Update: This post is receiving quite a few downvotes (no offense taken!). It is crucial for this intervention to understand what the reservations/criticisms are - there is likely important truth to them. Could you please let me know in the comment field below? As there are no comments I could see that it is sensitive, so I have made some comments you can simply agree-vote on - hopefully one of them reflects the negative reactions people have.

Epistemic status on the threat from mirror bio (I feel more confident about the solution): I’ve worked on this new threat scenario from a defense angle for a couple of years, but my microbiology knowledge is limited. More importantly, mirror bio is categorically different from previous pandemic scenarios. Unlike “wildfire” and “stealth” pandemics, which to some extent have been studied for years or even decades, little has been done to understand the details or implications of mirror bio. As a result, parts of this article may be inaccurate or entirely wrong. However, the proposed solution has been subject to extensive expert criticism and probably does not rely on every assertion in this post.

Please sign up here for ~bi-monthly updates on the shelter/refuges work described in this post.

Summary and main take-aways

• One particular threat shelters can defend against has recently become much clearer:
  • The motivating risk is the intentional or accidental release of catastrophically damaging mirror microbes (“mirror bio”).
  • We think that if we can design a shelter for an extreme scenario, it will also likely be useful in a range of other serious scenarios where the pathogen concentration is far lower.
  • We’re most concerned about non-state actors (like terrorists), but also researchers (who might have lab leaks).
  • The importance of preemptive isolation has decreased, but might not have gone away completely: a subset of threat scenarios would let people with “situational awareness” take protective measures sufficiently early to protect themselves.
• Thus, “civilizational refuges” have slowly evolved into “mirror bio shelters”.
• A positively pressurized plastic “bubble”, supplied by serial high-grade air filters seems robust enough and affordable.
  • This is inexpensive off-the-shelf technology.
  • At scale the material cost could be below $10k/person per year of protection.
• Work going forward:
  • Set up a company that designs, assembles and sells these shelters
  • Motivate policy makers and/or researchers to invest at least some resources into taking such pressurized shelters forward.
  • Test demand: approach the market to see if there are buyers for these shelters and if a “shelter delivery org” can be set up.
• Timing and deployment:
  • A mirror bio threat would probably unfold slowly.
  • Due to ease of manufacturing the shelters, construction and deployment could wait until after the threat crystallizes.
  • This is a much smaller ask of governments, since they would only need to prepare for production, not do actual production.
• What is needed:
  • If any readers know people in government emergency departments, research organizations (including military research institutions), or other institutions who might be interested in discussing these shelters, we would appreciate an introduction. With the low cost, we think it is worth constructing a significant number now to start building knowledge about how to make, test and use them.

pictured: a cut-through view of an inflatable shelter with a small room, large room, and airlock

My background

I'm an EA who has been working on shelters for over 2 years.  I’m considering manufacturing them commercially, and have recently started a company.  I wanted to share my theory of change in case I'm making important mistakes. I'm also interested in talking to people who might want to help, and especially people, whether in the public or private sector, who would be interested in buying one, or know someone that might.

Details on threat picture

The key threat we’re concerned about is mirror biology.

Others have done a much better job than I could do of summarizing the findings and explaining the danger of mirror biology, and the team behind the report have also created a short summary article. However, these summaries do not go into much detail about possible defenses, and the technical report doesn’t discuss the possibility of shelters. To be clear, shelters are in no way a desired outcome: they would be a desperate, last-minute effort to save what we could as the biosphere turned hostile.

What’s needed for shelter design is to understand environmental concentrations of the threat. Unfortunately, as the report explains, we’re still quite uncertain how a mirror biology catastrophe would play out. This means we will have a hard time guessing about what environmental concentrations we have to defend against. With uncertainty, it becomes necessary to create high levels of protection. The strategy we employ below is to use upper levels of currently observed “normal” microbial concentrations and then add a safety factor on top of that to represent the risk that, without natural ‘predators’, mirror bacteria might temporarily become overwhelmingly common.

The highest atmospheric concentrations we’ve identified are in connection with dust storms. This makes intuitive sense: soil is some of the most microbially dense widespread material we know of and high winds will bring large amounts of soil and dust into the air. We discuss concentrations in units of Colony Forming Units per cubic meter, CFU/m3^[1].The highest CFU counts recorded during dust events are around 10^7 CFUs/m3.

This is not as conservative an estimate as we would prefer, again due to the uncertainty about exactly how a mirror biology catastrophe would unfold. Mirror organisms would inevitably interact with the complex environment in a multitude of hard-to-predict ways, and if significant vegetation dies this could potentially lead to much higher erosion rates and more frequent dust storms in areas that have not previously experienced significant dust storms. Still, we think 10^7 CFU/m3 is still a generous upper bound when considered over the multiple years the shelter would be in operation.

