<\/p>\n
One of the greatest challenges we face during the Covid-19 pandemic is dealing with the abnormality of not being able to safely share indoor spaces with people who may, or may not, be infected. Indoor environments carry a particular risk if the SARS-CoV-2 virus that is responsible for Covid-19 is transmitted as aerosols through air which is trapped for a relatively long time in enclosed spaces.<\/p>\n
On September 10, Dr. Anthony Fauci, director of the NIH National Institute of Allergy and Infectious Diseases, reported in a Harvard Medical School Grand Rounds webcast on the status of Covid-19 transmission<\/a> that “The aerosol and particle physicists that have approached us now have told us that we really have gotten it wrong over many, many years. Bottom line is, this is much more aerosol than we thought.” He went on to explain that the medical community had been defining the maximum size of an aerosol \u2013 a particle small enough to float around in the air for a long time \u2013 to be about half that of actual aerosols, including those emitted by a person when they are talking (or just breathing).<\/p>\n For me, this revelation of aerosol classification was not news. What I did find surprising was that medical researchers had been misidentifying the size of aerosols when the physics and methodology for determining the size of aerosol particles and droplets has been in textbooks for decades. In fact, the scientist who essentially defined an aerosol, J.J. Stokes, published his work on the subject in 1851. Physicists are not the only ones who have known this. As a graduate student in mechanical engineering in 1997, I was studying droplet sprays similar to what comes from a cough and spent many hours characterizing how the different sized droplets moved around in a turbulent air stream as they evaporated. Many of the papers I have read on cough sprays were done through university engineering departments, such as this one showing mask effectiveness<\/a> which was published in June 2020 in the journal Physics of Fluids<\/em>.<\/p>\n The effort to figure out how to keep people safe from the Covid-19 virus in public is daunting and multidisciplinary. It requires action from a wide range of top experts in virology, epidemiology, chemistry, fluid mechanics, and other specialized areas. Programs within, or sponsored by, the CDC and NIH are supposed to bring together these knowledge sources to determine all the different mechanisms for transmitting a virus from one person to another through the air, contaminated surfaces, personal contact, or some other means.<\/p>\n Success in the war on Covid-19 is also dependent on the organizational directors, like Dr. Fauci, who oversee these concerted battles, as well as the journalists, politicians, and public safety authorities who provide critical information links between what the experts know and how that knowledge is used to protect the public. If too many of them make uneducated decisions or pass along misinformation resulting from bad assumptions or misinterpretation of research, the value of the work of the experts is greatly diminished. And if those in the most influential roles of government promote disinformation instead of responsibly guiding everyone through the uncertainties of the pandemic, expert analysis and advice has to compete with dangerously absurd speculation.<\/p>\n This is why it is crucial that the experts have a realistic grasp of the limitations of their own research and knowledge when responding to questions. There should be no expectation that any given doctor is also an epidemiologist with insights into how the price of chicken in Thailand may be an indicator of a virus outbreak in Southeast Asia. Likewise, asking an epidemiologist with no clinical experience to diagnose your sinus headache doesn\u2019t make sense. I\u2019m not sure if it is even possible for a single person to have true expertise in every discipline involved in fighting a pandemic, so it is imperative that journalists, government officials, and the public not expect Dr. Fauci or any other advisor to necessarily have all the specific answers at hand. Such spokespersons have a responsibility to refer specific questions to individual experts, including engineers and physicists who have been studying the fluid dynamics of sprays for many years. We often want one person that we trust to have all the answers, but that is an unrealistic expectation.<\/p>\n I want to emphasize these points about expertise before diving into a discussion of airborne SARS-CoV-2 transmission that falls into my own knowledge area. Since I have no formal medical training, my explanations of how virus particles travel through air and some potential remedies must be limited to my education and professional experience in spray behavior and fluid dynamics<\/a>. This examination begins at the point when droplets exit someone\u2019s mouth and stops where they land on a surface, get caught in a filter, or are exhausted to the outdoors. It is just one part of the puzzle, but a critical one as Dr. Fauci emphasized, that will allow us to figure out the safest way to get people back into public buildings.