National Academies Press: OpenBook

Air Quality in Transit Buses (2023)

Chapter: Day 2 Session 1

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Suggested Citation:"Day 2 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 2 Panel Sessions

Session 1

Existing and Emerging Technologies

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

Presenters

Ashley Shipley, EvergreenUV

John Gasparine, WSP

Katherine Ratcliff, U.S. Environmental Protection Agency

Ashley Shipley began with a brief history of UV-C being used in disinfection. The first use of UV-C was in 1895 for disinfection of water. In 1903, it was used for the control and treatment of TB. The first pandemic-level use of UV-C was to stop measles outbreaks in schools. The first patent for UV-C in air handling systems was in 1941. In the 1980s, the CDC worked with UV companies on using upper air lights and developing the best upper air system to combat TB, specifically for low- and middle-income countries where vaccinations were not readily available.

Shipley explained that effective use of UV-C starts with the understanding of the pathogen, because not all pathogens are the same. There are three basic categories: viruses, bacteria, and spores. Viruses are easy for UV to address because they are smaller in size and most, if not all, species of virus have no hard outer shell. UV pierces the cellular wall of the pathogen to attack the DNA and render the pathogen incapable of causing harms to humans. Bacteria are relatively or very susceptible to UV but are usually larger in size, and some species have hard outer shells. For example, Legionella bacteria are more like a virus in size and do not have hard outer shells. On the other end of the spectrum, the bacteria that cause bacterial meningitis are larger and have a hard outer shell. Spores are much larger, and most, if not all, have hard outer shells; it would take a very high dose to inactivate spore DNA. However, filtration can be relied on to address these larger spores and larger bacteria and the UV to disinfect virus and bacteria.

Shipley noted that another aspect to understand about pathogens is how long they stay in the air and that humans are always producing pathogens. The problem with a lot of infection rates is the pathogen loads that humans produce and the length of time that the pathogens can stay suspended in the air. For example, a sneeze can produce over 10 million microscopic particles, coughs can produce about 100,000 droplets, and

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Suggested Citation:"Day 2 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.
×

talking produces about 10,000 particles. The amount of time and the number of people within a given space, and the fact that some pathogens can stay suspended in the air for up to 2 days depending on humidity, all produce air quality issues.

The UV-C wavelength can be between 200 and 280 nanometers, and the UV-C wavelength, at 254 nanometers, is an optimal middle-ground wavelength to accomplish inactivation across all pathogen types. Each pathogen has a designated dose based on its size, makeup, and physical properties. Each pathogen has its own resistance, but there is no known pathogen resistant to its designated UV dose. The dose is calculated on the basis of extensive studies that have sought to determine what applications of UV for air, water, upper air, and surface are needed for deactivation of pathogens. These calculations are published—for instance, in the book, Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection by Wladyslaw Kowalski (https://link.springer.com/book/10.1007/978-3-642-01999-9). The kill rate, or K value, and the microwatt output are directly correlated. The number of microwatts produced per second for a specific unit, and then the number of seconds required to achieve a long disinfection, can also be used in an HVAC application.

Shipley further explained that upper air disinfection has been used for disinfecting areas before passengers get on the bus as well as for surface disinfection, which also contributes to air disinfection. There are a lot of options for UV-C air disinfection technology, some of which are more viable than others, which depend on how air disinfection is calculated and how success is defined. A request for some amount of removal per air pass is more feasible, and a system that has good filtration and UV has a better approach. For a smaller space such as a bus, a system that has fluorescent bulbs is more viable than a system that uses light-emitting diode (LED) lights.

Terminals and stations can use upper air lights to prevent the spread of infection where a lot of people are waiting for their transportation. Upper air disinfection relies on air currents coming from people’s body heat and, as someone speaks or coughs, the pathogens are going to rise to the top of the space. An upper air system, or even an HVAC application with a fan that is bringing in the air and pulling it across a UV system to treat it, can be useful. Surface UV disinfection applications have been used for 30-40 years in medical applications, such as ambulances, and can be used in transit as both portable and fixed options.

Shipley also noted that LEDs are efficient and that they are much safer because their emission of UV energy is not very far; because they are safer, they can be installed in the HVAC of a bus. LEDs do apply in smaller, more compact spaces, but the farther away from those diodes, there is far less output than the traditional fluorescent bulb. For now, there are some applications where LEDs would be extremely viable, and some applications where they are not the best option yet. UV LEDs are also more expensive to replace than the typical LED strip and more expensive than a traditional fluorescent bulb. The other wavelengths, UV-A and UV-B, are safer and do have some germicidal

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Suggested Citation:"Day 2 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.
×

properties, but they are not as optimal as UV-C, which is the most effective or optimal choice. The UV-C 222 wavelength is reportedly safer for human exposure, but it is not as effective as the UV-C 254 wavelength in disinfection.

