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Quickly and Easily Reduce Your Home’s COVID Risk
It’s easy, it’s not that expensive, and no mask required.
Note from Josh: Many of us are tired of COVID protocols, but the virus is still a major risk for some, especially the immunocompromised and the elderly. Memorial Day is coming up, which means parties and barbecues. How can you protect the most vulnerable without spoiling the fun?
What if I told you that there is a scientifically-backed method for reducing COVID transmission that won’t lead to fights about masking or testing?
Please welcome my friend Ari Allyn-Feuer to Unprepared. He has a Ph.D. in Bioinformatics from the University of Michigan and is an AI product manager at GlaxoSmithKline. In other words, he’s a legit scientist, and he’s here to tell us about a simple and inexpensive solution for a problem many homes now face.
Since the beginning of the COVID-19 pandemic, ventilation and air purification have been discussed as ways to help reduce the risk of COVID-19. Until recently, the scientific literature on the efficacy of ventilation for COVID-19 has been more theoretical than practical, and even then, good air purifiers were selling for inflated prices.
That’s recently changed, with cheap, high-quality purifiers flooding the market, and a recent study showing that good ventilation can reduce COVID-19 transmission by 80% or more. It’s surprisingly easy to replicate the conditions of the study, and know that you’re meaningfully protecting your family.
For the next section of this post, I’ll use the terms ventilation and purification interchangeably, but later we’ll discuss the differences between them and how to choose which path to take.
In short, here are two options for reducing COVID-19 spread by about 80% in your house:
Use air purifiers with a total smoke CADR of 800, running continuously on the highest setting, for every 1000 square feet.
Run one 20” or larger box fan, continuously on the highest setting, with the fan set to blow air outside of the house, for every 1000 square feet. Close the gaps around the fan tightly with tape or other barriers. Then, on the other end of the house, open a window or a door to bring in fresh air.
In terms of which equipment to buy, it’s not hard. Anything will do, as long as it is a legitimate HEPA filter or an air moving fan, and you buy enough of them for your space. Don’t focus on exotic features like ionizing filters, or bells and whistles like Wi-Fi and remote controls; they don’t make the unit push more air.
I went looking for the best throughput per dollar, and wound up with 20” Lasko box fans, and refurbished Winix air purifiers. Winix sells refurbished units with 232 to 310 CADR for only about $100 used, making it possible to cover a whole house for only a few hundred dollars. Medify does the same on their eBay account. If you can’t find used or refurbished units, you might have to buy new ones. I haven’t been able to find new air purifiers with the same CADR values per dollar as refurbished units, but you might. Among new, not refurbished purifiers, the Medify units on Amazon look cheap per unit of capacity, but are still much more expensive than the refurbished units.
Some other units that are reasonably priced among new units, and have CADR ratings listed on their product pages:
In any case, you’ll need to do some calculations to figure out how strong of an air purifier you’ll need and how many. Read on to calculate how to effectively ventilate or purify the air in your home.
Basic Facts about COVID-19 and Air Quality
Before we move on, here are some key facts to understand:
1) This is a big deal. A recent high-quality study, which we’ll discuss in more detail below, showed that schools in Italy with the most ventilation experienced a reduction in COVID-19 transmission of over 80% compared to schools with no special measures.
2) This is achievable by the average person. It’s likely you can attain the level of air throughput of the Italian schools in the study, without spending a lot of time or money.
3) Ventilation quantitatively reduces your risk of catching COVID. The number of people, whether they have COVID-19, and the viral load of infected people matters a lot too. If you are holding a risky event with great ventilation, the ventilation helps a lot, but it’s still a lot riskier than it would be if there were less inherent risk. And a very low-risk situation will be little helped by ventilation; if it happens that no one there has the virus, by definition it won’t help at all. Of course, the virus has a long incubation period and you can’t see it, so you can never be sure who has it until it’s too late.
4) Ventilation only helps where and when it happens. If your ventilation system is turned off, or you’re not in the rooms where it’s working, or the same people also congregate somewhere unventilated, then you’re not getting the benefit. And the vast majority of the time, people spread COVID-19 without knowing it. So to get the full benefit, you have to do it consistently when people are together.
5) Ventilation addresses the aerosol mechanism of COVID-19 spread, which appears to account for the vast majority of infections, but is not the only known mechanism. Infection by droplets, contaminated surfaces, and direct interpersonal contact will not be much helped by ventilation. However, most COVID cases are not thought to be spread by these vectors.
