Tag: coronavirus

How N95 Masks Work
You might imagine N95 masks as essentially a strainer intended to catch small particles, but as Minute Physics shows in this video, what these masks do is actually much more clever. A dense, strainer-like mask with tiny openings to block microscopic particles would be very tough to breathe through. Instead, N95 masks take advantage of one of the characteristics of tiny things: they’re very sticky. Thanks to van der Waals forces particles that touch a fiber will stick there.
By creating an array of fibers between the particle and a person’s mouth, N95 masks do an excellent job of catching both large particles and tiny ones. They have a harder time with medium-sized particles because airflow around the fibers helps these particles avoid them.
But, luckily, N95 masks have a solution for that problem, too. The fibers of the mask have an electric charge, which helps them attract particles of all sizes and capture them. Of course, as with all masks, they’ll work when worn as intended. (Video and image credit: Minute Physics)
July 20, 2020
How Well Do Masks Work?
Many mixed messages have been spread about the efficacy of masks in preventing transmission of COVID-19. Nevertheless, there is good evidence that they help, as discussed in this video from It’s Okay to Be Smart. Much of the video shows schlieren imaging of a (healthy) individual engaging in regular activities – like talking, breathing, and coughing — with and without a cloth mask.
Now, it’s important to note that what you see in these images is airflow, not the droplets that can carry the virus. However, research has shown that these airflows play a significant role in transporting droplets. It follows that disrupting those airflows can disrupt transmission of diseases passed via droplet. This is one of the key reasons to wear a mask.
Notice how far jets and plumes of air fly from a maskless person’s mouth and nose. We cannot even observe how far momentum carries that air because the area visualized in this schlieren set-up is smaller than the full distance the air moves! But wearing a mask breaks up that flow structure. It reduces the air’s momentum, and it forces any air that does escape to move in smaller, less efficient structures. Even without considering any filtering effects or the fact that masks catch large droplets coming out of the wearer’s mouth, it’s clear that mask-wearing keeps others nearby safer. (Video and image credit: It’s Okay to Be Smart; references)
July 9, 2020
How COVID-19 Affects the Lungs
One of the best known COVID-19 symptoms of this pandemic is difficulty breathing, and while you’ve likely heard a lot about ventilators used to help patients get oxygen, you may not know much about the processes that cause the breathing problems. This video from Deep Look provides a solid overview of the infection route and how lung damage occurs during infection. Perhaps unsurprisingly — this is FYFD, after all — fluid dynamics plays a major role in this process, both under normal conditions and when air sacs in the lungs get damaged by the body’s immune system responding to the virus. (Image and video credit: Deep Look)
May 7, 2020
COVID-19 and Outdoor Exercise
By now you’ve probably come across some blog posts and news articles about a new pre-print study looking at the aerodynamics of running and the potential exposure to exhaled droplets. And you may also have seen articles questioning the accuracy and validity of such simulations. I’ve had several readers submit questions about this, so I dug into both the research and the criticisms, and here are my thoughts:
Is this study scientifically valid?
I’ve seen a number of complaints that since this paper hasn’t been peer-reviewed, we shouldn’t trust anything about it. That seems like an unreasonable overreaction to me considering how many studies receive press attention prior to their actual peer-reviewed publication. This is not a random CFD simulation produced by someone who just downloaded a copy of ANSYS Fluent. This work comes from a well-established group of engineers specializing in sports aerodynamics, and long-time readers will no doubt recognize some of their previous publications. Over the past decade, Blocken and his colleagues have become well-known for detailed experimental and simulation work that indicates larger aerodynamic effects in slipstreams than what we generally recognize.
In this paper, they lay out previous (biological) studies related to SARS and droplet exhalation; they use those papers and several wind tunnel studies to validate computational models of droplet evaporation and runner aerodynamics; and then they use those inputs to simulate how a cloud of exhaled droplets from one runner affects someone running alongside, behind, or in a staggered position relative to the first runner.
