New online tool can calculate COVID-19 transmission risk in poorly-ventilated places

Researchers have developed a new online tool that can calculate the risk of COVID-19 transmission in poorly-ventilated places, showing that when two people are in such spaces and neither is wearing a mask, prolonged talking is far more likely to spread the novel coronavirus than a short cough.

The research, published in the journal Proceedings of the Royal Society A, also shows that the virus spreads further than two meters in seconds in poorly-ventilated spaces.

The researchers, from the University of Cambridge and Imperial College London in the UK, noted that when speaking, we exhale smaller droplets, or aerosols, which spread easily around a room, and accumulate if ventilation is not adequate.

In contrast, coughing expels more large droplets, which are more likely to settle on surfaces after they are emitted, they said.

Scientists agree that the vast majority of COVID-19 cases are spread through indoor transmission, whether via aerosols or droplets.

The researchers noted that it only takes a matter of seconds for aerosols to spread over two meters when masks are not worn, implying that physical distancing in the absence of ventilation is not sufficient to provide safety for long exposure times.

However, when masks of any kind are worn, they slow the breath's momentum and filter a portion of the exhaled droplets, in turn reducing the amount of virus in aerosols that can spread through the space, they said.

The team used mathematical models that show how the SARS-CoV-2 virus, which causes COVID-19, spreads in different indoor spaces, depending on the size, occupancy, ventilation and whether masks are being work.

Based on the results of their models, the researchers developed Airborne.cam, a free, open-source tool that helps users understand how ventilation and other measures affect the risk of indoor transmission, and how that risk changes over time.

The team used characteristics of the virus, such as its decay rate and viral load in infected individuals, to estimate the risk of transmission in an indoor setting due to normal speech or a short cough by an infectious person.

They showed that, for instance, the infection risk after speaking for one hour in a typical lecture room was high, but the risk could be decreased significantly with adequate ventilation.

Airborne.cam can be used by those managing public spaces, such as shops, workplaces and classrooms, in order to determine whether ventilation is adequate, the researchers said.

"The tool can help people use fluid mechanics to make better choices, and adapt their day-to-day activities and surroundings in order to suppress risk, both for themselves and for others," said study co-author Savvas Gkantonas, who led the development of the app with Pedro de Oliveira from Cambridge's Department of Engineering.

The researchers quantified the vital role of ventilation in the spread of COVID-19, finding that in poorly-ventilated spaces, the virus spreads further than two meters in seconds, and is far more likely to spread through prolonged talking than through coughing.

"We're looking at all sides of aerosol and droplet transmission to understand, for example, the fluid mechanics involved in coughing and speaking," said study senior author Professor Epaminondas Mastorakos, also from the Department of Engineering.

"The role of turbulence and how it affects which droplets settle by gravity and which remain afloat in the air is, in particular, not well understood.

"We hope these and other new results will be implemented as safety factors in the app as we continue to investigate," Mastorakos added.