COVID-19 Aerosols – Airborne Infection

Last Updated on April 21, 2024 by Art

Indoor Air is Dangerous!

COVID-19 Aerosols

COVID-19 aerosols have been largely ignored in public health guidelines. This blog gives you an early alert to a rapidly-developing story: the great risk of indoor airborne infection, due to COVID-19 aerosols.

Here are the major headings in case you want to jump forward to that section:

COVID-19 Spreading Advice is Incomplete!

The official guidance from CDC (Center for Disease Control) and WHO (World Health Organization) is that coronavirus spreads mainly through “droplets produced when an infected person coughs, sneezes, or talks.” You can protect yourself with a face mask, by staying 6 feet distant from others and, in case droplets land on a surface, by disinfecting your hands and objects that many people touch.

Scientists are now stepping forward to say, that guidance is not enough. Moreover, it’s downright risky! If someone asks, how does the virus spread?, a more accurate answer is it all depends.

Today’s blog explains why face masks and distancing are not very effective indoors; why WHO and CDC are so slow to publicize that risk; how businesses are addressing the problem; and how you and I can best protect ourselves at this moment.

COVID-19 Aerosols Raise the Risk Indoors

We have previously discussed how people with COVID-19 shed infection-carrying virions into the air when they breathe and talk. The virions ride along on bits of fluid from the infected person’s nose and mouth.

            What’s an Aerosol?

In today’s blog, we will use the term “droplets” for the larger fluid drops that fall to the ground within a few feet. The small fluid drops that float in the air and settle much more slowly, if at all, we will call “aerosols.” There are differing opinions about how small a drop has to be to allow it to float. However, it’s reasonable to think of 10 micrometers diameter as a dividing point.

Experts all agree that breathing droplets emitted by an infected person can give you COVID-19. And you are pretty well protected from droplets if one or both of you wears a face mask, and if you stay some distance apart, so that droplets will fall to the ground and you won’t inhale them. (Here again, health experts have differing opinions on what distance droplets can travel before they fall. Some people say 3 feet, some say 6 feet, some say 10 feet. Obviously, how far the drops can go also depends on factors like air movement.)

            Most Face Masks Don’t Block Aerosols

However, now there’s growing recognition that COVID-19 aerosols can also transmit infection, through the air over long distances. This is a matter of such concern that on July 6 over 200 scientists signed a letter to WHO urging WHO to add aerosols to their list of coronavirus concerns and recommendations.

If COVID-19 aerosols are produced by infected people, that is a very serious matter. Aerosols are so small that they pass right through all face masks except the N95 masks worn by healthcare providers. Moreover, aerosols float so long in the air that they can travel huge distances on air currents. It is not safe to be 3 feet or even 10 feet away from an infected person. Social distancing now requires you to not even share the same air they are breathing!

            Proof of COVID-19 Aerosols as Bad Actors

This is a science-based website so we want to ask, how strong is the science behind the risk of airborne infection, caused by floating aerosols? We’ll start with one “superspreader event” that has been carefully analyzed. Then we’ll summarize supporting studies and analysis.

Superspreading at a Restaurant

On January 24, 2020, a “superspreader event” of infection occurred in a restaurant in Guangzhou, China. One person infected at least 7, perhaps 9, others. Some of the infections required airborne virions to travel at least 8 meters (over 26 feet) in the air.

This event has been carefully analyzed in at least two articles. One article gives a careful report of the event. Authors of the other article set up equipment in the restaurant to replicate the circumstances of the infectious event, accounting for the patterns of air flow on that entire floor of the restaurant. My description here summarizes the key findings of those articles.

            Background of the Infection Event

On January 23, family A traveled from Wuhan to Guangzhou. Ten members of family A had lunch on January 24 on the third floor of Restaurant X, Guangzhou, China.

