Patent Application
20220221170
Not Applicable
This invention is related to the class of systems and devices to suppress transmission where a disease is transmitted primarily over air. Related areas include air handling systems, air purifiers, and air conditioning systems. Other related areas are laminar flow workbenches, laminar flow hoods, and laminar flow clean rooms.
The Covid-19 crisis has demonstrated the need for a rapidly deployable system that can effectively suppress transmission of airborne disease without requiring a complete lockdown of the population and shutdown of the economy. The classic reaction to detection of a highly lethal infectious disease is to quarantine a person when we realize they are infected because of the symptoms he or she displays. Once infection is detected, “contact tracing” is instituted to determine who the person has met, and what they did before they got the disease. The people they met before quarantine was started are also quarantined. This “contact tracing” and follow up data obtained during quarantine and treatment also provides information to determine the values of the various parameters, including its lethality, and basic transmission mechanism. Unfortunately some diseases (like Covid-19) are communicable even without showing symptoms (or the symptoms are very close to some other less lethal disease), and we only detect those cases when enough people become sick enough to require hospitalization, or show clear symptoms, or test positive for the disease (once tests are available). Contact tracing then fails because the links between cases are broken, or it is simply overwhelmed by the wide prevalence of the disease.
Epidemiology tells us that the important number to control is R. When it is greater than 1 an epidemic grows exponentially, when it is less than 1 the disease dies out. Unfortunately, at the beginning of an epidemic there is a serious lack of knowledge about the parameters of a disease, leaving few options for a crisis manager.
To get a better handle on this, we can break R down into components:—[T], the time for which a person is infectious; [f], how often people encounter or “meet” other people; [v], the probability that the person an infectious person meets is vulnerable (i.e. not immune); and [p], the probability that the infection is actually transmitted during an encounter with a vulnerable person. [T] and [v] are characteristics of the disease only the medical profession can do something about. [v] is also dependent of where we are in the total history of an epidemic, which is not generally controllable. [p] has components that are characteristic of the disease, but also has a component related to the density of the pathogen required to cause serious illness. A plot of [p] versus the density tends to look like a threshold function, so we don't try to split it out and instead have a goal of keeping the density below the threshold, recognizing that at least in the early stages we don't actually know what that threshold is.
[f] includes unintentional encounters that are not meaningful to people, such as being near a stranger in a subway car. Intentional meetings are vital to the prosperity of businesses, and meaningful meetings are essential to the financial and mental well being of people. We therefore distinguish between the frequency of unintentional encounters [fu] and the frequency of intentional or meaningful ones [fm]. Then, we can write:—
R∝[T]·[f]·[v]·[p]
[f]∝[fm]+[fu]
The ∝ is there to remind us that this is just a statement of composition, and the [ ] are there to remind us that the variables may have many components. Given the above breakdown, and assuming scalar values and multiplication is appropriate, the following is roughly true:—
[T][f] would be the number of people a contagious person meets
[T][f][v] would be the number of vulnerable people a contagious person meets
We can now try some a-priori ballpark estimates. For CoVid-19, [T] is of the order of 10 days. If not using public transportation or crowded venues, [f] is in the low 10s for an average person on a normal day, making [T][f] be in the low hundreds. Going to a single super-spreader party would add another few hundred to [T][f]. Because of the need to feed the quarantined person, and to do the medical diagnosis that establishes the need for quarantine, [T][f] cannot be less than 2. [v] would be close to 1, at least until perceptible herd immunity has been developed. [T][f][v] would therefore not exceed the mid hundreds. This means if we can keep [p] below 0.1%, R would not exceed 1. We don't really have control over [p], but we do have some control over the density of the pathogen a person encounters in a meeting with another person. The relationship between [p] and the density tends to look like a threshold function so we have to pick a value for the threshold. Initially this will be just a guess, and so we can just pick 0, i.e. contact with even one virus particle results in transmission. This then transforms the problem into one where we have to ensure that the probability of encountering a virus particle is less than 0.1%. This will result in an over-engineered solution, but is usable until our growing medical knowledge lets us use a larger threshold. [p] also depends on the contact time involved in a meeting.
