Dangerous air

The flow of SARS-CoV-2-laden aerosols through a Chinese restaurant/Physics of Fluids

News analysis

A year into the deadliest pandemic of the Digital Age, the U.S. government is finally being pressured to accept what has appeared obvious for a long time: SARS-CoV-2, the virus that causes COVID-19, is in the air.

Even the New York Times picked up on the issue of aerosols once notable experts publicly pressured the Centers for Disease Control and Prevention (CDC), although the nation’s best-known newspaper did seek to blunt the threat of deadly airborne pathogens with the suggestion that the immediate action should be “to limit airborne transmission of the virus in high-risk settings like meatpacking plants and prisons.”

As if your local supermarket and restaurant weren’t already high-risk settings. A peer-reviewed Harvard University study published at BMJ back in October reported one in five employees of a Massachusetts grocery studied in May showed signs of antibodies indicating they’d been exposed to SARS-CoV-2.

“There is limited research discussing non-health-care-worker essential workers in this pandemic, particularly retail employees and their exposure to customers,” the authors of that study wrote. “(But) the SARS-CoV-2 infection rate among these retail employees was significantly higher (15 to more than 20 times higher) than that of the local community around similar time period, which was 0.9 percent to 1.3 percent.”

The Harvard researchers concluded the most probable conclusion to be drawn from the data was that “a true work-related SARS-CoV-2 exposure.” They weren’t the first to make this observation.

Before that, the Dutch reported much the same thing, and before that Chinese researchers outlined a super-spreader event they believed was furthered by the heating, ventilation and air-conditioning (HVAC) system in a restaurant.

Fluid engineers at the University of Minnesota have now modeled how that HVAC system could have spread the virus in the restaurant.

“Our results have shown a remarkable direct linkage between regions of the high aerosol exposure index and the reported infection patterns in the restaurant, providing strong support to the airborne transmission occurring in this widely-reported incident,” they reported in the peer-reviewed journal Physics of Fluids earlier this month. “.oreover, using flow structure analysis based on the mean flow streamlines and the distribution of turbulent kinetic energy and reverse-time tracing of aerosol trajectories in the simulation, we are able to further pinpoint the influence of different environmental parameters (i.e., thermal, ventilation rate, and filtration efficiency) on the infection risks and various potential airborne transmission pathways that can lead to the infection. “

Those researchers warned against “local recirculation flows in low-ceiling/confined spaces that can substantially increase infection risks” and the risks of “returning aerosols from air conditioning/ventilation systems due to the limited filtration efficiency (which) can also cause aerosol exposure of individuals adjacent to or facing the ventilation outlets.”

They called for improved building ventilation. The value of ventilation in reducing infections caused by airborne viruses has been well-known since the Spanish Flu pandemic more than 100 years ago.


Why the CDC has refused to aggressively address the issues of aerosols and the need for ventilation is unclear. The agency’s conclusion that droplets were the primary cause of spread was tenuously based on the the relatively low attack rate of SARS-CoV-2.

“Diseases that are spread efficiently through airborne transmission tend to have high attack rates because they can quickly reach and infect many people in a short period of time,” the CDC Scientific Brief on Airborne Transmission noted in October.

“Thus, were SARS-CoV-2 spread primarily through airborne transmission like measles, experts would expect to have observed considerably more rapid global spread of infection in early 2020 and higher percentages of prior infection measured by serosurveys. Available data indicate that SARS-CoV-2 has spread more like most other common respiratory viruses, primarily through respiratory droplet transmission within a short range (e.g., less than six feet). There is no evidence of efficient spread (i.e., routine, rapid spread) to people far away or who enter a space hours after an infectious person was there.”

The report came just after peer-reviewed study in the Journal of Fluid Mechanics reported that “the data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors. In this paper we discuss the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved.”

The study noted serious risks and raised questions about the effectiveness of masks, noting that thermal imaging found exhaled air flowing through the gap between the nose and mask, “an issue common with many mask designs, with a low-momentum plume forming that hugs the forehead before merging with the body plume….It is reasonable to expect that the exhaled air will end up in the same layer as the majority of the heat from the surface of the body to form the main ventilated layer at the top of the room.”

