Reducing COVID-19 Airborne Transmission Risks on Public Transportation Buses: An Empirical Study on Aerosol Dispersion and Control

Abstract

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, as well as 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 also 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.

The Lancet Commission on lessons for the future from the COVID-19 pandemic

“…Research suggests that the risk of SARS-CoV-2 transmission on public transport can be substantially reduced by asking people who are unwell to refrain from its use, requiring or encouraging vaccination among passengers, changing transport schedules to reduce crowding, and improving ventilation or filtration. Buses can substantially improve ventilation by opening as many windows, doors, or roof hatches as feasible; 412 deploying recirculating-air cleaning systems that use mechanical filtration or ultraviolet germicidal irradiation; and requiring passengers to wear face masks.…”

Seroprevalence of SARS-CoV-2 antibodies among public transport workers in Sweden

“… Billingsley et al (2022) point out that buses may be hot zones for the virus as many passengers enter and exit over the course of a shift and efforts to train and provide Personal Protective Equipment (PPE) for such drivers is therefore important. Simulations and experimental studies have found that increased ventilation and face masks can effectively reduce the transmission of disease in buses ( Edwards et al, 2021 ; Zhang et al, 2021 ). Systematic reviews have also shown that wearing a face mask has a great potential in controlling airborne transmitted viruses including COVID-19 ( Abboah-Offei et al, 2021 ).…”

Control of airborne infectious disease in buildings: Evidence and research priorities

“…Studies have implicated specific settings for SARS-CoV-2 and other respiratory infection transmission. These include school lunchrooms, restaurants, and small, private gatherings 25,146 ; classroom learning 147 ; a variety of occupational settings and worker dormitories 24 ; meat processing 148 ; religious services 149,150 ; public entertainment 151 ; buses 152,153 ; nursing homes 154 ; and fitness centers [155][156][157][158][159] . Residential settings, which typically foster extended exposure at close proximity, lend to transmission risk.…”