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A sneeze or cough sends thousands of viral particles into the air. The  question is, how far do they travel? (Shutterstock) A sneeze or cough sends thousands of viral particles into the air. The  question is, how far do they travel? (Shutterstock)

A sneeze or cough sends thousands of viral particles into the air. The question is, how far do they travel? (Shutterstock)

Jon Greenberg
By Jon Greenberg September 23, 2020

If Your Time is short

  • The CDC issued guidance warning that airborne particles could transmit coronavirus. It later retracted that information.

  • Independent researchers say that airborne transmission seems more and more likely. 

  • In real world conditions, it is difficult to prove airborne transmission beyond any doubt.

The federal government did a quick pivot on the threat of the coronavirus spreading through the air, changing a key piece of guidance over the weekend. 

On Friday, Sept. 18, the Centers for Disease Control and Prevention warned that tiny airborne particles, not just the bigger water droplets from a sneeze or cough, could infect others. It cited growing "evidence."

By Monday, Sept. 21, that warning was gone, with a note that said it had been posted in error, and the CDC was in the process of updating its recommendations.

The move put the CDC in the middle of a debate over how the coronavirus infects people. Its guidelines could make the difference between restaurants, bars and other places where people gather, fully reopening sooner or much later.

And it raised more questions about politics at the public health agency and whether White House officials are dictating policy to health authorities. 

So what does the science on airborne transmission actually say? 

The emerging picture is a work-in-progress, but many of the pieces do point toward the potential for airborne transmission. 

The challenge of proving airborne transmission

The CDC’s retracted language said, "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)."

Why is this a big deal? It means the guidance for physical distancing might need to be increased.

Six feet is the benchmark for safety that has helped shape the reopening of schools and businesses nationwide. The number is based on the long-held finding that larger water drops from a cough are so heavy that most of them fall to the ground before the six-foot mark.

But much smaller droplets can hang in the air longer. The debate is whether they carry enough of the virus to infect another person. If the answer is yes, the implications for everyday life could be substantial.

University of Maryland School of Public Health professor Donald Milton sees plenty of evidence that airborne transmission is a major factor, but he emphasized that a definitive answer is hard to come by. 

No one disagrees that being near someone with the disease is the main threat. But Milton said what happens during that time is tough to untangle.

"It could be they cough and you get infected by getting a direct hit on your eye or mouth," Milton said. "Or could it be through an airborne particle that you inhale. Or you might have touched something and then touched your nose or your mouth. It's fiendishly difficult to sort that out."

That said, many incidents and studies point toward the idea that airborne particles play a bigger role than has been thought..

The research

An international group of researchers from China, Australia and the United States recently reviewed the evidence for airborne transmission. They concluded that it was highly plausible.

A study published in the Proceedings of the National Academy of Sciences reported that one minute of loud talking could produce "1000 virus-containing droplet nuclei that could remain airborne for more than eight minutes."

The authors’ conclusion? "These are likely to be inhaled by others and hence trigger new infections."

Bus rides are a key testing ground.

In China, scientists looked at 126 passengers on two buses making a trip that lasted about an hour and a half. One bus was virus free, the other had one infected rider. The people on the bus with the virus were 41.5 times more likely to be infected.

Many other researchers have noted the super-spreading event at the two-and-a-half hour-long choir practice of the Skagit Valley Chorale in Mount Vernon, Wash. Of the 61 people who attended, there were 53 cases and two deaths.

A University of Florida study sampled the air in the hospital rooms of two COVID-19 patients. They found aerosol particles carrying enough viral load to infect someone more than 15 feet away from the patients.

In July, 239 researchers co-signed an open letter that called on national and international health agencies to "recognize the potential for airborne spread" of COVID-19.

Credible studies, they wrote, "have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure."

Still, a July World Health Organization report found while airborne transmission was possible, more robust research was needed to confirm that it presents an appreciable risk.

If public health leaders take airborne transmission more seriously, Milton said there are a few implications. Most business activity could continue, but restaurants and bars — because masks don't fit with eating and drinking — would face a higher hurdle.

Beyond that, more attention to ventilation in more closed spaces becomes important, as does the supply of N-95 masks. Those masks continue to be in short supply

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Our Sources

Centers for Disease Control and Prevention, How COVID-19 Spreads, Sept. 19, 2020

Centers for Disease Control and Prevention, How COVID-19 Spreads, updated Sept. 21, 2020

New England Journal of Medicine, Droplets and Aerosols in the Transmission of SARS-CoV-2, May 21, 2020

Clinical Infectious Diseases, It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19), July 6, 2020

New England Journal of Medicine, Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1, April 16, 2020

MedRxiv, Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients, Aug. 4, 2020

Risk Analysis, Consideration of the Aerosol Transmission for COVID‐19 and Public Health, May 2020

Environment International, Aerosol transmission of SARS-CoV-2? Evidence, prevention and control, Aug. 7, 2020

World Health Organization, Transmission of SARS-CoV-2: implications for infection prevention precautions, July 9, 2020

Nancy Pelosi, Pelosi Statement on Withdrawn CDC Guidelines on Airborne Transmission of COVID, Sept. 21, 2020

PolitiFact, Post saying new coronavirus stays in the air for 8 hours is false, April 2, 2020

CNN, Updated CDC guidance acknowledges coronavirus can spread through the air, Sept. 20, 2020

Washington Post, CDC reverses itself and says guidelines it posted on coronavirus airborne transmission were wrong, Sept. 21, 2020

ABC News, After CDC whiplash, here's what science says about airborne transmission of the coronavirus, Sept. 21, 2020

CDC — Morbidity and Mortality Weekly Report, High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice, May 15, 2020 

Interview, Donald Milton, professor of environmental health, University of Maryland School of Public Health, Sept. 22, 2020


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