Breath `plumes' while in a queue
#18,828
Regardless of what pathogen sparks the next pandemic, the public's first line of defense - at least during the opening months - will be NPIs; Nonpharmaceutical Interventions.
We'll undoubtedly be told to avoid crowds, wash our hands, stay home if we're sick, and to cover our coughs. Many shops and queues will institute a 6-foot separation policy, and - assuming they are available - face masks may once again come back into fashion.
And while all of these are useful common-sense suggestions - and will be heavily touted by public health officials desperate to provide `something' to the public - exactly how much protection each individual action might afford is debatable.
- Washing (or disinfecting) hands is always good practice, even outside of a pandemic scenario. But most pandemics are caused by respiratory viruses, limiting the impact of handwashing (see Efficacy Of Hand Hygiene Alone Against Influenza Infection).
- Covering coughs (sneezing into elbows, tissue, or hand etc.) may redirect the spread of viruses, but may do little actually reduce it (see Study: Simulating the Environmental Spread of SARS-CoV-2 via Cough and the Effect of Personal Mitigations).
- And while staying home when sick is excellent advice, many respiratory viruses can be spread asymptomatically or by pre-symptomatic individuals (see EID Journal: Asymptomatic or Presymptomatic Transmission Of SARS-CoV-2).
The good news is that while individually the evidence is weak supporting their ability to reduce the risks of infection during a respiratory pandemic - when combined or layered - their impact is significantly magnified.
Dr. Ian McKay's famous `Swiss Cheese' analogy (below) illustrates the power of a layered defense.
The CDC's definition of a `close contact' to an infected individual read:
Someone who was within 6 feet of an infected person for at least 15 minutes starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to specimen collection) until the time the patient is isolated.
Many people assumed that this meant `6 feet was a safe distance', but there was scant scientific evidence behind it. By mid-2021 the CDC's updated scientific brief allowed that `Transmission of SARS-CoV-2 from inhalation of virus in the air farther than six feet from an infectious source can occur.'
While the transmission dynamics of the next pandemic virus could be far different from what we saw with COVID, now - during our interpandemic period - is the time to improve our understanding of the effectiveness of NPIs (individually and in concert).
To that end we have a fascinating, but at times quite technical, research article published in Science Advances that attempts to better understand how airborne virus plumes spread; particularly in a moving queue.
What they found was the dynamics of aerosol plumes are far more complex in a moving queue than during a static encounter. Warm exhaled air normally rises (buoyancy), but forward movement can cause downwash currents.
Researchers report that intermediate ambient temperatures (22-30°C) and the walking/moving speed of the queue can combine to increase the risk of transmission, causing infectious aerosols to linger at head height with minimal dilution, writing:
The findings presented here highlight the stark contrast to the transmission mechanisms in static social interactions, where flows generated by the kinematics of individuals are absent.
I've reproduced the abstract, and some excerpts from a press release from the University of Massachusetts Amherst. Follow the links to read them in their entirety.
I'll have a bit more after you return.
Fluid dynamical pathways of airborne transmission while waiting in a line
Ruixi Lou, Milo Van Mooy, Gabriel A. Tarditti https://orcid.org/0009-0003-6692-5372, Rodolfo Ostilla Monico https://orcid.org/0000-0001-7049-2432, and Varghese Mathai
Science Advances 6 Aug 2025 Vol 11, Issue 32
DOI: 10.1126/sciadv.adw0985
Abstract
Waiting in a line (or a queue) is an unavoidable social interaction that occurs frequently in public spaces. Despite its wide prevalence and rich parametric variability, few studies have addressed the risks of airborne infection while waiting in a line.
Here, we use a combination of laboratory experiments and direct numerical simulations to assess the flow patterns in a simplified waiting line setting. From observations of the transport of breath-like expulsions, we reveal the presence of fluid dynamical counter-currents —due to the competing effects of line kinematics and thermal gradients.
Depending on the walking speed, an intermediate temperature range can potentially heighten the infection risks by allowing the breath plume to linger; however, colder and warmer ambients both suppress the spread.
Current guidelines of increasing physical separation appear to have a limited impact on reducing aerosol transmission. This work highlights the need for updated transmission mitigation guidelines in settings where physical separation, interaction duration, and periodicity of movements are factors.
Waiting in line: Why six feet of social distancing may not be enough
Study, led by undergraduate physics majors at UMass Amherst and researchers at University of Cadiz, sharpens our understanding of how airborne-communicable diseases travel
University of Massachusetts Amherst
Credit: Lou et al., 10.1126/sciadv.adw0985
August 6, 2025
AMHERST, Mass.(Excerpt)
The results, published recently in Science Advances, grew out of a question that many of us may have asked ourselves when standing in marked locations six-feet apart while waiting for a vaccine, to pay for groceries or to get a cup of coffee: what’s the science behind six-feet of separation? If you are a physicist, you might even have asked yourself, “what is happening physically to the aerosol plumes we’re all breathing out while waiting in a line, and is the six-foot guideline the best way to design a queue?”
(SNIP)
“What we found was really surprising,” says Van Mooy.
Since warm air rises, there is a slight updraft surrounding our bodies—and so the team expected to see the aerosol plumes rising. But instead, they observed a “downwash” effect, where the simple act of walking and waiting in a line caused the plumes to sink. Even more surprising was that, if the ambient temperature is close to our body temperature, as would be the case in a non-air-conditioned room in summer, those aerosols could be pushed toward the floor due to air currents.
However, in a climate-controlled room, the difference in temperature between what we exhale and the ambient conditions are enough to drive those plumes aloft. If the temperature is in an intermediate range, it is quite possible that the aerosols can hover at just the right height for the next person in the line to inhale them as the line moves forward.
“Ultimately, there are no hard-and-fast rules about social distancing that will keep us safe or unsafe,” says senior author Varghese Mathai, assistant professor of physics at UMass Amherst. “The fluid dynamics of air are marvelously complex and general intuition often misleads, even for something as simple as standing in a line. We need to take space and time into account as we come up with our public health guidelines.”
(Continue . . . )
While I expect that keeping a 2-meter distance from others in line is safer than maintaining only a 1-meter gap, it is not a difference I'd be willing to bet my life on during the next pandemic.
While I expect that keeping a 2-meter distance from others in line is safer than maintaining only a 1-meter gap, it is not a difference I'd be willing to bet my life on during the next pandemic.
A `layered' NPI approach will provide the most protection, but for that you'll need to be well prepared before a threat becomes obvious.
For some tips what you and your family can do now to prepare for the next global health crisis, you may wish to revisit A Personal Pre-Pandemic Plan.
