National Academies Press: OpenBook

Air Quality in Transit Buses (2023)

Chapter: Day 1 Session 2

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Suggested Citation:"Day 1 Session 2." 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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Session 2

Optimal Airflow Patterns & Measurement Methods

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

Presenters

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

Jason DeGraw, Oak Ridge National Laboratory

Nathan Edwards and Jason DeGraw summarized airflow verification studies and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) guidance relative to buses.

Edwards noted that there is a lot of research on the spread of diseases that uses theoretical models or simulations, but many of the guidelines that have come out are based on best practices that have not really been verified. The challenge to the world of transit bus operations is how to verify the concepts for air quality, filtration, and reduction of risk. Only a few transit airflow verification studies have conducted field studies, and Edwards provided a summary of the history of these studies and highlighted some of the more recent ones.

Edwards reiterated that aerosols measuring 5 microns or less tend to stay afloat in the air longer and move around more. Unfortunately, in the transit environment, keeping passengers 6 feet apart is not feasible. TCRP funded a project in 1998 published as TCRP Report 41: New Designs and Operating Experiences with Low-Floor Buses (https://onlinepubs.trb.org/onlinepubs/tcrp/tcrp_rpt_41-a.pdf) that looked at new designs and operating experiences for low-floor bus models. The report found that several transit organizations experienced contaminated air inside the cabin and a leading-edge suction problem, meaning that a bus moving at greater than 20 miles per hour sucks the contaminants from the back of the bus all the way forward into the operator and front door space and then out. The low-floor bus models have rear-mounted engines and rear-mounted HVAC systems and filters, and the air may not actually be filtered as well. The low-floor bus models also suck in contaminants from the outside environment. In one of the tests, researchers set up a smoke bomb in the upper right corner of the air-conditioning unit and, with the bus moving at more than 20 miles an hour, the smoke was pulled inside the passenger compartment. Air quality improvements in buses need to be approached from a system level that considers the whole bus, not just with a filter or a UV unit.

Edwards noted that a 2009 TCRP report, Transit IDEA J-04/IDEA 53: Ultraviolet Germicidal Irradiation for Transit Buses, investigated the use of UV germicidal for transit

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Suggested Citation:"Day 1 Session 2." 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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buses (https://www.trbtss.org/?p=2131). The researchers conducted airflow studies with upper room ultraviolet germicidal irradiation (UVGI) and showed that it improved airflow by 31%, 24%, or 23%, depending on what style of bus and which HVAC unit was present. The primary reason for these results is that the UV germicidal reduced the buildup of biofilms on the HVAC systems and the condenser coils. The air filters are a critical part of the HVAC system, but higher filtration can also mean less airflow. Another study, which was not specific to transit buses, looked at particulate matter 2.5—meaning particle sizes of 2.5 microns and smaller—as a cumulative measurement used in air quality. The study focused on the subway systems, both in the stations and on the trains, from which researchers measured 292 locations and took 337 samples. The results showed between 37% and 87% reductions in the particulate matter exposure, which indicates that these HVAC systems on the vehicles can provide a key opportunity to reduce passenger and operator exposure.

Edwards noted that the Denver, Colorado, transit system equipped some of its buses with different airflow sensors and drove them at various speeds with various configurations, that is, some with the roof hatches open, some with the windows all open, or some with front windows open. Comparing the bus configuration changes showed the most effective one was open roof hatches and the front window areas. This configuration effectively created a positive pressure system and helped ventilate the buses, with airflows as high as 200 feet per minute. The Fresno State Transportation Institute looked at bacteriophages that are safe for environments with people. The researchers released three different bacteriophages—Phi6, MS2, and T7—into the air and measured them through collection plates. The collection plate was a way to collect and culture the bacteriophages and to understand the efficacies of these different decontamination or filtration mechanisms. The studies have shown that MERV 19 filtration and UV-C light are effective at mitigating the buildup of the biological contaminants. However, these studies do not show how to mitigate the person-to-person transmission for biological diseases.

Edwards also discussed a 2020 study using Colorado Springs’ Mountain Metropolitan Transit 35-foot Gillig low-floor-model buses. The study collected data with the buses in motion over a 2-week period. Considering the massive momentum of different particle airflows, the researchers wanted to test everything on regular driving routes, which resulted in about 124 miles of in-motion data. The study used 12 buses instrumented with anemometers to measure airflow throughout the bus, and 28 particle counters, which monitor a clean room environment and can measure down to 0.3 microns in size all the way up to 25 microns. A clean room is an engineered space that maintains a very low concentration of airborne particulates. Fluid dynamics modeling showed two vortices within the bus from the HVAC system.

Researchers wanted to make sure sensors were placed out of the central vortices, which would have had less contamination. To emulate a human cough or sneeze or

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Suggested Citation:"Day 1 Session 2." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

breathing, the researchers instrumented a cardiopulmonary resuscitation mannequin with an aerosol chamber driven by an air compressor and a medical device that can measure the peak airflow. The researchers used sodium chloride, which is an ASHRAE test method for generating aerosols, which was placed in the midline of the bus. The aerosol moved throughout the bus quickly; it did not matter if the HVAC system was running or if the bus was stationary or in motion. The aerosol made it all the way to the front driver area within a matter of minutes. There were fewer larger particles from this dispersion. With all the doors and windows closed, the particles were staying aloft longer than 10 minutes; the researchers stopped measuring after 10 minutes, knowing that the contamination would continue to build up. When both the front door and the rear passenger doors opened at bus stops, the researchers saw a significant reduction in those aerosols. The researchers also found that when a cloth mask was worn, there was no peak dispersion of the aerosols, and a mask was effective at slowing the particles down. When masks were worn, the HVAC system with MERV 13 filters was able to clear the aerosols out within about 2 minutes. Generally, the researchers found higher airflow with faster bus speeds. The researchers also found that if windows were open, the HVAC air conditioning unit would overwork and sometimes freeze up. Edwards summarized that verification testing is important to understand how things work in the real world, especially real-time monitoring of air quality within transit environments.

