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Part 2 - DIY Construction
Part 3 - Science & Engineering
Questions & Answers
No, not much. While HVAC systems do offer some prevention, it is far from ideal because an HVAC system will most often not intervene where the real mode of transmission is occurring. The closer people are to each other in an indoor space the lower the the intervention and mitigation effect of an HVAC system. That is because HVAC air vents are designed for broad dispersion rather than focused stream that intervenes in close-contact settings. In contrast, prevention based on the crosswind technique does work very well, and when added to an HVAC system, greatly enhances mitigation performance.
Yes, it can. As long as there is no direct interference from the A/C with the crosswind unit air stream, it will be just as effective. Nothing needs to be done to the window A/C. You may use it as you would in a room without A/C.
Yes, absolutely. An exhaust fan alone does not mitigate against worker-resident airborne infection. However, an exhaust fan will help remove some aerosols after they have been dispersed by this solution away from the close contact zone .
While a regular fan might help a little, the wide air pattern it emits [view here in Part 2] will not create a physical barrier between the worker and the resident unless the fan is close to the interaction. Such proximity would introduce operational discomfort for the worker, often getting in the way of maneuvers. Worse, a regular fan does not use an air filter therefore it does not reduce airborne contaminants like the crosswind unit by eventually absorbing most of them. Instead, a regular fan recirculates infectious microdroplets and aerosols back into the room keeping them suspended and available for infection. Although a regular fan may offer a small degree of initial defense if used properly, it is advisable not to use a regular fan to mitigate airborne pathogen transmission because it would give a false sense of security and therefore should not be relied on.
The W.H.O. stated the following (July 29, 2020):
- "Can fans be used safely inside during the COVID-19 pandemic? Air blowing from an infected person directly at another person in a closed space increases the risk of the virus spreading from one person to another."
This is relevant to the 'in-the-line-of-fire' scenario in the worst case condition when there is no air purification from the fan (see Question Q11 for an in-depth explanation). The crosswind concept stipulates that the air stream must be aimed transversely between two people. It is this important configuration that gives this solution such high efficacy, which is in apparent opposition to the W.H.O. statement above.
The W.H.O. also stated the following for this question:
- "If COVID-19 is circulating in your community and the use of a table or pedestal fan is unavoidable, it is important to regularly bring in air from outside by opening windows or doors, while minimizing how much air blows from one person (or group of people) to another person (or group of people)."
Definitely yes. Although the flu is a myxovirus with a seasonal reproductive number R of ~1.3, it is nonetheless an enveloped virus with spike fusion proteins similar to a coronavirus and is transmitted by similar mechanisms. Therefore this solution is very effective against the flu. To maintain maximum efficacy, all visitors to a resident room must sign a notice instructing them how to abide by the air barrier protocol [view here in Part 3].
Yes, definitely. All hospitals should consider installing this solution in a hands-free distributed configuration [view here in Part 3] in all patient rooms immediately. It is very clear that HEPA filtering and outside air in one local hospital alone did not prevent infection in October 2020. The fact that there were at least 7 infected in two different units at St Mary’s Hospital and an outbreak at the Montreal General Hospital supports the airborne infection paradigm:
https://montreal.ctvnews.ca/montreal-s-st-mary-s-hospital-reports-two-covid-19-outbreaks-in-two-units-1.5162703
https://montreal.ctvnews.ca/admissions-suspended-on-15th-floor-of-montreal-general-hospital-after-covid-19-outbreak-1.5174531
Because the crosswind air barrier is stand-alone and very effective at removing infectious aerosols, it would be the most beneficial to a building without forced-air circulation. In fact, it would be far more effective against airborne infection than the expense and waiting time needed convert your present system to HVAC with HEPA filtering. Even if you have the crosswind units preinstalled in a distributed configuration then later converting to forced-air heating and air-conditioning, adding those to your existing crosswind solution would only provide a marginal improvement against airborne infection, not justifying the time and expense. If you do convert, you should keep this solution installed as-is. An installation of distributed crosswind units would have almost surely prevented this large outbreak that resulted in one death in a residence:
https://ici.radio-canada.ca/nouvelle/1746866/cas-covid-19-matane-residence-batisseurs-eclosion-foyer
Infection spread in a building is a valid concern. The distributed crosswind solution will still be very effective and is highly recommend that it be installed. This solution is optimized for employees who wear surgical masks, therefore, the particles this solution is designed to mitigate against are in the E-1 and E-2 particle size ranges, with the majority being smaller than 5 µm. The key tenet to keep in mind is the main objective of this solution, namely, the immediate removal of concentrated aerosol clouds between the worker and the resident [view here in Part 3] during the moment of interaction. It is this function that provides the strongest first line of defense in infection mitigation. Since most E-1 and E-2 infectious particles stay suspended in the air, yes, an HVAC system would collect them from an infected room and distribute them throughout your building especially if your HVAC system does not have HEPA filters.