The researchers noted that while masks block droplets, they don’t change the volume of air people exhale. Where any SARS-CoV-2 viruses go when they hit a mask remains unclear. They may stick to the mask; they may be aerosolized and go into the air; how many viruses a mask can hold before it is overloaded and viruses are forced out, is unknown.

The effectiveness of masks remains a subject of debate, but they clearly do not offer full protection for the users or others despite the seeming belief of some that masks work as well as seatbelts and airbags.

The University of Washington’s Institute of Health Metrics and Evaluation (IHME), which has been modeling and adjusting COVID-19 death rates since shortly after the pandemic began, now estimates a “worse” case scenario of 3.78 million deaths around the world by June 1.

“Universal masks” are projected to lower the number of deaths to 3.44 million, a 10 percent improvement. The efficacy of masks, however, has a strong link to their proper use, and this is a big variable.

A peer-reviewed study published in Physics of Fluids in December concluded that a dirty mask could be worse than no mask at all.

“It is natural to think that wearing a mask, no matter new or old, should always be better than nothing,” Jinxiang Xi, the studies lead author told the American Association for the Advancement of Science (AAAS). “Our results show that this belief is only true for particles larger than 5 micrometers, but not for fine particles smaller than 2.5 micrometers.”

Those smaller particles are the aerosols that can accumulate in poorly ventilated spaces.


Correlations are no proof of causation,  but infections in the U.S. northern tier states appear linked to time spent indoors. Infection rates dropped when people were spending more time outdoors in summer and began going up as they moved indoors.

In Michigan, for example, the average daily number of cases peaked at about 1,800 per day at the start of April and fell to below 300 by June in response to government closures of bars and restaurants, according to the Worldometers COVID-19 tracker. Those facilities reopened to limited capacity and outdoor dining on June 8,  but the infection rate did not reach 1,800 again until mid-October.

For most of the summer, the average infection rate remained below 900 per day.  Other northern teir states – even New Jersey, the state hardest hit by COVID-19 – witnessed similar summer lulls in infections.

In Sweden, where most people were running around unmasked despite global expressions of horror, the summer also reflected this change. Daily new cases there continued to climb through mid-June, but then began falling and remained below 500 per day from early July to early October, according to the Worldometers counter.

By mid-December with Swedes back indoors, the number peaked at nearly 8,500 per day – 17 times the summer rate.

U.S. Southern states, it should be noted, did not follow this trend. Infection rates in most of them spiked as temperatures climbed to uncomfortable levels.

“As COVID-19 cases rise rapidly throughout the South, some scientists believe there could be an important, but overlooked factor in the spread of the virus in the region – air conditioning, WebMD repoted at that time.

On July 6, 239 authorities on respiratory infectious diseases published a letter in the journal of Clinical Infectious Diseases calling on “the relevant national and international bodies to recognize the potential for airborne spread of coronavirus disease 2019. There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several meters, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.

“Studies by the signatories and other scientists 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 at distances beyond 1 to 2 meters from an infected individual. For example, at typical indoor air velocities , a 5-μm droplet will travel tens of meters, much greater than the scale of a typical room, while settling from a height of 1.5 meters to the floor.”

Despite that plea and the steadily climbing rate of infection as Americans moved increasinlgy indoors at the end of summer, the CDC was in October sticking to the view that aerosal infections “appear uncommon and have typically involved the presence of an infectious person producing respiratory droplets for an extended time (greater than 30 minutes to multiple hours) in an enclosed space. Enough virus was present in the space to cause infections in people who were more than 6 feet away or who passed through that space soon after the infectious person had left.”

The CDC didn’t mention the need for improved ventilaiton until earlier this month and then offered only a list of “tools in the mitigation toolbox” that it left “up to the building owner/operator (obtaining expert consultation as needed) to identify which tools are appropriate for each building throughout the year.”

Or not.

In November, Dr. Justin Morgenstern, a Canadian emergency physician who writes at the website First10EM, labeled the U.S. CDC views on aerosols as badly misguided.

“The logic here is clearly faulty,” he wrote. “The argument being used has the basic format: ‘X is a Y. Z is not like X. Therefore Z cannot be a Y.’ This is somewhat like saying ‘a horse is a mammal, therefore that dog cannot be a mammal because they don’t look the same.'”