How would we defend against a concentration of 10^7 CFU/m3?  Air filters are the obvious choice, but how much filtration would we need?  The requirement for removal is to not let a single particle into the lungs or digestive tracts of the inhabitants, because we want to conservatively assume that if this happens the microbe will reproduce, killing the initial host and any other shelter inhabitants.

In microbiology and related fields, due to the extreme numbers of microorganisms as well as their exponential growth, one uses logarithms to talk about sterilization. Reduction by 90% of a microorganism is a 1 log reduction, 99% is 2 log and so on: the logs can be thought of as “counting” the numbers of 9s in the percentage efficiency number. Now, no number of log reductions will give us certainty that no CFU makes its way to the inside; we can only talk in probabilistic terms. Therefore, let us start with the requirement that we want a 1% chance or less that a single CFU is inhaled by an inhabitant.

Consider a shelter designed to protect four people for one year.  Each person needs at most approximately 20m3/day (see e.g. table 6-5 here) of fresh air, but assume 40m3/day to be conservative.  This would require an air intake of around 160m3/day, or approaching 10^5 m3 over a year.  At our target average atmospheric concentration of 10^7 CFU/m3 our filtration system will be faced with 10^12 CFUs.  To have just a 1% chance of passing a CFU through we would need it to pass fewer than one in 10^14 CFUs, a 14-log reduction.  This is a staggering reduction, but as we discuss below we think this is possible with sequential filtering.

Water concentrations are similarly hard to estimate, and in current shelter work we have accounted for consistent, extreme levels. Note that for water, concentration numbers can be much, much higher than for air (at some point the definition of “water” is cast in doubt - it could be mostly microbes mixed with a bit of water!). For example, in water just downstream of large amounts of feces or decomposing carcasses we would expect to see something in the range of up towards, and perhaps sometimes above 10^8 CFU/ml. The latter scenario could be a common occurrence in a worst-case mirror biology catastrophe. With heat sterilization, we think it is reasonable to assume one can sterilize to 10 logs, probably even quite a bit more. But this would be insufficient for extremely polluted water over longer time periods. Therefore, we would recommend sourcing water from an old aquifer - these can take more than 100 years to receive significant intrusion from the surface and on a per liter basis, especially over the long-term, such clean water supply is extremely cost effective compared to other methods of delivering safe water^[2]. 

Even if we built a shelter that could keep out this level of environmental hazard, we think this is unlikely to be a scenario where humanity can simply stay put and wait for the problem to go away. We see shelters meeting these requirements as only one component of a larger response, allowing more people to survive to a time when, through efforts elsewhere, it’s possible to live outside these shelters again. 

One question that is probably high on people’s minds and that is also very relevant to shelter work: Is it likely that we will as a global society develop dangerous mirror biology science? To this I can only say I really hope that we can keep a lid on this, but I want us to be prepared in case that’s not how it goes.

Details on the shelters

The current shelter design is fundamentally uncomplicated: A positively pressurized plastic “bubble” supplied by serially filtered air. That extreme levels of protection can be achieved with simple and relatively affordable protection makes this solution attractive.

pictured: a view of a complete but empty inflatable shelter with a small room, large room, and airlock

These shelters are a direct descendant of a lot of different strands of previous shelter work. They build on the civilian nuclear shelters in Northern Europe, continuity of government bunkers in the US and Russia, Collective Protection Units used in the military, and concepts of civilizational shelters or refuges discussed on this forum by various people since 2014. However, around 2021 there was an increase in action around this idea in EA and EA-adjacent circles. It is unclear to me exactly what drove this increased interest: it could have been the seeming availability of FTX funding, the gradually rising prospect of a mirror biology catastrophe, or something else. This post describes work that directly built on that increased activity, encouraged by ASB’s suggestion that shelters be pursued, using previous work as input and announced in my previous post declaring the commencement of my work on the topic.

The shelters were conceptualized as an answer to the following question: what would be the absolutely cheapest way to construct a space that had 14-log protection in terms of atmospheric aerosols? When the question is phrased this way a solution presented itself: serial air filters supplying a positively pressurized plastic bubble tent, inside a larger existing structure for protection from the elements. 