In the case of Covid-19 many health authorities resorted to “lockdown” or “stay-at-home” orders, i.e. they targeted [f] instead of [fu]. Unfortunately, given that humans are social animals, [fm] is the primary driver of the happiness of people, and also the health of the economy. Reducing [f] for the whole population thus results in a general malaise of the popular mood, and an economic crash. In fact, the poorer segments of the population with low reserves are very dependent on constant interaction to acquire the resources they need to survive, and reducing [f] generally could actually result in more deaths than the disease would have caused. Given all that, a strategy to reduce [f] for the whole population will meet with popular resistance and active non-cooperation, defeating the reduction of [f], as happened over 2020 Thanksgiving and Christmas in the US. It is also not recommended practice in the history of epidemiology. Because intentional meetings are vital, people will actively resist efforts that control [fm]. Since crisis managers will not know what meetings are of value, (e.g. people do get emotional sustenance just by being with other people, even if they are strangers!) crisis managers will want to avoid policies that reduce R by reducing [fm].
Reducing [p] normally requires changes in individual personal behavior over which health authorities do not have sufficient control or even visibility. Beyond personal hygiene, and use of simple masks to control ballistic transmission, most people do not have the understanding, resources, or discipline to control [p]. They especially do not have control over the environments they are forced into by their daily activity.
But businesses do. Unlike individuals, businesses are much more susceptible to targeted control by health authorities. They are motivated by their licensing and the economic impact of reducing [f] to find and enforce ways of reducing both [fu] and [p] within their businesses. By co-opting businesses, health authorities can feasibly attempt to reduce [p]. For contact based transmission, this can be done by “social distancing” and “cleanliness” rules (NPI in CDC parlance). If we can convince people to meet at businesses instead of uncontrolled environments, we can make sure the meetings are conducted “safely”, i.e. with low enough [p]. Of course, aggregating many unrelated meetings at a single site may increase the “unintentional” [f]. If we can prevent the unintentional [f] from increasing, then R can be reduced substantially.
The most difficult transmission mechanism to suppress today is airborne transmission. The most common methods are complete isolation in severe situations. In other cases use of adequate ventilation is recommended, and/or wearing masks. It has long been known that fresh air and sunshine are beneficial in limiting disease transmission. During the Spanish flu, patients were moved outside during the day for a better prognosis. In the 1940s, a technique called “upper room UV” was used to sterilize air in classrooms to suppress the spread of measles. This relied on high ceilings to provide an adequate volume (and dwell time) in which air could be sterilized. Many hospitals currently install UV lamps in their ventilation ducts for infection control. More recently CBS 60 Minutes on Oct. 31, 2021 had an article on the MASS non-profit architecture for hospitals in Rwanda, where the architecture creates naturally induced cross flow ventilation.
“Adequate ventilation” relies on proper mixing to dilute the concentration of pathogens to an acceptable value. It is interesting to examine the engineering required to maintain adequately low levels of CO2. CO2 (and other contaminant) levels are maintained sufficiently low by adequate ventilation. While recommended values vary a lot for different circumstances, the ASHRAE 2001 standard specifies a minimum of 15 cu ft/min/person of fresh air in the living space of a home, 20 cuft/min/person in high energy places like gyms & discos, and 5 cuft/min/person in office type locations. “Good mixing” is required to ensure that there will be no unlucky person who accidentally ventures into a spot with a high level of CO2. It also simplifies the calculation by using single numbers for the whole room, instead of needing to segment the space into small pieces.
A calm 2001b person breathes about 7-8 liters/minute, so if all the exhalations are vented and no rebreathing takes place, i.e. there is no mixing, each person needs only about ¼ cu ft/min of fresh air. Note that this is much less than the ASHRAE standard. This scenario would also deliver significantly cleaner air to each person. Unfortunately, it is extremely difficult to direct all the fresh air flow thru people's lungs, so the total fresh air requirement is higher, as much of it will bypass people. Current air conditioning systems rely on “good” mixing of air within a space to ensure even temperatures, O2 and CO2 concentrations, and humidity within that space. However, with an airborne pathogen, this “good” mixing has the effect of spreading it over the entire space, and the unintentional [f] is increased to include all the people within that space. Fresh air introduced in standard ventilation dilutes the concentration of the pathogen (thus lowering [p] some), but does not eliminate it. Cross draft fresh air is better, as it sweeps away pathogens.
