Airborne Transmission of SARS-CoV-2

Currently, the existing public health measures point to the importance of proper building and environmental engineering control measures, such as proper Indoor Air Quality (IAQ). This pandemic clearly raised increased awareness on airborne transmission of respiratory viruses in indoor settings. Out of the main modes of viral transmission, the airborne route of SARS-CoV-2 seems to have a significant importance to the spread of COVID-19 infections world-wide, hence proper guidance to building engineers or facility managers, on how to prevent on-site transmission, is essential.
For information on the Airborne Transmission of SARS-CoV-2, feel free to check out the special issue on the Interface Focus journal from Royal Society publishing: Interface Focus: Volume 12, Issue 2 and an CERN HSE Seminar:

What is CARA?

CARA stands for COVID Airborne Risk Assessment and was developed in the spring of 2020 to better understand and quantify the risk of long-range airborne spread of SARS-CoV-2 virus in workplaces. CARA comes with different applications that allow more or less flexibility in the input parameters: The mathematical and physical model simulate the airborne spread of SARS-CoV-2 virus in a finite volume, assuming a homogenous mixture and a two-stage exhaled jet model, and estimates the risk of COVID-19 airborne transmission therein. The results DO NOT include other known modes of SARS-CoV-2 transmission. Hence, the output from this model is only valid when the other recommended public health & safety instructions are observed, such as good hand hygiene and other barrier measures.

The methodology, mathematical equations and parameters of the model are published here in the CARA paper: Modelling airborne transmission of SARS-CoV-2 using CARA: risk assessment for enclosed spaces.

Note that the short-range component of the model has not yet been published.

The model used is based on scientific publications relating to airborne transmission of infectious diseases, virology, epidemiology and aerosol science. It can be used to compare the effectiveness of different airborne-related risk mitigation measures. The tool helps assess the potential dose of infectious airborne viruses in indoor gatherings, with people seated, standing, moving around, while breathing, speaking or shouting/singing. The model is based on the exponential dose-response of disease transmission, which assumes a fixed value for the average infectious dose. The methodology of the model is divided into five parts:
  1. Estimating the emission rate of virions;
  2. Estimating the removal rate of virions;
  3. Modeling the concentration of virions within a given volume, as a function of time;
  4. Absorbed dose of infectious viruses, inhaled during the exposure time;
  5. Estimating the probability of a COVID-19 infection (or secondary transmission) and the expected number of new cases arising from the event

What is the aim of CARA?

Although the user is able to calculate the infection probability of a stand-alone event with a pre-defined set of protection measures, the main utility of CARA is to compare the relative impact of different measures and/or combination of measure. For example:

Main Developers:

Andre Henriques1, Luis Aleixo1, Marco Andreini1, Gabriella Azzopardi2, James Devine3, Philip Elson4, Nicolas Mounet2, Markus Kongstein Rognlien2,6, Nicola Tarocco5

1HSE Unit, Occupational Health & Safety Group, CERN
2Beams Department, Accelerators and Beam Physics Group, CERN
3Experimental Physics Department, Safety Office, CERN
4Beams Department, Controls Group, CERN
5Information Technology Department, Collaboration, Devices & Applications Group, CERN
6Norwegian University of Science and Technology (NTNU)

Code Contributors:

Anna Efimova1,2, Anel Massalimova1,3, Cole Austin Coughlin1,4

1Summer Students, CERN
2M.V. Lomonosov Moscow State University
3National Research Nuclear University "MEPhI"
4University of Manitoba


Reference list can be found in the CARA paper: CERN-OPEN-2021-004


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We wish to thank CERN’s HSE Unit, Beams Department, Experimental Physics Department, Information Technology Department, Industry, Procurement and Knowledge Transfer Department and International Relations Sector for their support to the study. Thanks to Doris Forkel-Wirth, Benoit Delille, Walid Fadel, Olga Beltramello, Letizia Di Giulio, Evelyne Dho, Wayne Salter, Benoit Salvant and colleagues from the COVID working group for providing expert advice and extensively testing the model. Finally, we wish to thank Fabienne Landua and the design service for preparing the illustrations and Alessandro Raimondo and Manuela Cirilli from the Knowledge Transfer Group for their continuous support. Our compliments towards the work and research performed by world leading scientists in this domain: Dr. Julian Tang, Prof. Manuel Gameiro, Dr. Linsey Marr, Prof. Lidia Morawska, Prof. Yuguo Li, and others – their scientific contribution was indispensable for this project.