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Respiratory droplets carrying SARS-CoV-2 can last 30 minutes and travel up to 200 feet

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It’s unclear how far respiratory droplets, like those that spread COVID-19, can travel before becoming harmless. A modeling study looked at how far respiratory droplets can travel before becoming harmless.

Experiments dating back to the 1930s suggested that respiratory droplets, such as those produced by a sneeze or cough, may travel in two directions. Either they are large and heavy, tumbling to the ground with little possibility of infecting another person, or they are small and light. Or they’re so little and light that they dry out nearly instantly, allowing them to remain airborne but quickly becoming harmless. The dryness prevents “enveloped” viruses such as coronaviruses from infecting.

However, a new study from the Department of Energy’s Pacific Northwest National Laboratory indicates a third option: small respiratory particles can stay moist and airborne for longer and travel further than previously thought.

The PNNL team examined the mucus that envelopes the respiratory droplets that humans expel from their lungs in great detail. Mucus helps many viruses to move further than they would otherwise, allowing them to spread from one person to another, according to scientists.

The conventional opinion has long held that very small, aerosolized droplets of a few microns, such as those produced in the lungs, dry out practically rapidly in air and become harmless. However, the PNNL researchers discovered that mucus alters the equation.

The researchers discovered that the mucus shell that surrounds respiratory droplets inhibits evaporation, allowing virus particles within the droplets to stay moist for longer. Because encapsulated viruses, such as SARS-CoV-2, have a fatty coating that must be kept moist to remain infectious, the slower evaporation permits viral particles to remain infectious for longer.

Droplets wrapped in mucus could stay moist for up to 30 minutes and travel up to 200 feet, according to the researchers.

“While there have been many factors proposed as variables in how COVID spreads,” said the authors, “mucus remains largely overlooked.”

How the virus moves in a multiroom office building

Passing through respiratory droplets is the initial stage for the virus to become airborne and infect individuals who breathe it in. Carolyn Burns, a chemist, was tasked with making artificial, respiratory-like droplets in order to observe how the particles travelled from one room to the next.

Burns eventually chose two chemicals to transport fake virus-like particles. The first was bovine mucus, and the second was sodium alginate, a brown seaweed substance. In foods like ice cream and cheese, the chemical is often employed as a thickening agent.

In one area of a multi-room laboratory complex, the team utilized an airbrush to distribute droplets. The droplets and airbrush worked together to replicate a coughing attack, releasing particles in a source room for roughly one minute. Droplet levels were tested in two nearby rooms with controlled building ventilation by a team led by Alex Vlachokostas and Burns.

Both low and high levels of filtration were shown to be helpful in lowering respiratory droplets in all rooms, according to the researchers. Filtration quickly reduced the quantities of droplets in the adjoining rooms—within three hours, they were down to one-third of what they were before.

The study also discovered that increased ventilation in the source room lowered particle levels quickly. Particle levels in the other adjacent rooms, on the other hand, instantly increased; levels peaked 20 to 45 minutes later, with violent air changes exacerbating the increase. After the initial increase, droplet levels in all rooms gradually decreased over the course of three hours with filtration and five hours without it.

According to the researchers, enhanced air exchange for packed spaces may be useful in particular cases, such as large conferences or school assemblies, but it may actually raise transmission rates throughout all rooms of a building in routine work and school conditions.

“If you’re in a downstream room and you’re not the source of the virus, you probably are not better off with more ventilation,” add the authors.

Source: doi.org/10.1016/j.icheatmasstransfer.2021.105746

Image Credit: Getty

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