As an illustration, remembering that one cannot actually do this calculation since the input numbers are unknown, consider two people A & B working together in an office 8 hours a day, where A gets sick. Given the above (extreme) assumptions, how contaminated can the air be so that the probability B gets sick from A is less than 0.1%? Over the ten days they meet for 80 hours since they share the air in the office. B breathes 80×60×8 liters of air during a “meeting”. For the probability that A gets sick due to B to be 0.1% the amount of virus from A in the air after filtering/dilution should be 1 particle in 1000×80×60×8 liters of air, i.e. 1 in 3,840,000 liters.
We can reduce the total airflow requirement toward the minimum required by each person if we can locate a port that supplies clean air next to each person. The person breathes the clean flow emanating from the port, and what they breathe out is absorbed by the environment. The kinetic energy of the air emanating from the port generates eddies and turbulence when it encounters random obstacles; which spread the contaminants over the environment.
Alternatively, we can locate a port that removes exhaled air near each person's nose. Eddies that can spread contaminants are compressed, and are sucked into the exhaust port. This results in an environment that is generally clean, allowing occasional misalignment of the port, so is preferable.
Neither is convenient to implement. One way to approximate the latter scenario is to build the floor with tiles that allow air out of the space when a person steps on them. We can also identify spots were small groups of people meet and place an exhaust vent in the floor near those spots.
What we do by the methods of this patent is to structure the airflow within a space so every person breathes “clean” air from a pool above their heads, and what they breathe out is moved to the “dirty” space below their heads. The flow past their heads is fast enough that normal exhalations cannot reach the head of the person next to them. In effect, the methods of this patent reduce [p] and [fu] for airborne transmission within businesses to near zero by avoiding mixing. This is critical. Downward flow is preferable, since exhalations are a mix of large particles affected by gravity, and small particles entrained by the airflow. A downward flow ensures that gravity and the flow push particles in the same direction. A combination of gravity, physical barriers, and sufficiently fast flow can be used to block ballistic transmission. Note that since people are warmer than their environment, their exhalations will tend to float up; so there is a minimum downward flow required to suppress upward flow. Experiments with smoke generators show this to be about 3″/second.
The cost of establishing the total required downward airflow is likely to be too high for such systems to be active during normal times with normal packing, so it would be useful if they can be rapidly deployed during a crisis. This is especially true if health regulations require some level of preparedness during normal times. Interestingly, in a room rated to hold crowds of people 1 ft apart (such as a packed disco), the fresh air required by the ASHRAE standard (20 cu ft/min/person) distributed evenly near the ceiling would result in 4″/sec net downward flow, with each person using ¼cu ft/min of the 20 allocated to them, even without recirculation. A 3″/sec downward flow would require 15 cuft/min/sq ft of floor area. This is 250 times the minimum “area outdoor air rate” of 0.06 cuft/min required by ASHRAE standards.
This results in air conditioning costs that are too high, since the total air flow required is much higher than that required by normal fresh air replacement. All the heating or cooling and humidity control applied to the intake air is lost through the exhaust ports. If the removed air is cleaned by an appropriate cleaning system it can be re-circulated back to the clean pool, and the business can reduce the air conditioning cost. These cleaning/recirculating systems can be made portable, and be installed inside the business only when needed. In addition, cleaning stale air before venting it outside also prevents the business from “polluting the neighborhood”, and preserves our assumption that “outside” air is “clean”. This is especially important in high population density areas, like cities.
Health authorities could stockpile the portable recirculating units and necessary filters. In a crisis, health authorities would specify how much “downward air flow” is required for a business to continue operating, as well as how effective the filters have to be. Businesses that have installed such systems could even be allowed to advertise themselves as “a safer place to meet”. People will then voluntarily choose to meet at such businesses instead of in less controlled environments (such as at home!) To keep businesses honest about their implementation they should NOT be exempt from liability if a patron gets sick.