In other words, their work includes all the components one would expect of a scientific study, and it makes scientifically justifiable assumptions with regard to its methods. (That’s not, mind you, to say that no one can disagree with some of those choices, but that’s true of plenty of peer-reviewed work as well.) All in all, yes, this is a scientifically valid study, even if it has not yet undergone formal peer-review*.
Can simulations actually tell us anything about virus transmission?
One complaint I’ve seen from both biologists and engineers is that simulations like these don’t actually capture the full physics and biology involved in virus transmission. While I agree with that general sentiment, I would point out two important facts:
1) Blocken et al. acknowledge that this is not a virology study and confine their scientific results to looking at what happens physically to droplets when two people are moving relative to one another. Whether those droplets can transmit disease or not is a question left to biological researchers.
2) Most medical and biological research also does not account for the physics of droplet transmission and transport. For the past century, this research has focused almost exclusively on droplet sizes, with the assumption that large droplets fall quickly and small droplets persist a little longer. To my knowledge, some of the only work done on the actual physics of the turbulent cloud produced by coughing or sneezing comes from Lydia Bourouiba’s lab at MIT. And, to me, one of the fundamental conclusions from her work is that droplets (especially small ones) can persist a lot longer and farther than previously assumed. Can those droplets facilitate transmission of COVID-19? The general consensus I’ve seen expressed by medical experts is no, but, to my knowledge, that is based on opinion and assumption, not on an actual scientific study.
The bottom line
In my opinion, there’s a big disconnect right now between the medical/biological community and the engineering community. To truly capture the physics and biology of COVID-19 transmission requires the expertise and cooperation of both. Right now both sides are making potentially dangerous assertions.
Honestly, based on what I know about aerodynamics, I am personally skeptical as to whether 6 ft of physical separation is truly enough; whether it is or not seems to depend on how transmissible the novel coronavirus is through small droplets, which, again, to my knowledge, is unestablished.
Should we leave more distance than 6ft between us when exercising outdoors? Absolutely. Aerodynamically, it makes perfect sense that following in someone’s slipstream would put you inside their droplet cloud, which needs time and space to disperse. Personally, I’ve sidestepped the question entirely by doing all my cycling indoors while quarantined.
tl;dr: There are a lot of open questions right now about COVID-19 transmission and what qualifies as safe distancing, but it’s smarter to err on the side of more distancing. Don’t hang close to others when running or cycling outdoors.
(Image and research credit: B. Blocken et al.; submitted by Corky W. and Wendy H.)
*I will add that, with my training, I have and do occasionally peer-review studies such as this one, and I read the full paper with the same sort of critical eye I would turn to a paper I was asked to review.
April 14, 2020
Fluid Dynamics and Disease Transmission
Right now people around the world are experiencing daily disruptions as a result of the recently declared coronavirus pandemic. There is a lot we don’t know yet about coronavirus, though researchers are working around the clock to report new information. Today’s video, though a couple years old, focuses on an area of medical knowledge that’s historically lacking but extremely relevant to our current situation: the mechanics behind disease transmission through sneezing or coughing.
Lydia Bourouiba is a leader in this area of research. Her studies have focused not on the size range of droplets produced but on the dynamics of the turbulent clouds that carry these droplets and what allows them to persist and spread. If you’ve wondered just why healthcare providers are recommending masks for sick people, keeping large distances between individuals, and frequent hand-washing, the image above hopefully helps explain why. Droplets carried in these turbulent clouds can travel several meters, and the buoyancy of the cloud’s gas components can help lift droplets toward ceiling ventilation. Right now, social distancing is one of our best tools against this disease transmission.
My goal in posting this is not to panic anyone. Rather, I hope you leave better informed as to why these precautions are needed. With coronavirus, our detailed knowledge of its characteristics — how long it remains viable in the air or on surfaces, how much is needed for an infection to take hold, etc. — is limited. But from research like Bourouiba’s, we know that coughing and sneezing are remarkably efficient ways to deliver respiratory pathogens, and that’s why caution is warranted. Stay safe, readers. (Video credit: TEDMED; image credit: Bourouiba Research Group, source; research credit: L. Bourouiba et al., see also S. Poulain and L. Bourouiba, pdf)
March 16, 2020