John Gasparine discussed the range of bolt-on technologies that could be considered for existing HVAC systems or newly designed HVAC systems. There are three categories of technologies: technologies that agencies can consider for reducing the risk of airborne diseases; technologies that are helpful for other aspects of air quality, but not necessarily helpful for reducing the risk of airborne disease; and technologies that are either not well studied or that can pose unintended safety risks to customers and transit workers.

Gasparine then discussed filters. Higher-rated MERV filters and HEPA filters are great when air gets back to a return grill and before it gets recycled into the space. Higher-rated filters will remove harmful particles, biological or not, from the airstream, and the higher the efficiency of the filter, the better the job the filter will do. Avoiding recycling of harmful particles back into the space, whether it is a building or a bus, is important, but it is also important to make sure the filter is not so restrictive that it puts unnecessary strain on the equipment or voids any warranties. Working with a vendor or HVAC designer can help evaluate that issue. An ionization device is useful, but if it is not applied correctly, then it will not perform. In their most basic form, ionization devices generate static electricity, which will help a small particle such as a respiratory droplet stick to a filter better. Ionization devices can act as an enhancement to filtration efficiency without the filter media necessarily being upgraded. In a transit bus, especially if the bus is operating with windows open or with doors frequently opening and closing, the air changes rate varies, and there is concern about whether that is going to negate the ability of an ionization device to enhance filtration efficiency. Ionization devices are known to generate ozone as a by-product of their operation. An ionization device must have a UL 2998 certification, which indicates that it does not produce ozone. More study needs to be done in buses to understand the degree to which this will help with filtration, efficiency, upper room, and UVGI.

UVGI has been more historically used for surface disinfection and keeping mold from growing on the inside of ductwork or coils, and ASHRAE recently posted guidance on its website about how UVGI can be used to disinfect air as it moves through ductwork. The most recent ASHRAE guidance requires a large dose of UV light to kill germs as they move rapidly through the ductwork. There are a lot of engineering considerations that go into whether a system designed for in-line UV devices will be effective on pathogens. These in-line UV devices should be designed not only to make the environment and the air safer to breathe but also to keep the workers who are responsible for operating and maintaining the bus in and around these devices safe. This means that workers do not inadvertently have their eyes or skin exposed to harmful light.

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Suggested Citation:"Day 2 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.
×

Gasparine discussed the next group of technologies, which focus on managing overall air quality but not on reducing the risk of airborne disease. While humidity regulation is important to air quality, it plays less of a role in the speed of breaking down viruses, which is more about sunlight than humidity. Lowering the humidity increases the risk of even the common cold being passed between people. If the humidity is raised too much, there is a risk of mold growing on building materials. Humidity is also about comfort and the preservation of structural materials, and dehumidification devices have their place for that, but not for reducing the risk of disease transmission. Sorbent air cleaners are a type of filter that chemically absorbs moisture onto a substrate to remove odors from the airstream before the air is recycled back into the space. These air cleaners can be helpful for controlling odors, but there is nothing in the literature that suggests that they are going to reduce the risk of airborne disease transmission. Demand control ventilation uses CO2 sensors to help with energy efficiency. If more CO2 is detected by the sensor in the building, the sensor assumes that more fresh air is needed, so the electronics allow more fresh air to come into the HVAC system. It usually takes more energy to condition the outdoor air, and then, when there is less CO2 in the space, it goes into recycling mode to save energy. However, recycling air is not good for air quality, so these devices are not good for pandemic conditions.

Gasparine discussed the grouping of technologies that are either not well studied or pose unintended safety risks. Photocatalytic oxidation (PCO) devices are a large grouping of devices and are sometimes marketed as dry hydrogen peroxide devices. Photocatalytic oxidation purification is a process that involves a light-activated catalyst reacting with organic pollutants to oxidize them. ASHRAE has studied some PCOs and found that some are effective at removing harmful contaminants from the air but that others are not effective. It is difficult for somebody who is not an expert in public health to discern the good products from the bad in this category, and expertise in environmental engineering is necessary.

Also, the operation of some PCO devices can generate harmful byproducts, putting other toxic compounds such as ozone or formaldehyde into the air that would not have been there otherwise. Again, it is best to consult a public health professional, in particular an environmental engineer, to help discern which of these technologies are going to help to achieve an air quality goal. Vaporized hydrogen peroxide devices nebulize liquid hydrogen peroxide into the air, and then people breathe in the liquid hydrogen peroxide. While the Occupational Safety and Health Administration (OSHA) does have a permissible limit for hydrogen peroxide, more study would be helpful to prove that chronic long-term exposure to vaporized hydrogen peroxide is safe. Ozone is great for disinfection of air but can also kill cells in the human body; therefore, the recommendation is not to generate ozone in occupied spaces. Again, as with the PCO devices, those with public health expertise are needed to understand whether these technologies are going to help meet an air quality goal or if they are going to introduce unintended harm.

