The US Centers for Disease Control and Prevention updated guidance on its website to say coronavirus can commonly spread "through respiratory droplets or small particles, such as those in aerosols," which are produced even when a person breathes.
"Airborne viruses, including COVID-19, are among the most contagious and easily spread," the site now says.
Previously, the CDC page said that Covid-19 was thought to spread mainly between people in close contact -- about 6 feet -- and "through respiratory droplets produced when an infected person coughs, sneezes or talks."
The page, updated Friday, still says Covid-19 most commonly spreads between people who are in close contact with one another, and now says the virus is known to spread "through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks or breathes."
These particles can cause infection when "inhaled into the nose, mouth, airways, and lungs," it says. "This is thought to be the main way the virus spreads."
"There is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes)," the page now says. "In general, indoor environments without good ventilation increase this risk."
The CDC also added new measures to its information about protecting yourself and others.
Previously, CDC suggested maintaining "good social distance" of about 6 feet, washing hands, routinely cleaning and disinfecting surfaces and covering your mouth and nose with a mask when around others.
Now, it says "stay at least 6 feet away from others, whenever possible," and continues to direct people to wear a mask and routinely clean and disinfect. However, it also now says people should stay home and isolate when sick, and "use air purifiers to help reduce airborne germs in indoor spaces."
Masks, it notes, should not replace other prevention measures.
The update also changed language around asymptomatic transmission, shifting from saying "some people without symptoms may be able to spread the virus" to saying "people who are infected but do not show symptoms can spread the virus to others."
Scientists pushed for acknowledgment of airborne transmission
For months, scientists have noted the likelihood of coronavirus transmission through viral particles in the air, and pushed health agencies to acknowledge it.
In April, a prestigious scientific panel told the White House in a letter that research showed coronavirus can be spread not just by sneezes or coughs, but also just by talking, or possibly even just breathing.
"While the current [coronavirus] specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing," according to the letter, written by Dr. Harvey Fineberg, former dean of the Harvard School of Public Health and chair of the NAS' Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats.
"Currently available research supports the possibility that [coronavirus] could be spread via bioaerosols generated directly by patients' exhalation," the letter said.
And in July, 239 scientists published a letter that urged the World Health Organization and other public health organizations to be more forthcoming about the likelihood that people could catch the virus from droplets that were floating in the air.
"The current guidance from numerous international and national bodies focuses on hand washing, maintaining social distancing, and droplet precautions," scientists wrote in the letter, published in the journal Clinical Infectious Diseases.
"Most public health organizations, including the World Health Organization, do not recognize airborne transmission except for aerosol-generating procedures performed in healthcare settings. Hand washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people," they added.
After the letter published, WHO released a report that detailed how the coronavirus can pass from one person to another, including through the air during certain medical procedures and possibly the air in crowded indoor spaces.
On Sunday, one of the lead authors of the letter, Donald Milton, a professor of environmental health at the University of Maryland who studies how viruses are transmitted, said the CDC's new language was a "major improvement."
"I'm very encouraged to see that the CDC is paying attention and moving with the science. The evidence is accumulating," Milton wrote in an email to CNN.
He described a pre-print paper released in August -- in which scientists described culturing viable virus from air in a hospital -- as "an important addition to the reports of large outbreaks that were clearly a result of transmission by aerosols that travel more than 6 feet."
"It is time for WHO to acknowledge these advancements in the science," Milton said.