There are lots of unanswered questions about the transmissibility of the coronavirus. One of the most controversial has to do with whether you can catch COVID-19 by breathing air that contains viral particles. It all boils down to droplets vs. aerosols.
What’s the Difference Between Respiratory Droplets and Aerosols?
Both the CDC and the World Health Organization maintain that droplets created when we exhale, talk, sneeze or cough will only travel a short distance and then fall to the ground. We’re safe if we stay 6 feet away from other people. But what if respiratory aerosols travel much farther?
Understanding Droplets:
Droplets are greater in size than five microns. What’s a micron you ask? It is a micrometer (μm) or one-millionth of a meter.
To give you some idea of what we are talking about, here are some examples:
Thick human hair: 200 microns in diameter
Sheet of toilet paper: 100 microns thick
Red blood cell: 8 microns
Fine human hair: 5 microns
E. coli bacterium: 2 microns
Staph bacterium: 1 micron
Influenza virus: 0.1 micron
Coronavirus particles: 0.06 to 0.14 microns
A respiratory droplet that is greater than 5 microns is still relatively small. But it is big enough to fall to the ground from the pull of gravity within about 6 feet. So, if all we had to contend with involved droplets greater than 5 microns, the 6-foot rule would probably be fine. That is especially true if we are wearing a decent mask.
Understanding Aerosols:
Aerosolized droplets less than 5 microns in diameter are where the controversy lies. Remember, coronavirus particles range in size from 0.06 to 0.14 microns. They can be contained in small aerosolized respiratory droplets that are produced when we exhale or talk.
When these droplets are less than 5 microns in diameter, they:
“…can remain suspended in air for significant periods of time, allowing them to be transmitted over distances >1 m [meter].
(in Natural Ventilation for Infection Control in Health-Care Settings, Atkinson, J., et al, World Health Organization, 2009).
The big question is, how far can they go? Judging from research we have read, significantly farther than 6 feet! More to the point, can you catch COVID-19 by breathing in some of these very tiny droplets? Are there enough aerosolized coronavirus particles in several of these floating droplets to transmit the disease?
New Chinese Research on Aerosolized Particles:
A study from Wuhan, China, suggests that aerosol particles may be involved in the transmission of the SARS-CoV-2 virus that causes COVID-19. Researchers collected air samples from two hospitals actively used to treat COVID-19 patients.
They found tiny droplets with viral RNA in the air, particularly in the toilets and in the room where health care providers removed their PPE. The patient rooms were well-ventilated. The investigators also sampled air in a supermarket and a residential building and found no virus particles in that air.
Although RNA from the virus was detected, no one knows whether this represents viable virus that would be infectious or whether it is simply particles.
The scientists conclude in Nature:
“Our results indicate that room ventilation, open space, sanitization of protective apparel, and proper use and disinfection of toilet areas can effectively limit the concentration of SARS-CoV-2 RNA in aerosols. Future work should explore the infectivity of aerosolized virus.”
If the virus is indeed transmitted by aerosol, wearing a mask becomes even more important as a means of protecting against infection. Nature, April 27, 2020
Evidence that People Can Catch COVID-19 By Breathing:
This is not the first time researchers have suspected aerosol spread of SARS-CoV-2 particles. Last week we reported on a study from Guangzhou, China. People lunching in a restaurant were infected by a diner at another table. The analysis suggests that air conditioning carries viral particles father than six feet.
You can read the details of that research and the People’s Pharmacy Perspective at this link. You will see a detailed diagram of the tables at the restaurant and in particular, the airflow from the air conditioning unit.
Pushback from Readers!
We were delighted to receive so many comments from readers of that article. At last count, 46 people left thoughts about the possibility you could catch COVID-19 by breathing. Many were convinced that the flow of air over the asymptomatic infected woman from Wuhan was not the culprit:
Margaret thinks it was the wait staff that spread the virus:
“Could the waiter or server have been the vector through which all three tables were infected? That seems logical to me since they would go from table to table and cross contaminate by touching the utensils and plates and glassware for the separate tables.”
