Jose-Luis Jimenez, a University of Colorado, Boulder aerosol scientist and one of the authors of that report, said in an interview Sunday that the CDC's updated guidance represented a major shift. Until now, he said, agency scientists have said that the virus is transmitted through the air when droplets shoot out of one person's mouth or nose in the form of projectiles, directly infecting another person.
"They changed it and didn't tell anybody," he said.
Donald Milton, a University of Maryland environmental health professor and an expert on aerosols, said in an interview Sunday that the CDC has gradually come around to the concept of airborne transmission as evidence has accumulated, and he noted that the agency has made unannounced changes to its guidance in the past.
"They've been paying attention and moving in response to research, so I'm glad to see that they're continuing and that there's nobody getting in the way," he said.
Without notice in May, the CDC altered guidance for reopening houses of worship, deleting a warning posted the previous day that said the act of singing may contribute to coronavirus transmission - a switch reportedly due to pressure from the White House. Friday's updated guidance identified singing as one of the activities that could produce infectious aerosols.
Jimenez and Milton said it's important to wear masks to reduce the risk of spreading and contracting COVID-19. They said it's crucial to make sure face coverings fit properly, so that aerosols don't escape or enter through gaps in the mask around the nose or mouth.
"Aerosols can travel farther than 6 feet, but they're more concentrated the closer you get, so standing as far away as possible reduces risk," Milton said. "The reason that bars have been such a big problem is that people get loud when they get alcohol onboard and move close together to hear, and you can't drink a beer or a shot with a mask on."
Milton and Shelly Miller, another aerosol researcher at the University of Colorado, Boulder, are studying ways that singing and playing wind instruments might be made safer through distancing, ventilation and masking with various types of materials. The research is funded by national choral and instrumental associations whose members have been unable to gather during the pandemic.
Good ventilation reduces risk indoors, as does simply opening windows to let air circulate, the researchers said. Ceiling units that use ultraviolet light to kill the virus are also showing promise, they said.
Milton and Jimenez were among a group of researchers who drafted an open letter to the WHO, ultimately signed by 239 researchers from 32 countries, that urged officials to accept the possibility that aerosols play a major role in spreading the virus. The WHO revised its guidelines after receiving the letter July 6, saying that airborne transmission had not been definitively demonstrated but recommending that people avoid poorly ventilated, crowded spaces.
The CDC has now taken another significant step to acknowledge the role of aerosols, Jimenez said.
"The entire field of aerosol science is telling them that the understanding of ballistic droplets is outdated, and it's really aerosols that are spreading the virus," he said.
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