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

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

In contrast, coughing expels larger 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, masks 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 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 worn.

Based on the results, 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.

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.

"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.

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