<\/p>\n <\/p>\n You have questions.<\/p>\n The trillion-dollar question of 2020 is how exactly does the SARS-CoV-2 virus jump from one person to another and what is the best way to stop that from happening? Right now, stopping the airborne virus literally means putting up physical barriers, including space, fabric and other sundry filters, plastic sheets, and walls. But with a better understanding of how the virus is transmitted, can we apply a dose of innovation and improve that list of barriers?<\/p>\n What is needed to make classrooms, offices, restaurants, and auditoriums safe for occupancy? For public buildings, guidance usually comes from the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) and local building and health codes. But are those standards and rules good enough?<\/p>\n Then there are the challenges of actually getting to another building from your home without being exposed to the virus. The number of people who rely on public transportation for at least some portion of their commute is not insignificant and should be taken seriously, especially when it comes to children depending on virus-free busses to get to school. Even if a bus is 100% decontaminated before everyone gets on, will the virus particles from an infected rider get vented out before floating past or landing near other passengers?<\/p>\n <\/p>\n We should start looking for answers by first asking one more question: what do we already know about virus transmission through the air and how do we know it?<\/p>\n Fortunately, we have the technical ability to look directly at the physical details of an infectious SARS-CoV-2 particle which is a fairly spherical assembly comprised of the virus RNA strands surrounded by a fatty lipid envelope that also incorporates a few different specialized proteins that help with the infection process. When the entire package is intact, it is infectious and called a \u201cvirion\u201d. When hunting down virus particles, researchers will mostly come across separated virus RNA or broken envelope proteins, but it is the intact virions that we are concerned about. This species of coronavirus is new and unique, but it is not entirely different from many other viruses, so we can lean on 90 years of virology and fluid mechanics research to get a pretty good picture of how an airborne virion moves from one person to another.<\/p>\n Rendering of a SARS-CoV-2 virus virion. It is approximately 0.1 micrometers in diameter. Let\u2019s take a look at the basics. There are several things that we know for sure:<\/p>\n <\/p>\n <\/p>\n I should note that the debate over whether Covid-19 is spread through the air or by contact with surfaces has a lot to do with the size of droplets that the SARS-CoV-2 virions are typically found in. If we had reason to believe that infectious virions were almost exclusively in large droplets, then yes, it would make sense that transmission through surface contamination would be of singular concern. But the research so far seems to confirm that nearly all the virions exit in droplets of all sizes larger than 5 micrometers.<\/p>\n There are also a few things that have been studied extensively but are difficult to characterize:<\/p>\n <\/p>\n Before getting into the current state of indoor air ventilation, let\u2019s look at what happens to your exhaled breath outdoors. First, it is helpful to get a feel for a couple of technical measures, such as wind speed, and the time it takes to wash virions away before reaching another person. Did you feel that breeze? Not quite or just barely? Then, according to the Beaufort Wind Scale that is used for weather events, that very light breeze is air moving at approximately 5 mph. At that speed, a clump of air moves past your face in less than half of a second. When you are talking normally to someone who is 6 feet away, the sound waves reach them almost immediately, but the puff of air from you loudly pronouncing the letter \u201cp\u201d takes a little less than 2 seconds to go that far. But wait, if the breeze flows across your face in 0.4 seconds, how does that puff of air ever reach the other person? It doesn\u2019t! In fact, even if the breeze is so slight that you cannot feel it, say 2 mph, the air is still moving fast enough to flush the virions away.