On that day, there were 18 tables on the third floor. One table was empty; the other 17 held a total of 89 patrons at tables ranging from 2 to 10 persons. There were four air conditioners cooling the restaurant area, plus an additional air conditioner in the rest room. The wall opposite the air conditioners contained four exhaust fans but they were not operating. There was no outdoor air supply in the restaurant area. Each AC unit blew air across 3 to 5 tables at ceiling level. The air hit the far wall and then returned to the AC unit at table level.

covid-19 aerosols

Restaurant X infection event, January 24, 2020. Individual A1 showed symptoms on the day of the lunch. The other 9 persons circled all showed infections between then and February 5. (B2 and B3 could have caught it from B1, which might explain their later dates.)

The third floor of the restaurant is 17 meters long and varies from 6 to 9 meters wide. The entry, elevator and rest room are on the 9 meter wide wall of the room. The drawing above shows the part of the floor farthest from the entrance. Along the upper wall there are three tables. The center table had family A. Seven members of family C occupied the table to the right, beneath the air conditioner; four members of family B occupied another table at the left.

            One Person Infects Nine More at Lunch

Later that day, one member of family A, designated A1, had fever and cough and went to the hospital. By Feb 5, 4 more members of family A had come down with COVID, plus 3 members of family B and 2 members of family C. Family B was dining from 11:37 am to 12:54 pm; family A’s lunch overlapped with B for 53 minutes, and family C’s lunch overlapped with B for 51 minutes. None of the other 79 patrons on the floor, nor any restaurant staff, nor patrons on other floors of the restaurant were infected.

Families B and C were Guangzhou locals who did not know each other, nor did they know family A. A study of the video record shows no significant interaction between the families in the restaurant or elsewhere, such as in the elevator. Thus the only known chance for A1 to infect the other families was during the lunch.

            Airflow Analysis Indicts COVID-19 Aerosols

covid-19 aerosols

Restaurant X infection event. Blue arrows show direction of air flow from air conditioner across tables and return. Exhalation from individual A1 initially crossed table B and when returning to the air conditioner intake, also crossed table C.

The simulation study showed that the air circulating around tables A, B and C was mostly separate from the rest of the third floor. Thus the air passed over table C, then table B, then table A and then returned across tables B and C to the air conditioner intake. A simplified version of the air flow appears in the drawing above.

The air over tables A, B and C mixed with other parts of the room only slowly and gradually, and no one at those other tables was infected. It appears that the extended (over 50 minutes), concentrated exposure to A1’s exhalation infected 9 persons in his immediate area and no one else.

The persons at table B are about one to two meters away from person A1. Although A1 would not be facing table B directly, it’s possible that he happened to turn his head while exhaling or coughing and produced droplets that might travel to the table B people. For that matter, since the air flow went across table A and then to table B, perhaps A1 could cough toward the center of table A and have some of his droplets carry as far as table B.

            The Infection Traveled At Least 26 Feet By Air

However, what about table C? The infected people at Table C are about 3 meters away from A1, and upstream, against the air flow. Large droplets can only travel a few feet before they fall, and will not travel upwind. The only way Table C can get infected is if person A1 exhales drops that float in the air for a long time, that is, COVID-19 aerosols. Those aerosols then have to float in the air conditioner air moving toward the left and then return across the width of the restaurant to reach the air conditioner’s intake. The infecting drops would have to travel at least 8 meters (over 26 feet) to reach from person A1 to persons C1 and C2.

The Restaurant X analysis clearly finds that:

  • Restaurant X’s event involves airborne infection far beyond the range of droplets.
  • The event is best explained as spreading by COVID-19 aerosols.

Additional Evidence Pointing to COVID-19 Aerosols

The Restaurant X infection event is suggestive, and it’s not the only evidence fingering aerosols as a culprit:

  • Scientific American points out that most superspreader events occur indoors, not outdoors, suggesting circulating aerosols.
  • A long but very readable article in The Atlantic details how COVID-19 aerosols help explain many infections that are not being prevented merely by social distancing. In addition, it notes the higher rate of healthcare worker sickness in UK compared with US and ascribes it to UK’s older hospitals with poorer ventilation compared with US hospitals.
  • A widely-quoted article by immunologist Dr Erin Bromage reviews additional infection events that clearly indict poorly circulating indoor air.
  • An article in Nature treats virus contamination in quarantine and isolation environments and reports: “It is likely that the positive air samples in the hallway were caused by viral aerosol particles transported or resuspended by personnel exiting the room.”
  • And I have previously pointed to research showing that there is 19 times lower risk of acquiring coronavirus outdoors compared with indoors.

