Assessment of Sars-Cov-2 airborne infection transmission risk in public buses

TL;DR

This study assesses SARS-CoV-2 airborne transmission risk in public buses using a combined proximity and room-scale approach, highlighting how ventilation, crowding, and mitigation measures influence contagion risk and bus occupancy limits.

Contribution

It introduces a novel combined risk assessment method for public transport environments, evaluating infection risk and transmissibility for different bus types under various mitigation scenarios.

Findings

Urban buses require masking or reduced occupancy for safe operation.

Long-distance buses can operate safely with filtration, masks, and high immunization levels.

Ventilation and crowding significantly impact SARS-CoV-2 transmission risk.

Abstract

In this study, we apply a novel combination of close proximity and room-scale risk assessment approaches for people sharing public transport environments to predict their contagion risk due to SARS-CoV-2 respiratory infection. In particular, the individual infection risk of susceptible subjects and the transmissibility of SARS-CoV-2 (expressed through the reproduction number) are evaluated for two types of buses, differing in terms of exposure time and crowding index: urban and long-distance buses. Infection risk and reproduction number are calculated for different scenarios as a function of the ventilation rates (both measured and estimated according to standards), crowding indexes, and travel times. The results show that for urban buses, the close proximity contribution significantly affects the maximum occupancy to maintain a reproductive number of < 1. In particular, full occupancy of the bus would be permitted only for an infected subject breathing, whereas for an infected subject speaking, masking would be required. For long-distance buses, full occupancy of the bus can be maintained only if specific mitigation solutions are simultaneously applied. For example, for an infected person speaking for 1 h, appropriate filtration of the recirculated air and simultaneous use of FFP2 masks would permit full occupancy of the bus for a period of almost 8 h. Otherwise, a high percentage of immunized persons (> 80%) would be needed.