6) Ventilation is a sliding scale. It matters how much you do it and how you do it, not just whether or not you ventilate at all. A little bit of ventilation helps, but not as much as a lot of ventilation.
7) You can’t rely on intuition or air purifier advertising to figure out how much ventilation you need. The amount of ventilation is more than most people think, and more than most air purifier claims suggest.
In scientific terms:
Ventilation addresses the aerosol mechanism of spread of COVID-19, where an infected person emits aerosols, tiny liquid droplets with SARS-CoV-2 virions in them which stay suspended in the air. They are emitted continuously but have a half-life, which results in the air attaining an equilibrium concentration over time, and the risk of catching COVID-19 from the aerosol mechanism is approximately proportional to this concentration and the time spent breathing the aerosols.
Replacing the air with fresh air that doesn’t have SARS-CoV-2 aerosols (ventilation), or filtering them out (purification), effectively reduces the half-life of the aerosols and reduces their equilibrium concentration, as well as allowing levels to drop faster after an infected person leaves.
The math governing how much the risk will be reduced, in theory, is governed by two numbers: the half-life of infected aerosols, and the turnover rate of air, which can be expressed as a half-life, but is more often expressed with the number of effective air changes per hour (ACH).
Until recently, academic estimates of SARS-CoV-2 aerosol persistence differed dramatically, with one set of estimates converging around a half-life of an hour to two hours, and another on numbers as short as six minutes. But a recent high-quality study in schools, which used actual infections in humans as its outcome variable instead of aerosol concentration measurements, suggests values between these two extremes, likely between 12 minutes and 34 minutes.
In that study, schools in Italy with better ventilation systems were found to have much lower COVID-19 spread compared to unventilated classrooms. Infections were reduced by about 40% with 2.4 ACH, about 67% with 4 ACH, and about 82% with 6 ACH. These numbers, which correspond to half-life estimates of 12, 22, and 34 minutes, are the best estimates we have of how effective ventilation is.
How Many ACH to Aim for
You can assume that you’ll get about a 40%, 67%, or 82% reduction in COVID-19 risk from 2.4, 4, or 6 ACH, respectively. But these numbers have some nuance to them. The study authors assume…
…that your space will be as well mixed as an Italian classroom. If it’s not well mixed (e.g. if the flow of ventilation misses some areas, or some areas are further from the air purifiers than others), then some areas will get less benefit.
…the entire area in which air will mix has been purified the same amount. If you put an air purifier in one room and calculate the ACH value for just that room, air will circulate between that space and other spaces, reducing the effective ACH. You have to calculate based on the whole space in which air will mix (including by circulation through the HVAC system).
…that the purifiers are highly effective or…
…if a fan is used, that the air is fresh from outside. If the input air has problems (e.g. is partially composed of the exhaust from another space), or the filters aren’t close to 100% effective, then the effective ACH will be lower.
…that the density of people in the space is approximately equivalent to, or less than, the density of Italian classrooms, which, as near as I can estimate, is about the equivalent of one person per 50 square feet with an 8-foot ceiling. If you are putting 12 people into a 200-square-foot dining room, you might need more ACH to get the same benefit. In theory, the number of ACH needed to attain the same benefit should go approximately as the inverse of the density of people, so if you have one person per 25 square feet, you could use double the number of ACH and expect approximately the same benefit, but this is theoretical and will not be precise.
For a recent family gathering, which featured a higher density of people than an Italian classroom, my family chose to run with 45 ACH; 15 from purification and 30 from ventilation. The math suggests that this may have been extreme overkill, but we wanted to be careful.
The tradeoff between the cost and effort of ventilation, and the risk reduction, will be a personal tradeoff. You might choose a lower or higher number. But 6 ACH is a pretty good analytic starting point; it’s not hard to attain, and there is high quality, peer-reviewed evidence that it offers a large benefit.
But how do you figure out if you’re hitting your target ACH number?
How to Calculate ACH
Start by deciding on the number of ACH you want and measure the air volume of the space you’ll be purifying (including all the rooms where the air is likely to mix with the rooms people will occupy, including by circulating through the HVAC system), in cubic feet. You can multiply room dimensions by ceiling height to get volume and add up different spaces.