In a more complete answer to your question, infectious aerosol spread would happen regardless whether or not you have crosswind units installed throughout your building. As such, there would be no difference between an installed distributed crosswind solution and no solution, but only on the first air exchange by your HVAC. If you install a distributed crosswind solution building-wide, on subsequent building volume exchanges, every room that contains a crosswind unit would contribute to the reduction infectious aerosols. A distributed crosswind solution would begin to behave as a large high-MERV filter helping your central HVAC, and therefore be of benefit to all rooms.
That means all other rooms would participate in removing the infected particles from the initial cloud emitted inside one room, rather than subjecting every resident to continued suspension and buildup of infectious aerosols throughout your building. Furthermore, because crosswind units keep air in motion within each room and are placed in the vicinity of every resident, there is no chance of aerosols aggregating into hazardous stagnant clouds.
In short, your HVAC system would help to disperse the initial infectious cloud into its building volume diluting far more than in one room alone without HVAC, and then, your distributed crosswind units would provide the subsequent removal.
Surprisingly, this hand-in-hand solution would provide faster and lower the risk of infection even further in a building equipped with HVAC than distributed crosswind units installed in a building with baseboard heaters, where a potentially infected worker generally may typically visit several rooms over his shift.
Yes, for the following reasons. At the moment an infected worker comes into proximity of a resident, the air stream will deflect his infectious aerosols away from the resident during the interaction. It is at this moment and the few minutes that follows after the start of an interaction, that protection against infection is the highest, while in contrast without the solution, protection would be at its lowest.
That initial protection from the crosswind unit could even be better than 1 infection in 10000 interactions (compared to the relative baseline of 1 infection in 1 interaction without the solution). Because it takes 20 minutes to circulate the room’s volume though the crosswind unit once, during middle of that period the protection will drop from 1 in 10000+ to 1 in 1500 short-term improvement, the worst case in the time progression for protection. But that short-term worst case on its own is already a huge improvement over no solution.
Therefore, because the crosswind unit is continuously clearing out the infectious aerosols, after 10 minutes of initial contact, the "1 in 1500" statistic begins to improve. Let us say the interaction was 5 minutes and the worker left the room. So after 20 minutes, the probability of infection drops to 1 in 3000, and after 40 minutes, it drops further to 1 infection in 6000 interactions, and so on (always comparing to 1 guaranteed infection in 1 interaction without the solution). That means if the resident did not get infected in the first hour, the chances of her getting infected are lower than 1 in 6000 after one hour following the departure of an infected employee. If the infected employee does not return, after 7 hours the air purity in the room will reset back HEPA filter quality, and will maintain HEPA filter quality if no infected worker enters.
Because no one knows when an infected worker can enter a room only find out days later from screening tests that he was infected, the crosswind unit has to operate 24/7.
