Representative image
Small coronavirus respiratory particles can remain moist and airborne for a longer time and at greater distance than scientists have thought so far. Scientists at the US Department of Energy's Pacific Northwest National Laboratory estimates that droplets encased in mucus could remain moist for up to 30 minutes and travel up to about 200 feet. "There are reports of people becoming infected with a coronavirus downwind of an infected person or in a room several minutes after an infected person has exited that room," said Leonard Pease, the corresponding author of the study.
"The idea that enveloped virions may remain well hydrated and thus fully infective at substantial distances is consistent with real-world observations. Perhaps infectious respiratory droplets persist longer than we have realised," Pease added.
The findings were published in the journal International Communications in Heat and Mass Transfer.
The team analysed the mucus that coats the respiratory droplets that people spew from their lungs. Scientists know that mucus allows many viruses to travel further than they otherwise would, enabling them to journey from one person to another.
Conventional wisdom has been that very small, aerosolised droplets of just a few microns, like those produced in the lungs, dry out in air almost instantly, becoming harmless. But the team found that mucus changes the equation.
The team found that the mucus shell that surrounds respiratory droplets likely reduces the evaporation rate, increasing the time that viral particles within the droplets are kept moist.
Since enveloped viruses like SARS-CoV-2 have a fatty coating that must be kept moist for the virus to be infectious, the slower evaporation allows viral particles to be infectious longer.
"While there have been many factors proposed as variables in how Covid spreads," said Pease, "mucus remains largely overlooked."
The focus on mucus also helped the team, in a separate study published in the journal Indoor Air, to address how the virus moves in a multiroom office building.
In the study, Chemist Carolyn Burns created artificial, respiratory-like droplets to study how the particles moved from room to room.
The scientists found that both low and high levels of filtering were effective at reducing levels of respiratory droplets in all rooms.
The team also found that increasing ventilation rapidly reduced particle levels in the source room.
But particle levels in the other connected rooms jumped immediately; levels spiked 20 to 45 minutes later with vigorous air changes increasing the spike.
Ultimately, after the initial spike, levels of droplets in all the rooms gradually dropped after three hours with filtration and after five hours without it.
The scientists noted that increased air exchange for crowded spaces may be beneficial in certain situations, like large conferences or school assemblies, but in normal work and school conditions, it may actually increase transmission rates throughout all rooms of a building.
"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," said Pease.
