of barriers on operator infection levels, including barrier efficiency on both transit riders and

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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operators. In response, although not aware of statistics on barriers, Milton stressed the importance of having a clean air supply behind the barrier where the operator is located, so that the operator is in a positive pressure area. Otherwise, barriers can act to defeat air flow and cause problems as much as they can help solve them. Lindsley agreed that, unfortunately, in solving one problem and avoiding big droplets, another problem can be made worse, and that the barrier can actually block the distribution of air and make aerosols accumulate rather than be swept out by the ventilation system. Lindsley described a National Institute for Occupational Safety and Health (NIOSH) study that simulated the use of barriers at a checkout line [Jacob Bartels et al., “Laboratory Study of Physical Barrier Efficiency for Worker Protection Against SARS-CoV-2 While Standing or Sitting,” Aerosol Science and Technology, 2002, 56(3), available at https://www.tandfonline.com/doi/full/10.1080/02786826.2021.2020210]. The researchers found that, depending upon the airflow, the barriers could be very useful or they could make things a little bit worse. For example, a barrier is a great defense against a squirt gun, but cigarette smoke has small aerosols that can go around the barrier, and that barrier can stop the airflow and cause the cigarette smoke to build up.

In response to an audience question on how the pandemic circumstances can lead to supporting the development of new technologies to improve air quality inside transit buses, Lindsley and Ramsey agreed that the pandemic has been a learning experience on filtration, ventilation, and air disinfection, and that these biothreats are not going away.

In response to a question on how to capture the movement of aerosols when test agents such as sodium chloride or potassium chloride cannot be differentiated from sea salt in the air, Lindsley noted that there is a variety of different tracers that can be put in an aerosol. However, to find the best tracer, the aerosol needs to be detected easily. For example, barcoded segments of DNA can be put into an aerosol and examined with a polymerase chain reaction (PCR) test. Lindsley described another approach, in which a NIOSH conference room was set up as a mock classroom and cleared of particles, so that whatever was detected in the room was something the researchers knew came from the simulator. Ramsey reiterated that the barcodes are fluorescents that have a specific handle on the tracer and that it is important that what the researcher is releasing is mimicking the most important aspects of what the researcher is looking for in that scenario. Tracers are not a one-size-fits-all solution, meaning that testing the effectiveness of a filter requires the right particle, and testing the effectiveness of a far UV decontamination system requires that the particle can be killed or impacted by UV. Therefore, the experimental design is not a trivial challenge. Milton added that there is great potential for using these surrogates, including recent experiments in which people’s hands were painted with bacteriophages, a virus that infects and replicates within bacteria, to see whether they washed their hands well.

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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Chats during this session included the question of whether there are statistics concerning the efficacy of physical barriers in diverting aerosol transmission within bus cabins and statistics on limiting bus operator infection levels. A responder noted that because barriers can worsen problems, there is a need to supply clean air to the operator behind any installed barrier. On this topic, Lynn Donovan noted that barriers have not prevented drivers in King County Metro from getting COVID-19 at all and that some contracted COVID-19 for a second time. Andrew Krum noted that the Virginia Tech Transportation Institute (VTTI) has provided an engineering controls summary of attempted technologies titled “Layered Approach to Reducing the Impacts of COVID-19 on Transportation,” available at https://www.vtti.vt.edu/covid-engrg-controls/. VTTI has also published a summary of external air impacts and use of existing air handling features on transit bus airflow, available at https://www.vtti.vt.edu/PDFs/Transit%20Bus%20Engineering%20Controls.pdf.

A chat question by Erik Christensen asked whether any studies have addressed the effectiveness of filtering out fentanyl “smoke” from fentanyl use on a bus. Ken Bogen (of Versar, Inc.) replied that fentanyl is a white crystalline powder with virtually zero vapor pressure at room temperature; so, to be respirable, the powder (or dissolved liquid solution) must be aerosolized. Bogen noted that a size distribution of aerosolized fentanyl is listed in Table 2 of the article by Yadov et al., “Acute Inhalation Toxicity of Smoke of Fentanyl and its 1-Substituted Analogs in Swiss Albino Mice,” Cellular and Molecular Biology 2014, 60(3):1–9, DOI: 10.14715/cmb/2014.60.3.1. This paper indicates that nearly all fentanyl particles are <5 microns in diameter. N95 masks filter at least 95% of particles of the most penetrating particle size (0.1 to 0.3 microns) and have greater filtration efficiency for larger particles [see Quian et al., “Performance of N95 Respirators: Filtration Efficiency for Airborne Microbial and Inert Particles,” American Industrial Hygiene Association Journal, 1998 59(2):128–132]. John Howard and Jennifer L. Hornsby-Myers noted that “An important potential occupational route of exposure to opioids occurs by breathing air contaminated with airborne opioid particles. During response situations, powder-like fentanyls can become easily airborne by disturbing opioid-contaminated surfaces, brushing opioid powder from clothing, or other incidental activities that cause powder or liquid aerosolization” [“Fentanyls and the Safety of First Responders: Science and Recommendations,” June 26, 2018 (https://blogs.cdc.gov/niosh-science-blog/2018/06/26/fentanyls-and-first-responders/)].

A chat comment by Kevin Carmody concerning air treatment (rather than filtering or exchange) noted that the U.S. Environmental Protection Agency (EPA) has provided Section 18 Emergency Exemption to some states that have applied to use a product that has been tested independently and by EPA using the COVID-19 surrogate virus bacteriophage MS2 (ATCC 15597-B1). As a nonenveloped virus, MS2 is expected to be more resistant to chemical inactivation than enveloped viruses such as the virus SARSCoV-2 that causes COVID-19. Testing showed extremely effective treatment in very little time (see Figure 1). This particle solution may offer a very high (≥99%)

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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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effectiveness, but the deployment for bus HVAC application is still in development and has only had limited testing so far. The corresponding original EPA figure and legend information from the source cited below replaces the figure that was presented in this chat comment.

Image
Source: U.S. EPA, Results for Aerosol Treatment Technology Evaluation, July 19, 2021.
https://www.epa.gov/covid19-research/aerosol-treatment-technology-evaluation.

Figure 1. Concentration of MS2 at each sampling time point during tests of an antimicrobial air treatment product. The product was dispersed in a test chamber at an EPA specialized Aerosol Test Facility in Research Triangle Park, North Carolina, prior to MS2 aerosolization. The tested product holds a Section 18 Emergency Exemption for use in certain indoor spaces in Georgia, Maryland, Nevada, Pennsylvania, Tennessee, and Texas. Error bars represent the standard deviation in averaged MS2 recoveries from each test condition. The average percent reduction in log10 recoveries between the test and control conditions are also noted for each sampling time point.
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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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Suggested Citation:"Day 1 Session 1." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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With a major drop in U.S. transit ridership since the start of the COVID-19 pandemic, an increased understanding of infectious disease in confined spaces and the role of droplets and particles in transmission has been increasingly important to the bus industry. A combination of experiments, models, and simulations in fluid dynamics has been employed to understand how aerosols move in spaces containing people.

TRB's Transportation Insights 2: Air Quality in Transit Buses provides a summary of a June 2022 in-person TRB Transit Cooperative Research Program (TCRP) Insight Event.

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