DeGraw discussed ASHRAE’s guidance efforts. HVAC is important because spaces are uninhabitable without it, and an acceptable level of indoor air quality (IAQ) must be maintained. Achieving acceptable IAQ is about controlling contaminants, not eliminating contaminants. Contaminants include aerosols, dust smoke, radon, and volatile organic compounds. Contaminants are controlled primarily with ventilation and filtration. There are cultural variations in what people deem acceptable. IAQ is the factor that connects HVAC to the management of pathogens. HVAC is expected to provide at least acceptable conditions, and a failure to do that is a failure at its one job. Early in the pandemic, there was a concern whether HVAC was part of the problem. ASHRAE, as a standards-developing organization, determined that 62.1 is the standard for acceptable IAQ in commercial buildings. ASHRAE set up the Epidemic Task Force in March of 2020 to respond to the pandemic and develop guidance in several subareas, including in transportation. ASHRAE guidance was posted in the middle of 2020 (https://www.ashrae.org/about/news/2020/ashrae-epidemic-task-force-releases-updated-building-readiness-guide).

DeGraw outlined four themes in the ASHRAE guidance that are applicable to buses. First, HVAC in buses is serving the same purpose, but because there are different situations and uniform guidance is not always possible, it is important to recognize situational variations. Second, it is important to be aware of the stakeholders and to choose better guidance for each of the constituents, because not everyone involved in getting passengers from Point A to Point B has the same requirements. Third, it is

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Suggested Citation:"Day 1 Session 2." 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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important to recognize that there are many unknowns, such as the exact infectious dose and shedding rates of the virus. However, the industry cannot necessarily wait for those unknowns and must go ahead and provide guidance and look to the solutions that are already available. Finally, the industry should consult with the manufacturer, or an accredited professional, and other experts because of the different conditions that buses can encounter, to ensure that interventions get done correctly.

DeGraw noted that guidance specific to buses is based on the fact that buses are nearly always in constant motion, and there are a lot of common touch surfaces. This is another example of situational variation being unique to buses. If possible, those at transit agencies should consult with the manufacturer or another accredited professional or expert before making changes. There are positive steps that can be taken, but DeGraw stressed that if they are not done right, there is the possibility for harm. For example, there are lots of potential risks with UV because these are electrical systems.

Finally, even though there is a shared goal to get a passenger from Point A to Point B, the passengers may have different priorities from the operators and the management. For the ASHRAE guidance related to buses, the first item in ventilation, filtration, and air cleaning is to provide and maintain at least the required minimum outdoor airflow rate for ventilation, as specified by applicable codes and standards.

Ventilation using a combination of filters and air cleaners that achieve MERV 13 or better levels of performance for air recirculated by HVAC systems should be the first solution, before other alternatives are considered. MERV 13 filters have a good efficiency against the sizes of aerosols of concern, but if the bus system can handle more and does not have problems with the additional pressure drop associated with MERV 16 filters, then they can be considered. This is an example of situational variation, in which experts can try to find a way to achieve targets without relying on one solution.

DeGraw advised attendants to use only air cleaners for which the evidence of effectiveness and safety is clear. There are many potential air cleaning systems, and UV is one that has a more established record of effectiveness in the published literature. This is an example of working with the solutions that are available by selecting control options, including stand-alone filters and air cleaners that provide desired exposure reduction while they minimize associated energy penalties. It is important to consider those situational variations and leave room for experts to try to balance the energy penalties with the desired risk targets.

DeGraw noted that, with regard to HVAC system operation, maintaining temperature and humidity design set points may reduce introduction of additional risks. HVAC system operation should also maintain equivalent clean air supply required for design occupancy whenever anyone is present in the space served by a system, meaning that the clean air delivery is not modulated. Every occupant is potentially a source of

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Suggested Citation:"Day 1 Session 2." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

infectious aerosols, so it is not worth the risk to modulate clean air supply to save energy.

An audience member asked for recommendations concerning the extent to which user- or operator-controlled increased ventilation (i.e., opening a window) is applicable as a practical manner. DeGraw responded that it would need to be determined that opening the windows was a positive action to take but should be taken in consideration of specific environments and stakeholders, citing the West Coast wildfires and concerns about smoke inhalation for children on school buses.

Chat comments by Nizar Madanat asserted that UV-C ultraviolet light is a myth for transit applications and that studies that were done showed it did not meet the 3-log reduction required. Madanat also noted that electrostatically, rather than actively, charged MERV 13 filters lose their charge and effectiveness, dropping to MERV 9 very quickly. Sam Brauer asked why UV-C would be used for this purpose rather than less damaging UV-B light, to which Brian L. Sherlock replied that UV-C is at a frequency that destroys DNA and RNA.

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Suggested Citation:"Day 1 Session 2." 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 2." 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 2." 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 2." 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 2." 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 1 Session 3 »
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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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