Why Have WHO and CDC Dropped the Ball?

You might wonder, why have WHO and CDC not spoken up to warn us about the risks of aerosol transmission indoors? Medical journalist Michael Jorrin reports:

In early April, a group of 36 experts on air quality and aerosols met with WHO to discuss the likelihood of aerosol transmission, but the discussion was dominated by other experts who were staunch proponents of hand-washing and decontamination of surfaces, and felt that those should be emphasized over aerial transmission.

I have not participated in healthcare rule-making, but I have seen similar activities, such as when a group of technologists tries to agree on engineering standards to ensure inter-operability of different pieces of equipment. The participants all have full-time jobs, but they have squeezed out some time as volunteers. Each one is an expert who may be mentally or financially committed to a specific approach. Some are more articulate or persistent than others, and louder voices have more influence. Moreover, the final report containing the group’s conclusions is written by a few folks who have the time to give to it. And even if they valiantly strive to capture everyone’s perspective, the writers impose a filter and a context that affects the results.

Jorrin’s report sounds just like this sort of group process. So it does not surprise me if we have a set of health recommendations from WHO and from CDC that does not capture all the issues that some experts consider critically important.

How Businesses Are Combatting COVID-19 Aerosols

In the absence of government guidance, what are businesses doing about aerosols? Healthcare facilities often use HEPA filtering, which removes aerosols. Commercial aircraft also circulate air locally, close to your seat, with HEPA filtration.

Other businesses – not very much! Most restaurants and bars recirculate the air, which saves heating and cooling expense. That approach is deadly because it circulates rather than removes aerosols.

Nevertheless, if you look closely you can find a few businesses that are actively cleaning their air:

            Business Examples That Reduce Airborne Infection

  • When I visited my dentist recently, I found that Dr Robyn Mullinger had acquired an air cleaning device that she used when disinfecting treatment rooms between patients. I also learned that ADA recommends cleaning treatment rooms with HEPA filtration. HEPA filters do a good job of removing aerosols from the air.
  • My friend Phil Wolken alerted me to the air filtration installed by a Seattle restaurant chain, Duke’s Seafood. Rather than HEPA, their system ionizes the air so that aerosols clump together to be easily filtered out. [There is another chain of Duke’s restaurants based in Hawaii, named after Olympian Duke Kahanamoku. Duke’s Seattle is a different chain, named after its founder Duke Moscrip.]
  • New York City public and parochial schools are improving air circulation and upgrading their air filtration to protect students.
  • In Wailea, Maui, resorts are adding air cleaning devices. Four Seasons Resort has installed ionizer units in each guest room, and Wailea Beach Resort has added HEPA filters to its air systems.
  • A recent interview with the CEO of Knoll discusses how the business office will change in the pandemic era. “Heavy-duty air filters” are one essential element in new office designs, so Knoll at least acknowledges the danger of airborne infection.
  • Nola and I own a condominium in Maui, which unfortunately we can’t visit right now because Hawaii is closed for business! However, I am currently president of the homeowners association so I am aware of our actions. Our property management firm recommended HEPA filtration for our rental office. We acquired an inexpensive Coway unit that can clean the air quickly in our small office to protect employees and visitors.

Readers, if you know of other businesses stepping up and filtering their air, speak up!

Update: Good News About Clean Air!

Note added September 26, 2020:

I’m happy to report some progress on the clean air front:

  1. In my city of Grosse Pointe, Michigan a favorite local restaurant, City Kitchen, has added an air purification and filtration system to their HVAC. Their system is Reme Halo, which posts impressive test results about viral and bacterial removal. The operating principle appears to be similar to the system mentioned above at Duke’s Seafood.
  2. Dr. Emanuel Goldman is a Rutgers microbiology professor who has railed against “Hygiene Theater,” in which people waste their time by obsessively cleaning surfaces while ignoring the more hazardous airborne viruses. Goldman reports that a teacher prompted him to write to her local school board, which was planning to close schools for one day per week for expensive cleaning. He says, “I wrote the school board for her and miraculously, they listened to me, and redirected their budget to ventilation systems, because breathing is where you catch this disease.”
  3. CDC “almost” caught up with the science and posted guidance warning about COVID-19 airborne transmission through aerosols. Unfortunately, their advice was abruptly withdrawn, reportedly due to election-year political interference. The Administration may have felt that businesses would object to the cost of cleaning the air their employees breathe.