<\/p>\n 2 mph is another good reference point as that is approximately how fast air needs to move to make a candle flame flicker. Here is a fun experiment that you can do to get a good feel for how far your breath travels when you talk, yell, or cough: In a very still room, light a candle \u2013 preferably a taper on a skinny candlestick \u2013 and position it on a table about 1 foot from your face. Start talking in a normal voice and you will see the flame jump a lot. Now move it 2 feet away and see if the flame shudders when you talk normally or yell. Then 3 feet away and see what happens when you yell or cough. Now move the candle 6 feet away and see if the flame flickers when you cough (note the delay from the time you cough to when you see the flame react). Have someone else do the experiment and see if the results are the same. When the candle flame just barely flickers, the aerosols (mainly droplet nuclei at that point) in your breath keep traveling a few inches farther than the candle before wandering off in other directions.<\/p>\n <\/p>\n Ah, but what if the outdoor breeze is blowing into or away from my face or into someone downwind? Good point. This is the one aspect of being outdoors that poses a risk. Fortunately, wind rarely flows so directly and actually swirls and mixes a lot more than we are aware of. This disperses aerosols, making them less likely to reach another person; but it also means that it\u2019s not a bad idea to wear a mask outside if you are going to be within just a few feet of other people in any direction.<\/p>\n Now let us compare being outdoors to what is experienced indoors. The most important thing to understand is that standard HVAC (heating, ventilation & air conditioning) systems for schools, offices, restaurants, stores, gyms, and auditoriums are not necessarily designed for removing virus-sized particles from the air. Building codes require commercial HVAC to bring inside a certain amount of outdoor air (ventilation), but those rules are only meant to keep CO2 concentrations below a prescribed level. For most HVAC systems, while conditioning a space with cold or warm air, the air blowing through the room is mostly re-circulated and the required outdoor air is mixed in such that it may make up only a small portion of the total HVAC air flow.<\/p>\n However, the air circulating through a building\u2019s HVAC system is supposed to be filtered. In fact, the air filtering requirements in most locations in the U.S. are effective enough to remove a little less than half of particles the size of a typical saliva droplet nucleus. Many locations now require or provide incentives to use filters that can trap about 75% of the nuclei when properly maintained. Filters that catch nearly all nuclei flowing through the system are commercially available, though rarely installed because of the higher maintenance and greater power consumption of the circulation fans.<\/p>\n The total filtered and conditioned circulating air flow through a building will vary depending on the local climate, how a given space is being used, and how the HVAC fans are controlled. Within a reasonable range, we can at least get an idea of how indoor circulation compares to being outdoors through the use of ASHRAE guidelines for HVAC design.<\/p>\n The minimum volumetric flow rate of outdoor air, or ventilation rate, required for office spaces by ASHRAE (the standard for nearly all building codes in the U.S.) is equal to about 80 times the breathing rate of an average adult sitting at their desk. So, CO2 levels are well regulated. But when it comes to meeting our challenge of removing SARS-CoV-2 aerosols from the air around each occupant, we have a problem: this required ventilation flow rate is only 0.2% of what is needed to completely flush the air between two people sitting 6 feet from each other before their virions can reach each other. In reality, the HVAC engineering best practice is to set minimum air circulation at 3 to 6 times the required ventilation rate, but that still only gets us to 0.6% to 1.2% of the flow rate needed to flush SARS-CoV-2 from between 2 occupants.<\/p>\n Why not just increase the building air flow? Moving more air means increasing the size or number of the fans and fans use energy. More or bigger fans and the additional electricity to run them can be expensive, so flow rates are generally kept as low as possible.<\/p>\n In a classroom, higher ventilation rates are required, typically resulting in flow rates about 5 times higher than for an office space, but still only around 5% of what we need flowing past each student, assuming they are all 6 feet away from each other. And then after the air flows past one student, what keeps it from continuing on past another student and another and then the teacher on its way to the exhaust vent?<\/p>\n The issue is that these engineering standards are for an entire room and largely ignore what happens at each spot where someone is sitting. Even if the room was designed with an average flow rate of our outdoor example, the local flow rate might not be nearly good enough. Depending on the shape of the room and where you put furniture, equipment, desks, people, etc., that \u201cfresh\u201d air might head for the exhaust vent along one wall while leaving stagnant pockets in an opposite corner. For the person sitting in that stagnant pocket all day, it could be a serious health issue.