Airborne disease transmission risks on public transit buses: Impacts of ridership, duration, and mechanical filtration using a relative risk metric

Although public transportation vehicles often provide reasonable amounts of ventilation and air filtration, their small volumes, and the close proximity of passengers may cause them to be significant sites of airborne disease transmission. Since the COVID-19 pandemic, several studies have investigated such risks on public transit vehicles for specific well-constrained case studies. This study generalizes these efforts by benchmarking the statistical bounds of relative airborne transmission risks onboard transit vehicles independently of a disease's particular infectivity and prevalence in a community. We examined which factors drive relative onboard transmission risks and compared these risks to those of other common indoor environments and activities, using a novel adaptation of the Wells-Riley method for modeling indoor airborne infection risks. Specifically, simulations were evaluated for five different HVAC scenarios and were carried out for a domain representing three rapid transit bus routes in Southeastern Pennsylvania using empirical ridership and schedule data provided by the region's transportation authority. From these simulations, continuous fan operation using a MERV 13 filter were modeled to reduce risks approximately fivefold compared to intermittent fan operation (25% runtime) and 1–2 orders of magnitude less as compared to no HVAC recirculation and filtration, highlighting the need for mechanical system upgrades and regular maintenance for transit vehicles. By comparing relative risks between activities done in different indoor environments on both a per-time and per-activity basis, our results demonstrate that riding transit buses with fully operational filtration systems poses less risk, on average, than indoor dining and office work.