Reducing COVID-19 airborne transmission risks on public transportation buses: an empirical study on aerosol dispersion and control

This study is one of the first COVID-19 related bus studies to fully characterize cough aerosol dispersion and control in the highly turbulent real-world environment of driving regular bus routes on both a school bus and a transit bus. While several other bus studies have been conducted, they were limited to clinical contact tracing, simulation, or partial characterization of aerosol transmission in the passenger areas with constraint conditions. When considering the risk of transmission of SARS-CoV-2 (COVID-19) and other highly infectious airborne diseases, ground based public transportation systems are high-risk environments for airborne transmission particularly since social distancing of six feet is not practical on most buses. This study demonstrates that wearing of masks reduced the overall particle count released into the bus by an average of 50% or more depending on mask quality and reduced the dispersion distance by several feet. The study also demonstrates an 84.36% reduction in aerosol particles and an 80.28% reduction in the mean aerosol residence time for some test cases. We conducted 84 experimental runs using nebulized 10% sodium chloride and a mechanical exhalation simulator that resulted in 78.3 million data points and 124 miles of on-the-road testing. Our study not only captures the dispersion patterns using 28 networked particle counters, it also quantifies the effectiveness of using on-board fans, opening of various windows, use of face coverings or masks, and the use of the transit bus HVAC system. This work additionally provides empirical observations of aerosol dispersion in a real-world turbulent air environment, which are remarkably different than many existing fluid dynamics simulations, and also offers substantial discussion on the implications for inclement weather conditions, driver safety, retrofit applications to improve bus air quality, and operational considerations for public transportation organizations.