Protecting Transportation Employees and the Traveling Public from Airborne Diseases (2024)
Chapter: 7 Conclusions and Suggested Research
Chapter 7
Conclusions and Suggested Research
The following is a brief summary of the project followed by a communication artifact that effectively communicates important project results, including key findings and recommendations to reduce COVID-19 transmission in public transportation, an action-based range of effective strategies that addresses policies and behavior, and techniques to evaluate different strategies and practices.
Project Summary
Tightly enclosed public spaces can be considered high-risk environments for virus transmission. The development and adoption of advanced cabin air control systems and measures can lower the risk of transmission of airborne diseases substantially which can protect transportation employees and the traveling public from airborne diseases. This research was jointly funded by the National Highway Cooperative Research Program (NCHRP) and TCRP. A literature review was conducted. Various strategies to mitigate exposure to airborne diseases through measurement of airborne particle concentrations were analyzed and divided into two categories. One used retrofit accessories, and another used a redesigned ventilation system. The experiments were conducted using two public transit buses and were supplemented with computational fluid mechanics models (CFD) and on-road testing. Results include the effect of cabin filtration improvements using new and pre-existing MERV 13 filters, air change rate (ACH) during stationary and on-road experiments, particle removal rate by an equivalent air exchange (eACH) rate, particle arrival times, and CFD analysis. Finally, the results were extended to other modes of transportation using additional CFD and another user-friendly modeling approach. The following are key findings and recommendations for implementation or further study, based on this work.
Project Key Findings and Recommendations
- The research showed that installation of two standalone HEPA air purifiers at the front and back sections of a bus cabin could lower overall concentrations of airborne viruses by at least a factor of 3. A standalone HEPA air purifier can lower overall concentrations by at least a factor of 1.5. This method is recommended as an immediate response upon the event of a COVID-like outbreak or pandemic. The standalone HEPA filter system is easy to acquire and install. It is not recommended to consider this as a permanent solution as putting HEPA air purifier in the middle of the cabin would likely be obstructive to passengers. Further study is needed to identify ideal location for permanent installation of a separate HEPA air purifier. It was beyond the scope of this study.
- Plastic “barriers” were evaluated. Barriers alone did not slow down the arrival time of the airborne viruses into the target subject but they were effective in lowering the concentration of surrogate virus particles. Barriers are most effective when used in conjunction with a parallel air ventilation system. The installation of barriers as a retrofit solution is not recommended.
- A parallel flow air ventilation system, which differs from a standard bus HVAC system that uses a single air return vent at the back of the bus, was evaluated. Results showed this approach significantly lowered the concentrations of the airborne virus surrogate particles, which would result in lowering the risk of virus exposure if used in a real-world application.
- Results showed that opening doors to board and un-board passenger significantly increases the AER. Although not tested in this project, opening windows likely have the same effect. As such it is a tentative recommendation to increase the time windows are open to increase indoor ventilation.
- New cabin filters have a fresh electrostatic charge and are more-efficient. The charge, and therefore efficiency, decline with time. In the event of a new outbreak of airborne disease, it is recommended to replace the cabin filter with a new cabin filter. Changing to a new filter with a fresh charge will enhance the filtration efficiency for the time being as opposed to continue to use an in-use filter.
- Excel-based, modeling tools (COVID-19 Aerosol Transmission Estimator) are available to the public and could be used to optimize cost and effectiveness of various bus ventilation scenarios. This report shows how this tool could be used to conduct a cost/benefit analysis in the context of reducing airborne virus transmission in buses. This tool/model could be easily applied to a wide range of other worksite environments to evaluate cost/benefit scenarios related to ventilation, space, occupancy, etc., to identify the effectiveness of virus mitigation approaches in other contexts.
- Throughout the pandemic, public transit authorities deployed a range of non-pharmaceutical interventions on buses to reduce COVID-19 infection risks for operators and passengers. Although many of these mitigation methods were effective in containing and reducing transmission risks, they also have other implications, such as the environmental impacts of disposable facemasks; the ecological and biological impacts of surface cleaning; the social acceptance of distancing and barriers; and the economic implications of non-ridership. Additionally, many of these strategies relied on administrative controls, which rely on the willingness and ability of individuals to modify their behaviors in the transit environment. Therefore, it is necessary to continue to find long-term solutions to reducing airborne disease transmission in public transportation. Designing new engineering controls (such as the parallel flow ventilation system), which separate people from potential hazards, is essential for improved risk reduction. This recommendation is consistent with the HoC paradigm that is promulgated by the CDC – which indicates that engineering controls are generally more effective than administrative controls or PPE. The suggestion to increase the amount engineering controls applies to all modes of transportation as well as worksites and other environments.
- Consistent with the recommendation above, more widespread adoption of ISD principles is recommended. ISD is a set of principles that focus on eliminating and mitigating hazards during the design process. Risk reduction can be addressed more effectively when “risk assessment is introduced early in the process design and engineering stages to avoid or eliminate hazards, instead of controlling them” (Gonzales-Cortez et al. 2022, 276).
- The preliminary results showed that the parallel flow ventilation system concept could be successfully applied to subway and tram cars, demonstrating the approach is a viable method for airborne virus transmission beyond just buses.
There are three caveats. First, there is a tradeoff for improving air change rate by opening windows and doors as air pollutants from the roadway will be included in the fresh air incoming to bus cabin, while simultaneously lowering the concentration of airborne viruses. Secondly, increasing fresh air exchange impairs the ability to control comfortable cabin air temperature. It is known that on freeways or during elevated speeds, the open windows may cause wind buffeting that makes customers uncomfortable. A solution to this would be to add low porosity metal window screens to all windows to allow air exchange but prevent buffeting. Thirdly, the positive impact of window opening from a ventilation perspective is not known exactly and could be minimal.