Health authorities should also make sure home environments are sufficiently isolated from each other, so unintentional boosts to [f] are avoided. It is not very helpful if two adjacent apartments share air, as can be detected by the ability to smell a neighbor's cooking, or worse, their bathroom emissions.
In the method contemplated by this invention each patron (4) breathes in “clean” air that comes down from the pool (1). The potentially contaminated air they exhale is entrained by the flow, taking it below their heads and thus preventing re-breathing of the possible contaminants by other patrons. The diffuser acts to evenly distribute the air from the clean pool over the patron space, and to prevent any eddies that would cause one patron to breathe unfiltered air exhaled by another patron. It can be as simple as a cloth screen between the pool and patron space. Even adequate distance between the ports that bring air into the clean pool and the patron space can act as the diffuser. Note that diffusers in standard ventilation practice are usually designed to enhance mixing—the opposite of what we want to do here! Even without any recirculation and using the ASHRAE “people outdoor air rate”, each patron in this airflow will be breathing air that will be cleaner than that provided by a “well mixed” room.
Downward flow is best for most situations. Gravity pulls down large particles, and the airflow pulls down small particles. However for a situation like people on a choir stand you want the flow to be perpendicular to the average plane of heads, and/or have a physical barrier to prevent the throw of droplets by people in back rows over the heads of the people in front of them. Note that with the recommended airflow, face gear that directs exhalations downward instead of over the front row is a sufficient physical barrier.
A downward speed of 3 inches per second should be sufficient to prevent exhaled air from a calm seated person reaching the mouth or nose of another person 2 feet (or more) away. This is close enough for people to reach out and touch each other. The ambient cross/downward flow is extremely important. Without an ambient flow there is mathematically no limit to the range of an exhalation. This is especially true given the possibility of ring vortices being generated by a cough. It has been demonstrated that in still air a cough or sneeze could spread contaminants up to 22 ft away. This range is reduced if there is a cross flow that overwhelms and sweeps away the eddies caused by a cough—essentially the range where the energy of the cough results in eddy velocities less than that of the cross flow. Where a person can be facing another, ballistic distribution of larger drops is a problem; so a physical barrier can be installed between them that at least drops below neck level, and an exhaust port can be located close to the bottom edge of the barrier. With the recommended downward flow, face gear on a person that directs exhalations downward would be quite effective as this physical barrier. Generally it does not need to cover the nose when there is an ambient downward flow, as emanations from the nose are also directed downward, and tend to fill the space emptied by the reduced chest and abdominal volume generating the exhalation. Another advantage of face gear covering the mouth is that it will reduce the frequency at which a person touches their own mouth, reducing transmission via contact. A fast moving sheet of down flowing air can also act as a physical barrier. This will allow people to see each other, and to even hold hands under the barrier. If the patrons in the establishment are expected to be more active, the down flow speed should be increased.
The filter needs to be able to eliminate particles large enough to show a laser beam, like smoke particles. Ideally, it would also clear the target pathogens from the air stream flowing through it, making the sterilization section unnecessary. The CDC says the SARS-Cov-2 virus mostly transmits via respiratory droplets, which are about 5 μl, but the virus itself is estimated to be 60-140 nm. A regular high grade filter available at retail should be good enough to suppress droplet based transmission. However, if the health authorities decide live airborne virus needs to be removed, we would need a ULPA grade filter, turning the patron space (3) almost into a laminar flow clean room. Or we can use a sterilization section to kill pathogen that gets past the filter.