To understand the statistics better, consider the converse scenario, with no crosswind solution. Let’s say that the resident was lucky and she did not get infected by a masked but infectious employee in the first 20 minutes after his departure. Then his infectious aerosols would diffuse into the room volume and remain suspended in the air, available for infecting the resident at a probability of 1 in 1500, not only for the first 20 minutes, but also after many hours the probability would remain the same instead of continuously reducing [view here in Part 3]. If you compare the crosswind solution to a surgical mask, the mask by itself reduces aerosols by 90% emitted by the infected worker [view here in Part 3]. The crosswind solution further reduces infected aerosols emitted from his mask by up to 99.93% after the first 20 minutes of operation (see Question Q18 to find out where this statistic comes from).
The improvement is dramatic after only 20 minutes and continues to improve with time. The unit only consumes 30W of power, so it is very energy-frugal. Under normal conditions, that means any time a worker enters a room, he walks into an environment of the purest air possible. Please keep in mind that these statistics do not directly model the probability of infection. Instead, they model the improvement in the reduction of the probability of infection, if this solution is applied on top of current mitigation measures. For example, in October 2020, this region had 40 long term care centers infected with 320 cases due to the 2nd wave:
https://www.cbc.ca/news/canada/montreal/christian-dube-chslds-covid-19-1.5773049
If this solution had already been in place in September 2020 in all the centers in my region, there would have been no infections due to any kind of airborne transmission.
Placing a commercial HEPA air purifier in a classroom without analysis could be deceptive and could lead to a false sense of security. Schools are a very difficult problem to solve due to movement of active young people, a beehive of interaction especially in hallways.
For classrooms where students are seated, effectiveness depends on your choice of a HEPA purifier and its deployment, whether students are masked or unmasked, and whether students remain distanced at all times or not.
In a classroom, where all students are masked and distanced, a strong HEPA purifier would be effective and reduce transmission from an infected intruder to the others, so long as students do not crowd together. In addition, students should be advised to only wear surgical masks. For other scenarios such as where students are unmasked, greater consideration must be given to deployment engineering.
In general however, most HEPA purifiers have a wide projection pattern that is insufficient to be effective and could make matters worse instead of better, because without the wearing of masks there are 10 times more aerosols, and also, large microdroplets come into play when a student speaks creating a different set of dynamics. Therefore in this scenario, particle deflection is the key [view here in Part 2].
Simply placing a HEPA purifier in a classroom corner will not prevent transmission between students on the other side of the room of the installation, or even in the center of the room, even though the air can be considered purified. In this case, HEPA purifiers would be met with “Purifying the air in classrooms with HEPA purifiers doesn’t work”, not different from hospitals that are getting infected despite HEPA filtering in their HVAC systems (see references in Question Q5). Here, every student must feel a noticeable breeze at all times for mitigation to be effective, which is the first line of defense. In that respect, another consideration is that the physical properties HEPA purifiers that claim HEPA performance are in opposition: it is impossible to get HEPA performance and get enough airflow rate to deflect large microdroplets at a reasonable distance away from a commercial purifier.
Therefore, multiple HEPA purifiers placed strategically in a classroom where each student feels a strong breeze at all times would be effective but prohibitively expensive to purchase and maintain. For unmasked students, airflow direction would have to be downward, meaning an effective solution would require multiple overhead purifiers with focused air vents directed downwards. School hallways would benefit greatly with overhead strong HEPA purifiers, where spacing is optimized so that a strong downward breeze is felt all along the hallway. Consult Consumer Reports for a better understanding of commercial HEPA purifier performance.
However, if you want an expeditious solution for hallways, a series of downward-pointing ceiling-mounted crosswind units without air concentrators (baffles only [view here in Part 2]), would be very effective, and much lower cost to purchase and maintain. In this hallway case as an example of re-engineering, the wide airflow pattern would be preferred in order to minimize the number of units along the hallway because the distance between students' heads and air exit ports is not greater than 2 meters. See also related Question Q5.
You could, but it is not necessary. Electrostatic filters in the crosswind air barrier trap aerosolized particles permanently hence they do not present any danger to your residents. Any trapped fresh infectious particles will self-destruct exponentially to near-zero after 4 days without emitting infectious components, transforming themselves into non-infectious particles embedded in the filter. Only periodic filter replacement is required every 3 months.
