The model – developed by researchers at the University of Cambridge, Imperial College London, and the University of Leeds – uses monitored CO2 and occupancy data to predict how many workers are likely to be infected by an asymptomatic but infectious colleague.

Applications of the infection model have demonstrated that most workers in well-ventilated, quiet offices are unlikely to infect each other via airborne particles, but the risk becomes greater if the space is poorly ventilated or if the workers are involved in activities that require more speaking. For instance, the model predicts each infected person could infect two to four others in an adequately ventilated but noisy call centre. Risks are also likely to increase if the infected individual is a ‘super spreader’.

The model also suggests that halving the occupancy of an office could reduce the risk of airborne transmission four-fold. The results are reported in the journal Indoor and Built Environment.

In areas with lower ventilation rates and high occupancy, CO2 levels are higher, so monitoring them can provide a warning to building managers to identify areas where the risk of airborne transmission of COVID-19 are higher. Achievable interventions can then be made, for instance, to improve ventilation or change worker attendance patterns to reduce occupancy.

In shared spaces such as offices and classrooms, exposure to infectious airborne matter builds up, and room occupancy may vary. By using carbon dioxide levels as a proxy for exhaled breath, the model can assess the variable exposure risk as people come and go.

“Ventilation is complicated and airflow is invisible, so it’s hard for people to appreciate the effects in the home or workplace,” said co-author Professor Paul Linden from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “Commercially available CO2 monitors are being installed in schools and I would recommend their installation in the workplace.”

“Our work emphasises the importance of good ventilation in workplaces and in schools,” said lead author Dr Henry Burridge, from Imperial College London. “The model demonstrates that by managing the ventilation and occupancy levels of shared spaces we can manage the risk of airborne infection by a virus such as that which causes COVID-19.”

“The appropriate use of tools such as CO2 monitoring can give building managers a much better understanding of their own ventilation systems and how they are performing for each activity undertaken in the space,” said Professor Andrew Curran, Chief Scientific Adviser at the Health and Safety Executive and lead for the PROTECT study. “For most businesses, a COVID-19 control strategy will involve a blended combination of measures identified through a risk assessment – there is no silver bullet.”

The research was funded by the PROTECT COVID-19 National Core Study and the Engineering and Physical Sciences Research Council (EPSRC, part of UK Research and Innovation).

Reference: Henry C. Burridge et al. ‘Predictive and retrospective modelling of airborne infection risk using monitored carbon dioxide.’ Indoor and Built Environment (2021). DOI: 10.1177/1420326X211043564

Adapted from an HSE press release.