DURHAM, N.C. – Duke University researchers have developed a new online calculator that teachers, administrators and students can use to estimate the risk of airborne transmission of the COVID-19 virus in classrooms.

Though initially developed for use on college campuses, the calculator can also be used by K-12 schools to inform their decisions on school re-openings and assess the effectiveness of different control measures in various settings like classrooms, cafeterias and gymnasiums.

“The idea was to make it as easy as possible for non-scientists to assess the probable risk of COVID transmission from microscopic airborne aerosols containing the virus, which is a newly discovered route of infection,” said Prasad Kasibhatla, professor of environmental chemistry at Duke’s Nicholas School of the Environment.

To calculate a risk of infection, users enter information about the physical characteristics of the room to be used – such as its height and floor, the number of students who will be present, and the duration of each in-person class session or school activity – into highlighted fields on the calculator’s webpage.

Clickable links guide users to “how to” pages that help them answer questions that require a bit of technical knowledge, such as knowing a room’s ventilation rate. There is even a link to a page that explains how community infection rates – a required field – can be derived from current information on COVID case counts in the user’s area.

Based on the entered information, the model calculates the probable concentration of airborne virions, or infectious aerosol particles, that have been exhaled into the room by someone infected with COVID, as well as the probable concentration, or dose, of these virions that will likely be inhaled by uninfected people in the room, even if they are using protective masks and practicing social distancing.

The actual risk of infection from aerosols may be higher than calculated if social distancing, mask use and hygiene protocols are not strictly adhered to in a classroom, Kasibhatla stressed. And the calculator doesn’t measure the risk of COVID transmission by droplets or from contaminated surfaces, two other potential routes of infection.

“This is intended to be used as a first line of defense, rather than as a final assessment of all risks,” he said.

Kasibhatla developed the calculator with fellow Duke faculty members John Fay, a lecturer in the Geospatial Analysis Program; Elizabeth Albright, assistant professor of the practice of environmental science and policy methods; and Bill Pan, the Elizabeth Brooks Reid and Whitelaw Reid Associate Professor of Population Studies.

They based their model on a COVID-19 risk estimator developed earlier this year by Jose-Luis Jimenez, professor of chemistry and Fellow at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. Jimenez’s estimator assesses transmission risks associated with a wide array of settings and activities.

Consulting extensively with Jimenez, the Duke team created a user-friendly web version of his estimator designed for classroom settings. They also developed a probabilistic framework that allows their model to assess the risk of infection while accounting for uncertainties in the current scientific understanding of airborne transmission of COVID-19.

“Our goal was to use advanced science to create a tool that non-scientists can use,” Kasibhatla said. “We can’t predict or eliminate all risks, but using our calculator can help teachers and school officials zero in on which control measures – more ventilation? fewer people? shorter class durations? – might make the biggest difference in their classrooms.”