Let’s say you have a 20- by 30-foot party room with 10-foot ceilings and its own air system:
20 feet x 30 feet x 10 feet = 6,000 cubic feet of volume
If you multiply the volume of the space by the number of ACH you want, you’ll get the needed air throughput in cubic feet per hour (CFH).
6,000 cubic feet x 6 ACH = 36,000 CFH
Then divide by 60 to get cubic feet per minute (CFM).
36000 CFH / 60 = 600 CFM
That CFM number is what you want to look for in a fan or air purifier.
For air purifiers, this is often advertised or can be looked up, with a figure called Clean Air Delivery Rate, or CADR, which is measured in CFM and takes filtration efficacy into account. Sometimes there are separate numbers for dust, pollen, and smoke; if so, you want the one for smoke, which is most similar to viral aerosols. The Winix D480, for example, has a smoke CADR of 310 CFM. Bear in mind that these numbers are for the highest fan speeds, and purifiers should always be used on the highest setting.
For fans, this is also advertised in CFM. Most box fans advertise between 1,800 and 2,500 CFM. Bear in mind, however, that the advertised value assumes no pressure differential and unrestricted flow, and that in the ventilation use case, where the air has to find its way from the intake point to the exhaust, you will get a lower value, often about half as much. And to ensure that the air is circulating through the whole space, not just the area near where the fan is placed, you have to place the fans and intake points carefully and seal around the fans, as I discuss below.
Using the example above, if you needed 600 CFM, you could then conclude that two Winix D480s, or a single large box fan, would meet your needs.
Air Purifiers vs. Fans
Fans will be cheaper per unit of capacity, and they’ll never need a change of filter; where even good purifiers need approximately annual filter changes. But fans require you to stick them in a window (for exhaust) and open a door or window on the other end, where purifiers can simply be plugged in and turned on. Fans often add to the cost of heating and cooling, or even make the air too hot or cold inside, or too moist or dry, whereas purifiers will leave air temperature and humidity unaffected. There are a lot of tradeoffs, but they’re pretty well defined. And you can combine methods if you want; the ACH values sum up directly.
When installing your equipment, consider the following:
Air Purifiers: Spread them evenly around the room(s), and give them plenty of clearance near their intakes and exhausts.
Fans: Make sure to seal tightly around the fan in the door or window, make sure the intake point you intend is within a reasonable distance from the exhaust point you’re using, and that the intake point has a large cross-sectional area (e.g. larger than the fan you’re using), that there is a clear path between the intake and exhaust points, and that the thermostat and the rooms people will occupy are along this pathway. By doing this, you’ll avoid reducing throughput, causing a short circuit, or causing dead zones.
Don’t Guess Your ACH
Many people trying to do ventilation or purification wind up relying on intuition to determine how much to do, using their furnace fan or homemade purifiers, or relying on square footage claims in the marketing materials of air purifiers to decide how much purification to do. This is not a good idea.
It’s easy to feel intuitively like you’re ventilating a lot by opening some windows, or all the windows. The room will get cold or hot, you’ll smell the fresh air from outside, etc. But you won’t really know how many ACH you’re getting, and it may be a lot less than it feels like.
Other people rely on the filtration involved in their HVAC systems. This is also not a good idea. These systems are not designed for this purpose. Even good furnace filters, with a MERV rating of 13, the highest widely available, only have about 40% efficacy against aerosols, and a standard furnace fan in an ordinary-sized house will only give a few ACH, and only while the fan is running. It’s unwise to assume that the system you happen to have will meet your needs without doing the math.
Homemade purifiers based on box fans and furnace filters have become popular in some circles. But they’re laborious to make, don’t last very long, and need to be taped very carefully (including the portions of the intake duct not covered by the fan blades) to get them working right. They offer a lot less filtration than you might expect because the airflow is reduced by the filters and even MERV 13 furnace filters only have about 40% efficacy against aerosols. In the end, the math I did suggested that refurbished HEPA purifiers offered slightly higher effective ACH per dollar, with longer life and less noise, and without the hassle.
Air purifiers come with square footage claims, and they are usually correctly calculated and not fraudulent, but they are predicated on the assumption that you want a particular number of ACH, often much lower than you would actually want, and they assume a particular ceiling height. If you have 10-foot ceilings and want 6 ACH, a square footage number predicated on 8-foot ceilings and 2 ACH will be almost four times too high. And the numbers aren’t comparable between manufacturers. You will want to rely on throughput numbers (CADR, in CFM) instead.