There is no need to rebuild the air handling system of an establishment to achieve these goals. The cleaning system can be packaged with ductwork leading from ground level to ceiling level as a portable recirculation unit. For example, one can package a HEPA filter, a fan, and a duct leading up to ceiling level into a single floor standing portable device that cleans and recycles air. A weight near the bottom of the unit can be used to stabilize the device. Or one can hang such a combination from the ceiling, and that reaches down to a table, around which people meet. Or one can place units that contain the filter and a fan on the floor along a wall, and then install a false wall making the space between the false wall and the real wall the recirculation duct. This is particularly useful in a commuter train or bus. UV lights in the recirculation duct could make it a sterilization section too. Or one can place the units containing the filter and fan in a line in the middle of the establishment space, with two false walls on either side of the line, and the space between the false walls becomes the recirculation duct. These portable systems can be designed to carry advertising or decorative features on the recirculation duct, making them fit into the decor of the establishment. The only thing that will usually be necessary to retrofit in an existing air handling system is to install/replace a diffuser to defeat the mixing within the patron space that is designed into most air conditioning systems. If the existing air conditioning recirculation intake is near the ceiling, a simple floor standing (or hanging) duct can bring air up from floor level to the intake. One needs to ensure that there are enough recirculation units to generate an adequate downward flow everywhere in the patron space. For example, if the patron space measured 900 square feet (30′×30′), and the required downward flow velocity was 3″/second, then the total upward flow in the portable units must be at least 225 cubic ft/second. If the upward flow in the units was 9 ft/second, the total floor area of the units would be 25 sq ft. The ability to reposition the recirculation units, and/or to adjust the airflow in them, gives the business a measure of control over the laminar airflow within the patron space.
This system also suggests an improvement to home air purifiers that boosts the speed at which they can clean air in a room. Home air purifiers are generally floor or table top units that suck in room air, clean it, and eject the clean air back into the room at approximately the same position, thus mixing it with the remaining dirty air. They have to move multiple times the room volume through the device to achieve a certain level of purification through progressive dilution, and that is never complete. However, if we attach a duct so that the clean air is delivered at the diagonally opposite corner of the room from the intake (and achieve zero mixing and even flow), then only one room volume needs to be moved through the purifier to perfectly clean the air in the room.
One consideration when using UV light to kill pathogens is the total hold time of air illuminated with UV light. That can be traded against UV intensity. Hold time is correlated with hold volume. If one has high ceilings, this is easy to achieve, as in the “upper room UV” concept used in the 1940s. One can also use half of the business space as pathogen kill space, or use a neighboring closed business as the kill space. In the case of underground commuter rail, the tunnel between stations can be the kill space. Another thing to consider using UV is the mutagenicity of the pathogen; if the sterilization is imperfect, the UV radiation is likely to speed up the mutation rate. UV radiation can be carcinogenic, so one must avoid exposing patrons.
A big reason for not using laminar air flow designs is the uncertainty of whether a specific implementation is sufficiently good, or actually achieves the desired laminarity. This is typically determined using a computer simulation of the airflow in the space—something that is beyond the typical capability of the average small business. Also, the simulation may not include small details in high KE parts of the airflow, and so may be fatally incorrect. [This is less of an issue if the high KE parts are near the exhaust ports.] So this patent also presents a simple method to test whether an implementation is good enough. The method is as follows:—Cover a wall with black, non-reflective sheets. Install smoke generators (like a burning incense stick) at approximately the positions where patron's heads would be. Run the system for a few minutes, and then turn on the smoke generators. Observe with the smoke trails between you and the black sheeting. There should not be smoke visible anywhere except below the smoke generators, and trails leading down from them to a nearby exhaust port. After running this setup for a while, test the clean pool for contamination, and the space between smoke generators for diffuse smoke. Note that since many incense sticks are usually designed to be “smokeless” once their output has diffused, they are not appropriate smoke generators for diffuse smoke tests.
Note that this test does NOT confirm that the air is pathogen free, as those particles may be too small to be visible. It only confirms the airflow structure is proper. This simple method can be used by health inspectors and the business to determine whether they have properly implemented the airflow. They will still need to inspect the output from the filters and sterilization system. This method can also be used by the establishment to tune the airflow in and placement of the various recirculation units so the proper airflow is achieved in the patron space.
Restaurants using this system should be aware that plating will be very important, since flavors will be undetectable until food is actually placed in the patron's mouth. Similarly, bars should realize an individual's pheromones will undetectable until there is actual contact.
This patent application claims the benefit of the filing date of a previously filed provisional U.S. application 63/199,601
| Number | Date | Country | |
|---|---|---|---|
| 63199601 | Jan 2021 | US |
