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Updated at 6:03 p.m. ET

The Centers for Disease Control and Prevention posted guidance Friday evening saying that aerosol transmission might be one of the "most common" ways the coronavirus is spreading — and then took the guidance down on Monday.

The now-deleted updates were notable because so far the CDC has stopped short of saying that the virus is airborne.

The agency says the guidance was a draft version of proposed changes that was posted in error to its website. The CDC says that it is updating its recommendations regarding airborne transmission of the virus that causes COVID-19 and that it will post updated language once that process is complete.

Over the weekend, the CDC page "How COVID-19 Spreads" included among the most common modes of transmission "respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks, or breathes."

It continued: "These particles can be inhaled into the nose, mouth, airways, and lungs and cause infection. This is thought to be the main way the virus spreads." The guidance also stated that these particles might travel farther than 6 feet.

For a few days, researchers who have suspected aerosol transmission for months cheered the update as a long-overdue acknowledgment of accumulating evidence for how the virus transmits, particularly in indoor spaces.

Now the page has reverted to what it said before — that the virus spreads between people in close contact through respiratory droplets. The page makes no mention of aerosol transmission.

In July, the World Health Organization updated its guidance on aerosols after more than 200 scientists urged it to do so. WHO's guidance now states:

"There have been reported outbreaks of COVID-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing. In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out. More studies are urgently needed to investigate such instances and assess their significance for transmission of COVID-19."

What's the difference between respiratory droplets and aerosol particles?

Respiratory droplets are larger and fall to the ground quickly — hence the 6-feet rule that's generally considered safe for social distancing amid the pandemic. Aerosol particles are smaller and can linger in the air, moving with air currents from which they can be inhaled.

An airborne virus is considered to be a virus that spreads in exhaled particles that are small enough to linger in the air and move with air currents, from which they can be breathed in by passersby who then get sick.

Linsey Marr is a professor of civil and environmental engineering who researches airborne transmission at Virginia Tech. She was excited to see the CDC's changes on Friday, though she was also surprised at how strongly the new guidance was written — particularly in that it stated plainly that SARS-CoV-2 is an airborne virus.

Such a classification could require additional precautions in health care settings, she says.

Marr says that the changes mistakenly posted by the CDC could be significant if they are implemented.

"It means that nationally we need to do something about [transmission] beyond 6 feet, which means masks and ventilation and filtration," she says. "And if we do that, I think we can get a better control on the spread of the virus."

The posted and withdrawn transmission guidelines are just the latest in public reversals and controversy at the CDC.

On Friday, the agency reversed its new guidance on testing, published in August, that suggested people who have possibly been exposed to the coronavirus don't necessarily need to get tested for infection.

Also last week, Michael Caputo, the top spokesperson for the Department of Health and Human Services, announced he was taking a leave of absence after a social media tirade in which he falsely accused government scientists of engaging in "sedition." He had also come under criticism after reports that he and scientific adviser Paul Alexander sought to edit and delay public health reports from the CDC. Alexander is leaving the agency permanently.

These episodes, among others, have raised questions about the agency's consistency and credibility during the coronavirus pandemic.

Dr. Howard Koh is a professor at the Harvard T.H. Chan School of Public Health who served as assistant secretary for health during the Obama administration.

"The consistent inconsistency in this administration's guidance on COVID-19 has severely compromised the nation's trust in our public health agencies," Koh said in a statement Monday.

"During the greatest public health emergency in a century, trust in public health is essential — without it, this pandemic could go on indefinitely. To rectify the latest challenge, the CDC must acknowledge that growing scientific evidence indicates the importance of airborne transmission through aerosols, making mask wearing even more critical as we head into the difficult fall and winter season."

NPR's Pien Huang contributed to this report.