Findings show these mitigations may help reduce the spread of COVID-19

Colorado Springs, Colo., December 10, 2020—Opening windows and using existing fans on a bus was shown to reduce exhaled airborne particles by up to 84%, and thereby could lower the risk of COVID-19 exposure, according to research by MITRE using buses provided by the City of Colorado Springs Mountain Metro Transit and Colorado Springs School District 11. Wearing a mask alone can also reduce the spread of potentially infectious particles by 50% or more depending on the quality of the mask.

Those are the key findings of a MITRE study to test aerosol dispersion on buses and determine the best methods to control potentially infectious airborne particles such as COVID-19.  When an infected person exhales, sneezes, or coughs, they create small aerosol particles containing the disease, which then spreads or is dispersed throughout the air space in the bus.

The study included more than 84 test runs on both school buses and low-floor transit buses using a mechanical cough simulator that dispersed the test aerosol while the buses drove normal routes over 10 days between August 3 and September 4, 2020. The safe sodium chloride test aerosol generated a variety of particle sizes down to 300 nanometers—similar to a human cough—and were detected by 28 sensors placed throughout the passenger areas.

No passengers were on the buses during the study. Unlike a highly engineered passenger cabin of a commercial aircraft, buses have turbulent environments where the airflow is not steady, making this kind of study complex. MITRE has also posted a video that shows the experiments in action.

After 78.3 million data points and 124 miles of on-the-road testing, key results include:

  • Wearing of masks reduced the overall particle count released into the bus by an average of 50% or more and reduced the dispersion distance by several feet.
  • When masks were NOT worn, dispersed particles spread through the whole bus.
  • Having all the windows open and dashboard fans running reduced the particle counts by an average of 84% on school buses and 50% on transit buses.
  • Likewise, the windows and fans also reduced the time that particles remain aloft by 80% in school buses and 60% on transit buses.
  • Bus HVAC systems when used with MERV-13 air filters eliminated airborne particles within 4 minutes. (applicable only to transit buses)
  • The back row of seats tend to accumulate more particles.

The MITRE team, led by scientist Nathan Edwards, worked to understand the risk of airborne particle transmission while riding on school and public transportation buses, and to bring scientific clarity of the best options for all public transportation systems.

While the results and recommendations seem intuitive, the data may help inform decisions on the best COVID-19 risk mitigation options for public transit.

The key findings and recommendations from this study are:

  • Require all passengers and drivers to wear masks on buses.
  • Open windows partially or fully to make a significant difference in reducing airborne risks.
  • Drivers and operators can reduce their risk of exposure by using a dashboard fan to create airflow in the driver seat area.
  • Consider seating arrangements that only allow for same-household passengers to sit together.
  • Social distancing of six feet is not practical on most buses, but any extra distance allows the air movement to reduce exposure to infectious particles.
  • When it is safe and possible to do so, open transit bus doors at every stop to allow for better air exchange.
  • When it is not possible to open windows and doors due to weather, or to add better filtration to the buses, consider cancelling bus services.

Editor's note: View a video of the research.


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