<\/p>\n Computer modeling of air flow is known as Computational Fluid Dynamics (CFD). Besides walls and furniture, there are also thermal effects that can strongly steer air flow because warm air rises and cold air drops. In fact, one of the most common drivers of air moving around a single person sitting still is that person\u2019s body heat causing the air next to their skin to rise and pull lower air up with it. In fact, one well-known ventilation strategy called \u201cdisplacement ventilation\u201d specifically takes advantage of the heat rising from people and computers to create a vertical flow throughout most of the space.<\/p>\n So, the best thing is to open some windows, right? Most residential building codes in the U.S. assume that all ventilation for a home will be achieved by opening a window, although some recent code revisions, such as in NYC, are finally requiring supplemental ventilation under certain conditions. Those same codes will require exhaust fans for bathrooms and possibly kitchens, but the air that goes up into an exhaust fan must come from somewhere and that somewhere is through open windows. The problem is that the outdoors right outside your home is too often too cold, too noisy, too dusty, too hot, too smelly, too stormy, too\u2026.something, to leave open. Counting on people to open windows to maintain a healthy environment in their home, office, or classroom is short-sighted at best and dismissive at worst.<\/p>\n Another problem with depending on windows for ventilation is that it assumes the wind will always blow and that a breeze will make its way to the other side of the room. I have done numerous studies of strategies for architects to effectively use open windows for ventilation and it only works when the building is specifically designed to take advantage of natural ventilation. Some great examples of how this has been done are buildings in warm to moderate climates designed and built before the advent of air conditioning. They typically provided a way for the wind to blow all the way through the wing of a building by way of transoms above the doors.<\/p>\n![\"Rendering](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/coronavirus_structure-1024x455.jpg\")
<\/em><\/p>\n
\nThe spikey proteins are what attaches to a cell in order to infect it.
\nNote: The Envelope ( shown in red) supports the spherical shape of the shell and is made of a fatty lipid structure that attaches to soap or detergent. Wash your hands!<\/em><\/p>\n\n
![\"Exhaled](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/VirusEvap-1024x512.jpg\")
\nThis illustration shows how exhaled droplets with SARS-CoV-2 virions evaporate as they travel.
\nThe largest droplets fall away within a second or two, but smaller droplets stay in the air longer. The droplet shown as being \u201ctracked\u201d starts out large enough to not be considered an aerosol, but it quickly evaporates to a size that is an aerosol and can stay in the air for a long time as it continues to evaporate down to a \u201cnucleus\u201d. These smaller aerosol droplets disperse and go with the air flow until hitting a surface like an air conditioning filter.<\/em><\/p>\n\n
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![\"Outdoor](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/VirusDispersion-Outdoors-300x200.jpg\")
<\/p>\n![\"Candle](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/Candle-1024x668.jpg\")
Please ensure that you have proper supervision whenever conducting an
\nexperiment where fire is involved \u2013 safety first!<\/em><\/p>\n![\"CFD](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/OfficeCFD.jpg\")
<\/em><\/p>\n
\nThis CFD model shows how uneven air flow can be in a typically air conditioned small enclosed office.<\/em><\/p>\n
![\"Example](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/OfficeNatVent.jpg\")
This CFD model demonstrates the effectiveness of \u201cblind\u201d natural ventilation where windows on the same side of a building open outward to catch the wind.![\"Refrigerated](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/RefrigCase-1024x585.jpg\")
Modern open refrigerated cases use air curtains to isolate the cold products from the warm ambient store air. The constant sheet of flowing air will block aerosols as well as warmer air. The same strategy can be used to create room barriers to SARS-CoV-2 contaminated air.<\/em><\/p>\n![\"Displacement](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/PositiveDisplacement-1024x476.jpg\")
This illustration of \u201cdisplacement ventilation\u201d shows how exhalation-contaminated air can be flushed away above peoples\u2019 heads before flowing past other occupants.<\/em><\/p>\n![\"Desk](\"https:\/\/warkenergy.com\/wp-content\/uploads\/2020\/09\/VirusDispersion-Curtain-1024x512.jpg\")
One possible solution for isolating and removing SARS-CoV-2 virions in meeting rooms is the installation of an air curtain directly into conference table or between desks.<\/em><\/p>\n