By modeling the flow of SARS-CoV-2 respiratory droplets through space, researchers have added new findings to continuing debates on the appropriate safe distance for avoiding transmission of the virus that causes COVID-19, and also demonstrated that using ultraviolet-C irradiation to inactivate the virus may be a safe approach to attenuating its spread.

While much of the research around COVID-19 focuses on epidemiology and biology, physics can provide important insights into the factors that contribute to the virus' spread and to methods for combating it. This study, published March 9 in Physics of Fluids, takes a fluid dynamics approach.

"The airborne diffusion of saliva droplets is a heat and mass transfer problem," said first author Valerio D'Alessandro, an assistant professor of heat and mass transfer at the Marche Polytechnic University in Italy. "Irradiating with ultraviolet-C is a known germicidal technique, so we assessed the possibility of using it to inactivate SARS-CoV-2 particles that travel in saliva droplets in a time scale compatible with cough ejection."

If ultraviolet-C, or UV-C, could be used to inactivate the virus, it would be a simple and inexpensive way of reducing its spread in addition to other measures such as masking. 

A 2020 study appearing in Nature demonstrated that a range of UV-C frequencies can inactivate coronaviruses, the family that includes SARS-CoV2, by destroying the outer protein coating around the virus. And while studies suggest that the virus may be similarly vulnerable to UV-C, there are still major concerns about the safety of this method for the average person.

While UV-C from the sun poses little risk because it is filtered out by the atmosphere, even brief exposure to UV-C from germicidal lamps can cause severe burns to the skin or eyes. In the United States, UV-C lamps are regulated under the Radiation Control for Health and Safety Act, which requires manufacturers to maintain specific performance and safety standards on these products.

To examine whether UV-C could be employed safely against COVID-19, the researchers used a supercomputer hosted by the Italian Energy Agency to model both the flow of air through space and the movement of respiratory droplets produced by coughing. The team also included a parameter to account for the presence of UV-C light in a low-risk dose.

In addition to addressing the UV-C question, the team was interested in evaluating whether one meter or approximately 3.3 feet of distance, which is recommended by the World Health Organization to prevent spread of the virus, is actually a sufficient buffer compared to larger distance guidelines in countries such as the U.S., where the Centers for Disease Control and Prevention recommends 6 feet of distance.

"We found that a social distance of one meter is only sufficient to prevent the direct transmission of droplets emitted by an infected subject to a possible host," said D'Alessandro. "We worked only on coughing, without background wind and face masks. So, it's probable that the safety distance of 1 meter isn't so safe when you take those factors into account."

However, when the team accounted for ambient UV-C at a dose low enough not to pose a risk to humans, they found that one meter was a consistently safe distance according to their models. They reported up to a 50% reduction in the risk of contamination when droplets are irradiated with UV-C.

"One meter could be sufficient but, in general, is not sufficient in standard conditions. So face masks are crucial, especially in outdoor conditions," said D'Alessandro. "In the presence of wind, saliva droplets can travel a lot. However, UV-C is a promising technique for real-time disinfection, because when we irradiate saliva droplets, a distance of 1 meter is absolutely safe."

The team hopes that its work will lead to the adoption of more stringent social distancing guidelines in Europe. The team is also interested in expanding the work to explore the role of other forms of ultraviolet radiation that are naturally ambient outdoors.

The study, "Eulerian–Lagrangian modelling of cough droplets irradiated by ultraviolet–C light in relation to SARS–CoV–2 transmission," published March 9, was authored by Valerio D'Alessandro, Matteo Falone and Luca Giammichele, Università Politecnica delle Marche.