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CDC Acknowledges Covid-19 Can Spread Via Tiny Air Particles

The U.S. Centers for Disease Control and Prevention said tiny particles that linger in the air can spread the coronavirus, revising its guidelines on the matter just a few weeks after the health agency had acknowledged a role for the particles and then abruptly removed it.

The guidelines on how the coronavirus spreads were initially updated last month to acknowledge a role, and possibly the primary one, played by tiny aerosol particles in spreading the virus. But the agency removed the changes only days later, saying a draft version of the proposed changes had been posted in error.

In its latest revisions to the guidelines Monday, the CDC acknowledged that tiny airborne particles that can travel beyond 6 feet can be infectious, though the latest wording says they aren’t the main way the virus spreads.

The virus is primarily transmitted via respiratory droplets by people in close contact, including those who are physically near, or within about 6 feet of, each other, the CDC said. Those droplets, the agency added, cause infection when they are inhaled or deposited in the nose and mouth.

The agency also recognized, however, that some infections can be spread by exposure to the virus in small droplets and particles that can linger in the air for minutes to hours. Evidence indicates that under certain conditions people with Covid-19 seem to have infected others who were more than 6 feet away, the CDC said.

“These transmissions occurred within enclosed spaces that had inadequate ventilation,” the CDC said. “Sometimes the person was breathing heavily, for example while singing or exercising.”

In such situations, infectious smaller droplets and particles became concentrated enough to spread the virus to others, the CDC said. People who became infected were in the space at the same time as the person who was transmitting the virus, or shortly after the person with Covid-19 had left, the agency said.

“CDC continues to believe, based on current science, that people are more likely to become infected the longer and closer they are to a person with Covid-19,” the agency said in a news release that accompanied its updated guidelines. “Today’s update acknowledges the existence of some published reports showing limited, uncommon circumstances where people with COVID-19 infected others who were more than 6 feet away or shortly after the COVID-19-positive person left an area.”

At the heart of the back and forth is continued debate about how the virus spreads and the infectiousness of the various-sized droplets and tiny particles known as aerosols that are emitted when a person coughs, talks, sneezes, sings or breathes.

How Droplets Move

Droplets of all sizes are emitted when a person coughs, talks or sneezes. How they travel depends on many factors. Some research has found that droplets will be carried by a moist gas cloud, which a Massachusetts Institute of Technology researcher has said can travel up to about 26 feet after a sneeze. Some of the droplets will fall as the cloud moves. Others ultimately evaporate, producing aerosols that can linger in the air and travel with airflow patterns, a March article by the researcher published in the Journal of the American Medical Association said.

The cloud may travel as far as almost 20 feet after a cough, and about 6 feet after exhalation, according to the MIT researcher.

Scientists emphasize there is no distinct size cut-off between droplets and aerosols. Some disagree about size ranges for each. Researchers are working to better understand the infectiousness of various-sized droplets and aerosols, and how it may change over time.

Small Aerosols:

3 microns or less

Small Droplets and Large Aerosols:

100 microns or smaller

Large Droplet: 100 microns (diameter) or larger

These heavier droplets fall to the ground within seconds

Can linger in the air for 30 minutes or more

The cloud may travel as far as almost 20 feet after a cough, and about 6 feet after exhalation, according to the MIT researcher.

Scientists emphasize there is no distinct size cut-off between droplets and aerosols. Some disagree about size ranges for each. Researchers are working to better understand the infectiousness of various-sized droplets and aerosols, and how it may change over time.

Small Aerosols:

3 microns or less

Small Droplets and Large Aerosols:

100 microns or smaller

Large Droplet: 100 microns (diameter) or larger

These heavier droplets fall to the ground within seconds

Can linger in the air for 30 minutes or more

The cloud may travel as far as almost 20 feet after a cough, and about 6 feet after exhalation, according to the MIT researcher.

Scientists emphasize there is no distinct size cut-off between droplets and aerosols. Some disagree about size ranges for each. Researchers are working to better understand the infectiousness of various-sized droplets and aerosols, and how it may change over time.

Small Aerosols:

3 microns or less

Small Droplets and Large Aerosols:

100 microns or smaller

Large Droplet: 100 microns (diameter) or larger

These heavier droplets fall to the ground within seconds

Can linger in the air for 30 minutes or more

The cloud may travel as far as almost 20 feet after a cough, and about 6 feet after exhalation, according to the MIT researcher.