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Suggested Citation:"Day 2 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.
×

Plasma devices are like PCO devices in the way that they function. The process of generating plasma transforms some chemicals into other chemicals, including ozone and chloroform, and into other organic gases that are dangerous. Therefore, plasma devices are not recommended for use in the transit system. There are other types of UV light that are made more for disinfection of surfaces but should not be used to disinfect the air. The biggest concern is exposure to the eyes or skin of occupants; therefore, such devices are only recommended for unoccupied spaces. Black lights are on a different segment of the UV spectrum. Some of these black lights claim to help kill biological pathogens, but this is not well documented, so more study is needed to understand both the safety risks and efficacy. UV-C wavelength 222, also called “far UV-C” in laboratory conditions, seems to help accelerate the degradation of COVID-19 in laboratory conditions and offer lower risk for eye or skin exposure. Chronic exposure to this light does not show damage; however, this needs to be studied further, especially in field conditions, to understand efficacy and safety risks.

Finally, Gasparine noted that the various technologies all have caveats regarding safety and effectiveness but that all of them could still be considered for HVAC. Public health goals need to be part of a holistic consideration of the overall HVAC design. Air flow directionality is the single most important consideration and often the most ignored in the design of HVAC today. Traditional HVAC air distribution systems basically move air from the left to the right of the room and, if someone is standing downstream, they are at an elevated risk of getting sick. However, in only checking the direction of airflow to make it more columnar, there is a dramatic change in concentration of particles as they are distributed throughout that space, the same as in a bus. Bolt-on technologies, when understood, can be counted on as part of a holistic plan that considers the airflow direction, air change, and filtration efficiency.

Katherine Ratcliff discussed how EPA has played several different roles throughout the COVID-19 pandemic, from developing cleanup guidance, to providing technical support, to understanding how to be prepared against various types of pandemics or biological agents. From a regulatory standpoint, under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), EPA is tasked with regulating pesticide products as well as pesticide devices and with developing various test methods for evaluating these pesticide products and devices. On a research front, EPA has been conducting several different research efforts to support its response and regulatory missions as well as to provide technical expertise by conducting research on a variety of different topics and projects, including evaluating aerosol treatment technologies.

Ratcliff noted that there has been an increased focus not only on air quality but also on different types of technologies that can operate when people are present and treat the air in occupied spaces. At the same time, there are a number of related challenges. Pesticide devices are regulated in a different way from pesticide products under FIFRA. Pesticide products, which are chemicals that are potentially put in the air or in

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Suggested Citation:"Day 2 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.
×

disinfectants that are used on surfaces, go through a data-intensive premarket review in which EPA reviews the testing data related to the efficacy and safety of using those products. There are after-the-fact rules on whether the pesticide can be a registered pesticide device or not. Registered pesticide devices, which encompass UV, photocatalytic, bipolar, ionization, or any of those devices that do not have a chemical component, even if they do generate chemicals, are regulated under FIFRA. False and misleading claims cannot be made about their efficacy, meaning that whatever claims the company is making are supposed to be backed up with testing data. However, those data are not routinely reviewed by EPA or verified for efficacy, which is different from how EPA regulates pesticide products. Thus, there is no standard test method for evaluating these different types of technologies that allows EPA to compare UV technologies across an even playing field. There is no independent testing of data, so most of the testing and research are commissioned by the device companies or the technology companies themselves. Test conditions are set out to be favorable to that particular technology; therefore, realistic or field-type conditions are often not used in testing. The results are difficult to extrapolate to real-world or field settings.