Thus, it’s two steps forward and one step back. So goes the progress of science in the real world.

Protecting Ourselves Today from COVID-19 Aerosols

OK, this is the real world, today. Aerosols are being flagged as a risk by a lot of experts who ought to know. However, most businesses are not yet taking steps to protect us. What is a safety-conscious person to do?

  • Outdoors: Keep a distance from other people. At least 6 feet, but 10 feet is better. When you can’t keep that distance, wear a mask covering both mouth and nose.
  • Those few indoor spaces with aerosol filtration (either HEPA or similar) or lots of ventilation: Similar to outdoors.
  • All the other indoor spaces:
    • Try for fresh air and open windows
    • Share the space with fewer than 10 people (not counting people with whom you live)
    • Minimize your time there
    • Maintain as much distance as possible, up to 10 feet
    • Wear a face mask

What if the indoor space is your office, where you need to spend a lot of time? A full discussion requires another blog, but here’s a short version: (1) ask your employer for fresher, cleaner air; (2) ask that employees be temperature-tested daily; (3) buy an air cleaner and make it more effective by reducing air flow in your work area.

Each of these steps gives you partial protection – if you can’t do them all, then do as many as you can.

Official Guidance in Florida

We started this blog by warning that advice from CDC and WHO is lacking. However, I’m happy to report that Florida at least has promoted the hazards of indoor spaces to a number one position on a recent public poster. Florida advises residents to “Avoid the three C’s“:

Note that the first item says to avoid Closed spaces with poor ventilation. That additional phrase tries to protect the reader from infection by COVID-19 aerosols.

Does this mean that Florida is kissing off masks? No, not completely – because some versions of the poster have additional information near the bottom:

  • Wear your mask
  • Clean items with disinfectant
  • Wash your hands
  • Stay home if you’re sick

COVID-19 aerosols are an emerging threat and I predict that we will hear more about them. If this blog spreads the information sooner, it may save a life by making someone just a bit more cautious. Please stay healthy!

Image Credits:
Sneeze photo in public domain, courtesy of CDC
– Restaurant X diagrams by Art Chester, based on CDC public domain diagram in Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. with additional information from Yuguo Li et al.
– Three C’s poster courtesy of Florida Department of Health


COVID-19 Aerosols – Airborne Infection — 3 Comments

  1. A recent letter to Science from 6 scientists based across the U.S. is an important addition to the conversation about aerosols. It argues strongly that droplets as large as 100 micrometers should be also considered aerosols, because they can remain airborne for a long time. See Prather et al, The final paragraph of that short article is especially noteworthy:
    “Individuals with COVID-19, many of whom have no symptoms, release thousands of virus-laden aerosols and far fewer droplets when breathing and talking (4–6). Thus, one is far more likely to inhale aerosols than be sprayed by a droplet (7), and so the balance of attention must be shifted to protecting against airborne transmission. In addition to existing mandates of mask-wearing, social distancing, and hygiene efforts, we urge public health officials to add clear guidance about the importance of moving activities outdoors, improving indoor air using ventilation and filtration, and improving protection for high-risk workers (8).”

  2. Art,
    You have put forth an excellent analysis of the aerosol mode of Covid-19 spread. I didn’t fully appreciate the difference between an aerosol and a droplet and their respective ranges of infection.

    Thanks for the education and stay safe. Paul

    • Hi Paul, and thanks for your comment! I see that aerosols and airborne transmission are appearing in the news now as people ponder whether their kids will be safe at school. I didn’t want to write such a long blog, but I explained the restaurant case carefully because it’s the most convincing example of long-range infection. – Art