Scientists emphasize there is no distinct size cut-off between droplets and aerosols. Some disagree about size ranges for each. Researchers are working to better understand the infectiousness of various-sized droplets and aerosols, and how it may change over time.

Large Droplet: 100 microns (diameter) or larger

These heavier droplets fall to the ground within seconds

Small Droplets and Large Aerosols:

100 microns or smaller

Can linger in the air for 30 minutes or more

Small Aerosols:

3 microns or less

From the start of the pandemic, some scientists and infectious-disease experts, as well as the CDC and the World Health Organization, said the virus is transmitted primarily between people in close contact, through respiratory droplets, which the CDC said can land in the mouths or noses of people nearby, or possibly be inhaled into the lungs.

Those droplets, studies have shown, typically travel only a few feet before falling to the ground.

But as more studies began to show that people were also contracting the virus even when avoiding close contact with an infected person, many scientists began to conclude that tiny aerosols are playing a key role in spread.

Those minuscule aerosols, research shows, can suspend in the air, traveling for minutes or hours on air currents, far beyond 6 feet. They can be inhaled by people, especially those who are unmasked, in rooms with poor ventilation and exposed for a prolonged period.

After the guidelines on how Covid-19 spreads were updated last month to include airborne transmission, many scientists cheered the CDC’s decision, saying it was a necessary and long overdue acknowledgment of the role played by the tiny particles.

Scientists applauded the renewed acknowledgment, though some said they wished the airborne-transmission language was stronger.

“Airborne transmission is happening in normal situations—it doesn’t just have to be a choir practice or while someone is exercising,” said Joseph Allen, director of Harvard University’s Healthy Buildings program, which studies how buildings affect human health. “We’ve seen this in a restaurant, on a school bus, at a camp.”

“If you look at all of these outbreaks, they have a common thread: time spent indoors, no mask and low ventilation,” Dr. Allen added.

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Whether and how much airborne transmission plays a role in the virus’s spread has become fiercely debated, in part because decisions on reopening schools, businesses and other establishments hinge on the matter.

A full endorsement of airborne transmission would likely call for more stringent precautions for reopening businesses and schools, health and ventilation experts say. Everyone would need to wear masks, crowd sizes would have to be small and many buildings would require better ventilation systems, according to the experts.

Such steps could make it harder and more expensive for schools, businesses and other establishments to reopen, potentially impeding the economy’s ability to recover.

In its new guidelines, the CDC acknowledged the need to ensure proper ventilation of indoor spaces, saying that “being outdoors and in spaces with good ventilation reduces the risk of exposure to infectious respiratory droplets.”

The agency recommended that people stay at least 6 feet away from others, wash their hands and routinely clean and disinfect surfaces. It also advised wearing a mask, saying it helps reduce the risk of spread both by close contact and airborne transmission.

Governments around the world are debating the timeline for offering Covid-19 vaccines to the public, as drugmakers speed up development. WSJ’s Daniela Hernandez explains the potential health risks linked to fast-tracking vaccines. Photo: Siphiwe Sibeko/AP

The CDC’s Covid-19 guidelines, available on the agency’s website, effectively provide the federal government’s view on how the new coronavirus spreads and what people, schools and businesses can do to reduce their risk of infection.

Their recommendations have become a flashpoint in the partisan debate over the appropriate response to the virus, including whether people should wear masks, if schools should reopen and who should get tested.

The administration’s critics, including many scientists, say it is ignoring scientific evidence for political gain, and risking the health of Americans.

Caught in the back-and-forth is the CDC, the country’s pre-eminent public-health agency. It has now reversed course on a number of Covid-19 guidelines.

Under pressure from the Trump administration, the CDC in July added to its guidelines language stressing the importance of children returning to schools, and saying Covid-19 poses lower risks for children than for adults.

Then, in August, the CDC dialed back its testing recommendations, saying that close contacts of confirmed cases don’t need to undergo testing if they don’t have symptoms. The agency recently walked that back, saying close contacts should get tested, even if they don’t have any symptoms.

The latest problem with the CDC’s guidelines stemmed from confusion inside the agency, not interference from the White House or the Health and Human Services department, people familiar with the matter said.

Some staff posted an update, when it was still in draft form and hadn’t been vetted by senior agency scientists and approved, the people said.

Write to Caitlin McCabe at [email protected] and Betsy McKay at [email protected]

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