He went on to note that the flu, which looks nothing like measles, is known to spread via aerosols, as is “the prototypical airborne pathogen” – tuberculosis (TB). TB “has an Ro between one and three, exactly like COVID-19,” he wrote. The Ro is the rate of transmission.

Morgenstern did offer some advice that might help those who really feel they must be out and about – the pandemic has been hell for extroverts – with SARS-CoV-2 in the air. They could, he said, likely gauge restaurant and bar ventilation with the use of a carbon-dioxide (CO) monitor.

“The ambient concentration of carbon dioxide is about 410 parts per million,” he wrote. “A concentration above approximately 800 is an indication that you are rebreathing exhaled air and therefore other people’s aerosols. It is not a perfect surrogate. Filtration could effectively remove all aerosols, but leave the CO2 level unchanged. However, it is a reasonable concept that could potentially increase the safety of indoor dining dramatically.”

And you might want to wash that mask, too.









In early April, I wrote a long post covering all the science I could find about aerosols and droplets. The basic summary was that this is an area of medicine with lots of misconceptions, poor assumptions, and incomplete science. There was good evidence that previous coronaviruses were spread by aerosols. There was good evidence that influenza is spread by aerosols. Overall, it seemed very likely that SARS-CoV-2 or COVID-19 was being spread by aerosols, but the science was pretty weak. There is still a lot we don’t know, but as I update the evidence 6 months later, it is pretty clear that aerosols play an important, and unfortunately still widely ignored, role in the transmission of COVID-19.

Canadian emergency phusiciain. november.


8 replies »

  1. Eliminate airborne pathogens, and offensive. Paint a permanent air purification system, onto the walls of your home or office for less than 7 cents per sq. Ft. No electricity, filters or servicing required.

  2. Serious question here. If the vaccine protects you against Covid, why would the vaccinated need to fear the unvaccinated?

    • Consider the facts. COVID cases have dropped by more than 70% in recent weeks, more than the “experts” predicted. Fatalities, which lag cases, are also plummeting. Vaccine delivery is ramping up, and already some 15% of Americans have received a shot.

      Johns Hopkins’ Dr. Mark Makary, writing recently in the Wall Street Journal, concludes that the welcome collapse in virus cases, which began in January, cannot be attributed solely to the end of holiday festivities or the start-up of vaccines. He says it also suggests that the medical community is vastly underestimating the number of people in the country who have contracted the virus, are asymptomatic, and who have developed immunities.

      Makary thinks, assessing the data, that “The country is racing toward an extremely low level of infection.”

  3. Very interesting Craig. You do a great service to help people understand the more factual aspect of situations. I feel confident sharing your articles roundly due to your fact based research.

  4. I am constantly amazed by the amount of research Craig performs into these and other topics. Very impressive! It seems as if Craig must constantly read scientific journals on every subject imaginable.

  5. The corona family of viruses love cold temps. Maybe they thrive in ceiling corners –in the vortex of contrasted warm and cold air movements created when doors are opened and shut in cold weather. The virus that causes viral meningitis behaves this way.

  6. Hmmm…

    “Flu season came and went without ever actually existing this year. Doctors and mainstream media are putting forth every possible excuse for why it went from 400,000 hospitalized cases and 22,000 deaths last season to an infinitesimal 165 hospitalized cases this season, but very few of them are stating the most obvious reason.

    They claim that since many are on lockdown, few children are in school, most are wearing masks, and all of the other things associated with China Virus protocols, it effectively eliminated the flu this season. Sadly, many are buying this excuse. The reality is this: There was likely a sharp drop in cases, possibly as few as 100,000. But to say it was essentially eliminated down to an average of just over three cases per state all season long is ludicrous.

    For that to be the case, we would have also seen a tremendous drop in Covid-19 cases. The flu is nearly as contagious as the China Virus, so hitting the flu so effectively would have meant sharp drops in Covid cases. Instead, we saw spikes.

    What nobody in mainstream media and a handful of doctors are willing to acknowledge is the plain and straightforward answer: The flu didn’t disappear. It was merely misdiagnosed as Covid-19, sometimes intentionally and sometimes accidentally. Covid tests have been demonstrated to be inaccurate and the similarity to the flu and the common cold are enough to shift results. And if tests show Covid AND flu, then pretty much every doctor in America is going to label it as a Covid case.”

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