This is a valid concern and must be considered in applying and configuring the crosswind air barrier into specific environments like a restaurant. You must be referring to a Korean study that found customers who were in line with direct airflow from the air conditioner did get infected. However, just as interestingly, others outside the flow but as close to the index case, did not. So this is evidence that airflow can indeed carry the infection for several meters, while on the other hand this study demonstrates support that the crosswind air barrier has good mitigation promise in restaurants.
Why? It is unlikely that the Korean air conditioner had an effective high-MERV air filter so it recycled infected air back to the customers behaving like a regular fan. Instead of decreasing the viral load with time, it increased the load, throwing the ever-increasing viral load at the healthy customers. In a permanent hands-free installation crosswind units in a restaurant, the units can indeed coexist with the A/C and be very beneficial (see also Question Q3). Multiple crosswind devices can be ceiling-mounted where their air streams are pointed downward towards the floor hitting your customers, while A/C units should have their exit ports pointed away from your customers.
Indeed bathroom operations are very susceptible to airborne infection. Given that the CDC has officially declared SARS-CoV-2 an airborne pathogen on May 8, 2021, any interaction between a masked employee and an unmasked resident is considered high-risk to the resident.
Outgassing of infectious contaminants from feces is not an issue with a crosswind unit running in a room. Nevertheless, the highest risk does occur in the bathroom, that is, spaces involving toileting and bathing, where the space is a small enclosed volume and where interaction time can be many minutes. Therefore a resident's bathroom must have airflow directed into the bathroom from a crosswind unit during an interaction for continuous flushing of potentially contaminated air from an unknowing infected worker wearing a mask.
If you have one portable unit in the room, you should temporarily position it so that its air stream points through the bathroom door covering the area of closest physical interaction with the resident, and set to the highest airflow [view here in Part 3].
If specialized bathing rooms are being used in a respiratory epidemic they also should employ the crosswind device pointed through the door if possible, set to highest airflow during use, and set back to lowest airflow when not in use.
Absolutely yes. Nursing stations are notorious hives for employee-employee aerosolized infection, such as during rounds or meetings where multiple employees gather in tight groups. Even though employees all wear masks, I demonstrated that 1 in 10 infectious aerosol particles leaks through a mask [view here in Part 3]. For example, with one employee who is infected (relatively speaking), if there are just two employees in the station the chances of the other getting infected is 1 in 100 such meetings. However, if there are five employees in the station, the chances that one will get infected is 1 in 20 such meetings. So a crosswind unit with a wider beam (without air concentrator but with a baffle) suspended from the ceiling running at 3 m/s during a meeting will be very effective in further reducing infection probability in these settings. If all employees in the meeting feel a crosswind breeze in their faces, it will work very well. See also bathroom infections in Question Q12.
The Purified Crosswind Air Barrier
Synopsis
The Centers for Disease Control declared SARS-CoV-2 an airborne pathogen. That means infectious aerosols smaller than 5 microns in diameter are now the dominant transmission mode in healthcare centers where employees wear masks while residents and patients do not. Surgical masks are effective at reducing aerosols from workers; however, approximately 10% of aerosolized infectious particles are not blocked during speech and normal breathing exposing unmasked residents and patients to infection in a close interaction with an infected worker. In this city during October and November 2020, three hospitals and sixty-one long term care centers were infected, some re-infected after a summer green period, in a resurgent wave of the epidemic. The solution presented here was specifically engineered to mitigate infection in the interaction zone where a masked worker attends to an unmasked resident or patient by providing an active physical barrier of purified air between both individuals. This solution deflects and then filters infectious aerosols, instantaneously reducing the existing infection probability by a predicted value of approximately 99.93% during a 10-minute interaction in a typical residence room by an infected worker or by an infected patient in a hospital room. A distributed version of this solution should be immediately considered even for modern hospitals despite HEPA air quality and fresh air in their HVAC systems. Variations of this concept can be applied to any indoor space where two or more individuals interact. If every indoor interaction zone obtains a solution based on this concept, there is a good chance the epidemic can be brought under sufficient control to have commerce reopened. It would also relieve distress in citizenry to know there is an external technological solution that does not entirely rely on their behavior. Moreover, in future respiratory epidemics, preinstalled, this solution can always be on standby and brought into immediate mitigation service without having to wait for vaccines. Commercial HEPA air purifiers can be installed in classrooms, however, caution should be exercised with guidance demonstrated here to ensure all persons who interact can sense sufficient aerosol-deflecting airflow on their faces at all times. Without feeling airflow, a commercial air purifier placed ad-hoc in a classroom would be much less effective.