William has a similar message:
“Your analysis doesn’t mention the wait staff. They were around each of the tables a lot. Were they tested? Were they mostly young and didn’t catch or exhibit the disease? Could they have carried droplets to the other tables? These questions put the distance estimate in doubt.”
Kathy doubts that the diners in the restaurant could catch COVID-19 by breathing in viral particles from the air flow:
“Did you ever consider that some may have been in the restroom at the same time, or paid the bill at the same time or have been seated at the same time?”
You may want to look at the diagram of the room at this link to understand why we don’t think the wait staff or the bathroom led to additional cases of COVID-19. Here is a quick response to these comments:
1) The only person in that restaurant who had COVID-19 on the day people ate lunch there was the 63-year-old woman from Wuhan. She was the “index patient A-1″.
2) None of the wait staff had COVID-19 and none of the wait staff ever caught COVID-19 from her that day.
3) The wait staff had more tables than the 3 that were in direct line from the air conditioning. In other words, if the wait staff were vectors, carrying the virus from table to table and patron to patron, they should have carried the virus to all the tables in the dining room, not just the three tables in the direct path of the air conditioning unit. And not everyone at those three tables came down with the infection.
4) Some people hypothesize that the transfer of viral particles could have happened in the rest room. In other words, the index patient A-1 might have touched the doorknob or come too close to another patron. That is entirely plausible. But, no one else in the restaurant caught the virus. Only people in the direct line of the air flow from the AC unit became sick.
If you have additional thoughts about this restaurant study, please add them in the comment section at the bottom of this post.
Did People on Cruise Ships Catch Covid-19 by Breathing Viral Particles?
People who were quarantined on cruise ships also caught COVID-19 even after they were isolated for many days in their staterooms. Some scientists believe that ventilation on the boats may have carried the virus from room to room.
Did the Skagit Valley Chorale Catch Covid-19 by Breathing in SARS-CoV-2?
An uncontrolled experiment from the state of Washington adds some weight to the argument that people can catch COVID-19 by breathing in aerosolized viral particles. On March 10, 2020 members of the Skagit Valley Chorale gathered at the Mount Vernon Presbyterian Church in the state of Washington.
They were careful. Everyone applied hand sanitizer as they entered the practice room. There was no hugging or hand shaking. People kept farther apart than usual. Of the 60 singers who practiced that night, 45 came down with COVID-19. You can read more details at this link.
More New Research:
A research letter published this week in JAMA (April 28, 2020) reports surface contamination from a patient room in Singapore. Although the investigators did not find evidence of viral particles in the air itself, they did recover viral RNA from air exhaust outlets.
In their own words:
“There was extensive environmental contamination by 1 SARS-CoV-2 patient with mild upper respiratory tract involvement…Swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents.”
The conclusion:
“Significant environmental contamination by patients with SARS-CoV-2 through respiratory droplets and fecal shedding suggests the environment as a potential medium of transmission and supports the need for strict adherence to environmental and hand hygiene.”
The People’s Pharmacy Perspective:
None of the studies published this week has established that viral particles can actually infect people. Then again, no studies have ruled out this possibility.
The World Health Organization and the CDC seem certain that six feet of separation is all that is needed to prevent transmission of the coronavirus. We wish there was evidence to prove that.
What we do know is:
1) When people exhale or talk they can transmit viral particles on small respiratory droplets into the air.
2) Small aerosolized droplets less than 5 microns in size can travel farther than 6 feet.
3) Viral particles can eventually settle on a variety of surfaces. They may remain viable for hours or possibly even a day or three, depending upon conditions.
We cannot answer the question: Can you catch COVID-19 by breathing aerosolized viral particles? We wish we could give you a nice clear answer one way or the other.
Many scientists say that there is no evidence that you can catch COVID-19 by breathing small viral particles at distances greater than 6 feet. We have a favorite saying, though.
Absence of evidence is not the same thing as evidence of absence.
Designing well-controlled experiments to tell us if people can catch COVID-19 by breathing aerosolized viral particles would be, in our opinion, impossible. As a result, we may never have a scientific answer to the question.
Our parting words are, please be careful. Err on the side of caution. Please wear a mask in public and try to stay as far away from others as possible.
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