Coughing or sneezing may not be the only way that people transmit infectious pathogens like the new coronavirus. Talking can also launch thousands of droplets so small that they can hang in the air for eight to fourteen minutes, according to a new study.
The investigation, published Wednesday in The Proceedings of the National Academy of Sciences, could help explain how people with mild or symptom-free symptoms can infect others in nearby places such as offices, nursing homes, cruise ships, and other confined spaces. The experimental conditions of the study will have to be replicated in more real-world circumstances, and researchers still don't know how much virus has to be passed from one person to another to cause the infection. But their findings reinforce the argument for wearing masks and taking other precautions in such settings to reduce the spread of the coronavirus.
Scientists agree that the coronavirus jumps from one person to another more often through tiny respiratory droplets. These droplets tend to fall to the ground a few meters from the person who emits them. They can fall on surfaces like doorknobs, where people can touch the remaining virus particles and transfer them to their faces. But some droplets can remain in the air and be inhaled by others.
Elaborate experiments have revealed how coughing or sneezing can produce a sizzling blast of air mixed with saliva or mucus that can force hundreds of millions of flu and other virus particles into the air if a person is sick. A single cough can propel about 3,000 respiratory drops, while sneezing can generate up to 40,000.
To see how many droplets are produced during a normal conversation, researchers at the National Institute of Diabetes and Digestive and Kidney Diseases and the University of Pennsylvania, who study the kinetics of biological molecules within the human body, asked volunteers to repeat the words "stay healthy" several times. As participants spoke at the open end of a cardboard box, the researchers illuminated its interior with green lasers and followed the bursts of drops produced by the speaker.
Laser scans showed that about 2,600 small drops were produced per second as they spoke. When the researchers projected the quantity and size of the droplets produced at different volumes based on previous studies, they found that hBy blowing harder, larger droplets could be generated, as well as larger amounts of them.
Although the scientists did not record the speech droplets produced by people who were sick, previous studies have calculated exactly how much genetic material from coronavirus can be found in oral fluids in the average patient. Based on this knowledge, the researchers estimated that a single minute of speaking out loud could generate at least 1,000 virus-containing droplets.
Scientists also discovered that, sAlthough the droplets begin to decrease from dehydration as soon as they leave a person's mouth, they can still float in the air for eight to 14 minutes.
"These observations confirm that there is a substantial probability that normal speech will cause airborne transmission of the virus in confined environments," the authors wrote in the study.
The researchers recognized that the experiment was conducted in a controlled environment with stagnant air that may not reflect what happens in well-ventilated rooms. But they still had reason to believe that the values they reported were "conservative lower limit estimates" because some people have a higher viral load, which means they could produce drops with several thousand more virus particles than the average. .
The Centers for Disease Control and Prevention says that keeping at least six feet away from others can help people avoid contact with respiratory droplets and reduce the risk of infection. But Many scientists have argued that droplets can travel beyond six feet, depending on the force with which the droplets are released, the ambient temperature, whether there are air currents that can carry them further, and other conditions.
There is also debate over whether the coronavirus can also be transmitted through even smaller droplets - less than a tenth the width of a human hair - known as aerosols, and can remain suspended or travel through the air longer.
In another recent study, the same authors demonstrated that just articulating certain sounds can produce significantly larger amounts of respiratory particles. The "th" sound of the word "healthy", for example, was a very efficient droplet generator for speech. ORAnother work, published in January by researchers at the University of California, Davis, found that the sound of the vowel "e" in "need" produces more droplets than the "a" in "sierra", or the "o" in " state of mind ”.
What researchers don't know yet is whether all droplets of speech, cough, and sneeze that carry virus particles are equally infectious, or whether it is necessary to transmit a specific amount of virus for a person to become ill from breathing it.
However, the new study adds to arguments in favor of keeping a physical distance from other people to help curb the spread of the coronavirus, said Linsey Marr, a Virginia Tech professor of civil and environmental engineering who was not involved in the study. "Based on this and other evidence, it would be prudent to avoid lengthy face-to-face conversations with others unless you are far away and in a well-ventilated space, including outdoors," said Marr.
The study also highlights the importance of wearing masks during social and other interactions. "The risk of talking to others is probably less than the risk of being exposed to a person who is not wearing a mask and who coughs and sneezes openly," said Werner E. Bischoff, medical director of infection prevention and epidemiology of the Wake Forest School of Medicine health system. “Talking normally to a person while maintaining the recommended social distance will be fine. Putting on a mask will be even better. ”
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