Minimum Reporting Efficiency Value (for air filters). It is a standardized single rating value of an air filter, measured at a linear airflow speed of 500 fpm (approximately 8 m/s) that gives a relative metric of how well an air filter performs at removing differently sized particles from the air. Ratings typically range from MERV-6 (dust absorption) to MERV-20 (virus absorption). The actual MERV number of a filter describes a percent-of-absorption curve over a range of particle sizes [view here in Part 3].
High Efficiency Particulate Air (filter). This is a classification for air filters that have a MERV range of 17 to 20 and that are able to remove virus-sized particles in one pass. HEPA filters are used in modern hospital ventilation systems and can be found in some commercial air purifiers. Because HEPA filters are very dense, they require very strong air pressure to move a sufficient volume of air to be effective at deflecting airborne particles in close encounters that violate the 2-meter distancing rule. One cannot use a HEPA grade filter on the crosswind air barrier device because the fan motor is too weak to provide the required pressure so that air speed is 1 to 3 m/s at 2.5 meters away from the device. The effective efficiency of the crosswind device filter stack is approximately MERV-12.5. However, the crosswind device can still purge a room of infectious particles to HEPA quality, but not as quickly as a real HEPA filter with a very strong fan motor. See also Question Q17.
That's a great question. In my analyses I found that to mitigate infections in close encounters, it is far more important to have a strong airflow stream that is directed into the interactions between people than a HEPA filter.
A HEPA filter is made of high density fibers and would produce too much air resistance to be of any use when we are trying to project an air stream up to a distance of 3 meters with a low-cost low-power household box fan. Otherwise, the solution would need an expensive powerful fan motor that would also cause too much noise to achieve that kind of high airflow rate at a long distance. This solution has to be affordable in poor countries and also has to be energy efficient.
Instead, the crosswind unit uses a combination of three medium-MERV filters that together have an effective efficiency of approximately MERV-12.5 [view here in Part 3]. So the tradeoff is that it takes longer to scrub a room to HEPA quality in exchange for a strong reasonably clean air stream that instantly deflects the infectious particles [view the difference between here and here in Part 3]. Therefore a medium-MERV (see Question Q15) filter set is sufficient to achieve the desired objectives. See also Question Q16.
This is a common question. The 99.93% reduction seems unrealistic, but it is not. Translating from physical modeling into words and pictures, the statistic is based on an average room volume of 150 cubic meters with the crosswind unit running at 1 m/s, for a 5-minute close interaction of 0.5 meter between the worker and resident.
Some basic arithmetic will help in this explanation.
To simplify, it is a matter of comparing the most infectious condition against the least infectious condition during the interaction. The most infectious condition occurs around the first minute of interaction in the left room [view animation here in Part 3] where the diameter of the expanding cloud is approximately 0.6 m. Therefore the volume of that cloud is 0.1 m^3. So the ratio of the cloud volume to the room volume is 1/1500. That is where the '1500' number comes from in Question Q8, and creates the 99.93% statistic (0.9993 = 1 - 1/1500). Given that you can see in the right room how the cloud is completely dispersed by the crosswind, the initial infectious cloud density is transformed into a complete homogeneous dispersion into the room volume without ever first touching the resident in its original high-density form, the heart of this statistic. It is the equivalent of keeping worker and resident distanced at a great distance of far more than the 2-meter rule, while in reality being only a half meter from his resident. And this is even before the filter stack takes effect to scrub the dispersed particles (see Question Q8 for how the filters work).