Ratcliff described looking into how effective these different types of air treatment technologies are in inactivating airborne pathogens and how those results scale to real-world or field and applied settings. EPA has been focused on developing standard test methods that can be used to compare across technology types or within a technology. EPA has been using specialized facilities within the EPA labs in North Carolina, utilizing a large, 3,000–cubic foot test chamber that controls temperature and humidity and a mock HVAC system that is fully recycling the air in the test chamber. There is no added fresh air throughout the test, and these tests are aerosolizing a nonpathogenic virus, MS2—a bacteriophage. Air samples taken from the chamber over time can enumerate what sort of infectious virus is present in the air throughout the testing at various time points. Surface materials are also included to assess how much viable virus is settling on the surfaces. Time-matched control tests, meaning tests with no technology active that account for the natural decay and settling of these viruses over time, are important because, without these control tests, EPA is not able to understand what sort of reduction of virus in the air is attributed to the technology versus natural decay processes. Quantifying those natural decay processes through a control test is critical to understanding how effective the technologies are; in addition to monitoring how much virus is in the chamber and in the air, EPA can measure the particle size and count concentrations. An objective of the EPA test conditions was to create a high concentration of virus in the chamber to present a high bioaerosol challenge for these technologies. In part, this was done by design and not necessarily designed to mimic actual aerosol release processes. Throughout the pandemic, EPA has been working closely with large transit agencies, as well as with EPA regulatory offices, to hone in on the research priorities for testing.

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Suggested Citation:"Day 2 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.
×

Ratcliff described the first set of results that came from a three-stage air filtration and purification system that was of wide interest to transit agencies across the country. A roof-mounted HVAC unit from a subway car was retrofitted with three different components: an electrostatic filter, a UV-C bulb at 254 nanometers, and a bipolar ionization component at about 25 ACH. For one of these tests, the natural decay and settling of the virus accounted for a 99.9% reduction in the virus. Understanding the background amount of decay is important to understanding how effective these technologies are and to looking at testing data. There is inherent bias variability in working with these microorganisms, particularly in the air, so replicate testing is important.

Ratcliff noted that in another set of three replicates, in which EPA ran the test with the electrostatic filter component active, there was more virus removed from the chamber air more quickly than in the control tests. With the electrostatic filter, the bipolar ionization component, and the UV-C bulb all on, there was more reduction of virus in the air, but there was not a huge difference in how much viruses were moved between the different sets. Thus, adding those additional layers of bipolar ionization or UV did not change how much virus was in the air relative to just having the electrostatic filter running alone. The takeaway is that all the components that were tested are similar in efficacy to the MERV 13 filter data. In another set of studies, the bipolar ionization and the UV bulb did nothing on top of the actively charged electrostatic filter.

Ratcliff summarized that the actively charged electrostatic MERV 13 filters appeared to be about as effective as the three-stage system in removing particles over time, but those were clean MERV 13 filters. EPA worked with a large transit agency to get filters that had been deployed for varying lengths of time to look at results from 1 to 2 weeks and 4 weeks. The MERV 13 filters were compared with MERV 9 filters and with clean MERV 13 filters. The results showed that there was not much difference between the 1-to 2-week and 4-week MERV 13 filters. The used MERV 13 filters still performed better than MERV 9 filters under the test conditions. There are multiple factors that need to be considered in deciding whether to deploy these technologies, and EPA’s research supports understanding just how effective they are against bioaerosols. However, EPA’s particular effort is not to evaluate anything related to being chronically exposed or to evaluate the safety of any particular product.

Ratcliff concluded that, additionally, utilizing bipolar ionization and UV-C under EPA’s test conditions did not improve any bioaerosol removal from the air beyond what was already being done by an electrostatic filter. The electret MERV filters that were tested did lose their efficacy relatively quickly in a transit environment. It was important to compare these technologies, understand how effective they might be, and in what setting they might be most applicable.

In response to an audience member asking what organizations can do to look for certifications or certain test procedures that would give a sense of efficacy and safety,

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Suggested Citation:"Day 2 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.
×

Ratcliff stated that there are no standard test methods for evaluating and comparing across technology types. However, across different sectors, including government, various organizations, and ASHRAE, there is a realization that this is an issue, and they are working toward developing standard test methods to help inform efficacy and to make comparisons across technology types or even within the same technology type. In the meantime, it is critical for decision-makers to ask the companies for the testing data and to think about certain conditions relative to the test conditions. From a safety standpoint, it is best to rely on CDC, ASHRAE, and OSHA recommendations.

Shipley agreed that there are EPA, ASHRAE, CDC, NIOSH, and OSHA regulations to follow and explained that customers have the power to demand information from their manufacturers regarding these technologies.

A chat comment by Ariel Freedman during this session noted that it is difficult for many to understand the difference in effectiveness between a system that generates a 3-log reduction in a short period, such as 10 minutes, versus a 3-log reduction in a longer period, such as 60 minutes. Freedman commented that people seem to gravitate toward large reductions as being more effective, regardless of the speed and time, and that there is room for improvement in the communication of these results so that field users can more intuitively understand the relative effectiveness between a product and a different intervention, such as improving dilution ventilation by opening a window.

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Suggested Citation:"Day 2 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 2 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 2 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 2 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 2 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 2 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 2 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 2 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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Next: Day 2 Session 2 »
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 Air Quality in Transit Buses
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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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