The National Academies

TCRP E-15 [Active]

Air Quality Inside Buses

  Project Data
Funds: $150,000
Staff Responsibility: Mariela Garcia-Colberg
Research Agency: University of Maryland
Principal Investigator: Jelena Srebric


The COVID-19 pandemic brought air quality to the forefront of the transit industry. In the summer of 2022, due to the immediate need to understand the issue, the Transit Cooperative Research Program (TCRP) conducted its first Insight Event. TCRP Insight Event--Air Quality in Transit Buses explored how air quality inside transit vehicles, especially buses, may contribute to the spread of infections. During that event, presentations and conversations about dispersion, ventilation, circulation, and filtration issues were held. Presentations from the event and the literature search results are available on the TCRP Insight Event web page at https://www.nationalacademies.org/event/06-21-2022/trb-tcrp-insight-event-air-quality-in-transit-buses. 

Since that event and the end of the pandemic’s restrictions, the public transit industry has learned that air quality inside transit buses matters to bus operators and passengers. There is an increased focus on reducing the concentration of pathogens containing respiratory aerosols and other harmful pollutants in the air inside a bus cabin. Transit agencies are working to increase operators and passenger confidence in air quality. With increased confidence, riders will return, and it will be easier to hire bus operators. 

Transit systems are analyzing the current air quality inside their buses to better understand current airflows and possible risks. Measuring and controlling air quality inside the bus cabin has proven difficult.  Buses idle, constantly open their doors to embark or disembark passengers, and are exposed to traffic-related air pollutants, such as  exhaust.  

This reality has not hampered the desire to improve air quality inside cabins. Transit agencies are employing the following solutions to mitigate poor air quality: dilution, which consists of bringing more fresh air inside the bus while sending indoor air out; filtration with the use of better-quality filters; and cleaning, including the use of ultraviolet light and photocatalytic oxidation.  

Research is needed to find clear solutions to improve air quality on buses for operators and passengers. Transit systems are doing the best that they can to protect their employees and passengers. However, research can provide information to improve air quality and standardization of practice. 


The objective of this research is to create a research document that helps transit agencies understand air circulation inside a typical 40-foot heavy-duty transit bus and finds solutions to protect employees and passengers without decreasing passenger comfort, safety, and reliability of the system. The research should help guide the development of future design and performance criteria to support better transit rolling stock procurement and heating, ventilation, and air conditioning (HVAC) operations in emergency conditions (e.g., airborne diseases and wildfires).

The research deliverable should include the following:

  • A comprehensive review of existing studies about air circulation inside buses and other relevant passenger vehicles that summarizes:
    • Challenges and concerns, and
    • Methods for improving air quality inside buses, such as outdoor ventilation, HVAC filters, and other technologies that manage indoor air pollutants.
  • Report on the existing air management systems installed or retrofitted by transportation agencies during the COVID-19 pandemic, their benefits, and how those benefits were validated.
  • Summary on the constraints and considerations for potential retrofitting of bus air management systems, considering the interior and external envelope of the bus.
  • Recommendations for the design criteria of the optimal air management system for a bus vehicle in revenue service, such as outside air exchange rate, recirculated air, airflow patterns, and filtration efficiency.
  • Recommendations to move research forward and for future standardization, including recommendations for different bus typologies (cutaways, articulated, etc.). 


The research plan shall be divided into tasks that present, in detail, the work proposed in each task. The research plan shall describe appropriate deliverables that include the following (which also represent key project milestones):

  • An Amplified Research Plan that responds to comments provided by the project panel at the contractor selection meeting;
  • An interim report and virtual meeting that occurs after the expenditure of no more than 40 percent of the project budget;
  • A draft final report;
  • A final report; and
  • A technical memorandum, titled “Implementation of Research Findings and Products” (see Special Note K).

A research agency has been selected for th project. Research in progress.

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