Anytime a worker violates protocol of maintaining the air stream between him and his resident during a close interaction, that 99.93% is degraded. That is why good adherence to the protocol is important to maintain that improvement.
Now here is something even more interesting related to your question: the closer the worker is to the resident, the better is the statistic. It feels counter-intuitive. However, consider this: if the distance is 0.3 m instead of 0.5 m and the unit is running at 2 m/s or more making sure that the dispersion condition in the right room is maintained, then the statistic becomes 99.98% improvement! The reason is that at the closer distance without a crosswind unit, the probability of infection would be greater by about 2X. However, in both cases, the aerosol is completely cleared away from the resident.
Yes, it is very good idea to install the crosswind device. Although the best COVID-19 vaccines have an efficacy of around 95%, elderly are far more frail to risk being in that 5% who can contract this disease. If your building has 100 residents, 5 of those can become infected if there is an asymptomatic young employee who is spreading the virus even though he may be vaccinated himself. So adding the crosswind solution will reduce those random residents at 5% risk to near-zero, a welcome reduction for such a low-cost and easy-to-implement solution.
Keep in mind that there are other benefits, like seasonal flu reduction (see Question Q4) and very clean healthy air not just for residents, but for your employees as well, at very low maintenance cost.
Yes, certainly. This is one the best close-encounter settings where mitigation performance will be at maximum from the crosswind air barrier. That is because both the hair stylist and your customer are not moving relative to the air stream and the air barrier can be maintained in one direction. In this setting it is best to aim the air stream so it blows from the top down [view here in Part 1], and your customer can safely remove her mask as long as your stylist is not directly in front of her.
There were two seminal moments. The first came on January 30, 2020 when the W.H.O. called the Public Health Emergency of International Concern (PHEIC). A bit of research told me way back then that this one was much more serious than SARS-1. I immediately rushed out to get a box of 50 F2100 surgical masks. I was the first Caucasian to wear a mask in indoor public spaces in my district and I encouraged others to do the same. Most thought I was crazy. It was obvious why: there was absolutely no reaction from my government or the media to the PHEIC for the entire 6 weeks until the W.H.O. called the pandemic on March 11. That lack of acknowledgment made me distressed and by then, everyone now knows, it was too late to react effectively.
The second more prominent seminal moment came mid-July 2020, after I read the Los Alamos paper which pegged the R0 of SARS-CoV-2 at 5.7 [view here in Part 3], because until then the known value was lass than 3. Not only did this 5.7 put the pandemic on the verge of being airborne, it also inspired the development of the crosswind air barrier. By late October 2020 a crosswind air barrier prototype was already running at my mother's residence [view here in Part 1] saving her from a major outbreak in her building. It did not take long for more transmissible mutations to appear and we can recognize that the B.1.1.7 variant has an R0 of at least 8 and possibly as high as 10, putting COVID-19 in the same league as measles which has an R0 of 11, a disease well-known to be airborne. An Israeli study showed that a single dose of the pfizer vaccine is only 36% effective against B.1.1.7. This means that jurisdictions who have extended the delay between mRNA doses beyond label are at high risk of propagating this pathogen nonetheless, even via vaccinated people who may be transmitting it unknowingly making matters worse when those governments are deceiving themselves into expecting better.
Yes. In fact because of the very high transmissibility of the omicron variant, probably greater than measles, it can have a much greater effect on reducing COVID-19 spread in enclosed spaces than any other mitigation method, but should applied in conjunction with other methods. We strongly suggest implementing the crosswind solution or one based on the concepts shown in the videos. It will surely have a positive impact on reducing local case rates and reducing hospitalizations.
With deep appreciation to PMW Ltd for their artistic and technical contributions to website creation. ~Kristof~