But now a group of scientists is calling on the World Health Organization to acknowledge that the virus may remain airborne for much longer than previously believed.
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More than 200 scientists from 32 countries are backing a letter published this week in the journal Clinical Infectious Diseases. In the journal, two scientists from Australia and the U.S. wrote that studies have shown "beyond any reasonable doubt that viruses are released during exhalation, talking and coughing in microdroplets small enough to remain aloft in the air."
That means people in certain indoor conditions could be at greater risk of being infected than was previously thought.
The WHO has long maintained that COVID-19 is spread via larger respiratory droplets, most often when people cough or sneeze, that fall to the ground. It has dismissed the possibility of airborne transmission, except for certain high-risk medical procedures, like when patients are first put on breathing machines.
In a statement on Monday, the U.N. health agency said it was aware of the article and was reviewing it with technical experts.
A Southern California doctor spoke to Eyewitness News about his take on the letter and his own observations from treating patients.
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Dr. Anthony Cardillo, an ER specialist and CEO of Mend Urgent Care, said if it turns out to be true that the virus can remain airborne, that means it is "incredibly infectious," more so than we already believed.
Cardillo said in a discussion of airborne viruses, one comparison can be made to measles. If someone infected with measles sneezes in a room, those particles can remain airborne and infectious for several hours, he said. If a patient turns out to have measles, an examination room would be evacuated, he said.
But for now, it doesn't appear that the coronavirus is quite that infectious, he said.
"We have not thought that this virus is like the measles virus," Cardillo said. "All the studies suggest so far that it is too big to be airborne."
That said, he noted that some studies have found the virus may be mutating, and that could mean the particles are becoming lighter.
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There are some scenarios, he noted, where even the heavier droplets can be spread through the air. For example, if an infected person has asthma and is using a nebulizer machine, that creates a mist that can aerosolize the virus. Also some air systems on airplanes and other locations may recirculate virus particles before they drop to the ground.
But for now, he said, those are exceptions to the rule.
"There are scenarios where it is airborne," Cardillo said. "But in its purest form, it is really a respiratory droplet and you have to be six feet close to someone to pick it up."
The Associated Press contributed to this report.
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