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 Control of Metabolic CO2 in Public Transport as a Strategy to Reduce the Transmission of Respiratory Infectious Diseases

“…However, computational studies point to a 1.5–1.6% attack rate [ 79 , 80 ]. Even though numerous works have been aimed at evaluating the behavior of bioaerosols in collective transport by computational fluid dynamics [ 81 , 82 , 83 , 84 , 85 ], extrapolation to actual conditions is an enormous limitation. One of the strategies that allow the indoor ventilation rate to be quantified in situ is the measurement of CO 2 , which has positioned itself as a standard for air control [ 22 , 86 , 87 ].…”

“…Favoring natural ventilation (opening windows), the HVAC system, and the use of masks have been shown to significantly reduce the risk of transmission [ 30 , 79 , 84 , 89 , 90 ]. Masks reduce the bioaerosols emission variably, depending on the type of mask and the aerodynamics of the scattered particles [ 91 , 92 , 93 , 94 ].…”

A method for assessing the COVID-19 infection risk of riding public transit

“…The developed model could be used as a decision-making model for risk assessment at a workplace and it needs the information on the daily routines of each employee and the workplace setting. Edwards et al (2021) also conducted a study that captures aerosol dispersion patterns from a mechanical exhalation simulation. This research quantified the effectiveness of using onboard fans, the opening of different windows, the use of face masks, and the use of the transit bus AC system considering turbulent air and any effects of momentum inside a moving bus.…”

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.…”