While the concept of a positively pressurized shelter isn’t new, we’re not aware of earlier work that uses serial filters.  Moreover, this concept of a shelter is extremely minimal, which has two additional benefits:

• Very cost effective
• Possibility of rapid scaling of production

Serial filtration has been shown to achieve extreme levels of performance^[3]. During the Cold War there were plants that generated plutonium dust and needed to vent dust-containing air to the environment. Due to concerns about radioactive pollution, air was passed through a series of HEPA (protection factor of 2000 which is 99.95% efficient) filters and the efficacy of this treatment was finally tested at the Los Alamos lab that demonstrated an average of 12-log performance and a worst-case performance of 10-log. We are therefore fairly certain that this performance can be extended to 14-log and perhaps even higher.

For the positive pressure, no similar empirical experiments at the required level of performance have been found. But talking to an engineering professor in cleanroom technology who has investigated contaminant transport into cleanrooms, they thought it impossible for even a single particle to enter a positively pressurized space through the space envelope. Moreover, calculations were performed on diffusion speeds and likelihoods based on established physics and these similarly showed that practically speaking, the chance of a particle entering “against the flow” through a 0.5mm wide and 2mm long hole was, for all intents and purposes zero^[4].

An important factor here is wind. Simple calculations with Bernoulli’s equation show that one can quickly get pressures of more than 100 Pa with wind gusts that appear with some frequency in most locations. If the pressure generated by wind exceeds the pressure differential from the inside to the outside, there is a significant risk that outside aerosols might be pushed inside. This is why these shelters are envisioned being deployed inside a larger protective structure. Due to the inflatable plastic structure, there are few requirements on such spaces and they can be anything from garages and large living rooms to farm buildings and warehouses. 

pictured: A fully equipped shelter along with two inhabitants deployed as intended inside a larger structure (in order to protect from particle intrusion by wind gusts)

While the main concept of these mirror bio shelters is a smaller positively pressurized space supplied by serially filtered air, there are more components needed for long-term survival. 

• Waste is ejected via a specially designed waste system that similarly to BSL 4 labs do not let potentially contaminated air go “the wrong way” (e.g. bubbles or biofilm going from the dirty to the clean side of the shelter).
• Entry and exit is perhaps the most vulnerable part of the intervention. The only empirically tested, high-log decontamination found was for germ-free laboratory animals (“gnotobiotics”). Here, Vaporized Hydrogen Peroxide (VHP) is used to decontaminate, and animals are transferred between cages by VHP sterilized and air purged tunnels. One can imagine people being transferred in a similar fashion especially between shelters and vehicles in order to facilitate a functioning society.

Above, the following items have been covered:

• Air supply
• Protective structure
• Water supply
• Waste handling
• Decontamination/airlock

In addition, the following items are likely required:

• Protective gear for habitation transfer and outside missions
• Power
• Food
• Less critical but important items like bedding, exercise equipment, etc.

On protective gear, the highest protection factor gear found has been >50,000 protection factor which is 4-5 log of protection. Note that this is far short of the required 14 log for the protection. Some of this gap can be bridged by limiting the amount of time spent outside (if needing to survive for only 1 hour, the required log reduction would be “only” 7 log). Also, if combining a suit with protective tunnels to transfer personnel between habitation and transportation, it might be that the tunnel + suit will offer sufficient protection. Moreover, these suits will be supplied by stored, compressed air so the tunnels could be filled with VHP, further increasing the log reduction.

For power, it is hoped that the government will protect the utility workers so that power will be available via the grid. But in case one would like to prepare for the eventuality that this fails, or even to have protection against interruptions, an off-grid system might be good. The most cost effective set-up will depend on geography. In areas with sufficient sunshine during winter, solar and batteries will provide the main bulk of power while a propane generator will provide power during any prolonged periods of cloud cover. Note that the most costly components of an off-grid system (solar and batteries) can be used during regular periods to offset utility bills and therefore partially (or in special cases fully!) pays for itself.

For food, it is fortunate that the Church of the Latter Day Saints has been developing cost effective ways for long-term storage of food. There is some uncertainty about especially vitamins and oxidation of fats, but it is hoped that refrigeration will go some way to solve this issue. In any case, based on a growing base of information from space missions and Mars analogues, it seems very wise to make a small investment in an ability to grow plants indoors. Organic waste will be ample, and there will be water. As such, at least for some time, it should be possible to at least grow some foods that could help alleviate especially problems around vitamin deficiencies.

Other items are important too, even though they might not directly relate to the rule of 3. Long durations of isolation places very high burdens on people and the lockdowns many experience during COVID was quite benign compared to being sealed in bio shelters for months, if not years. Luckily, Tereza Flidrova has done excellent work on what is needed to increase the likelihood that significant psychological problems do not happen and the shelter design should heed as much of this advice as possible. Luckily, due to the flexible and low cost material, many such design aspects can quite easily be accommodated at only modest increases in cost of production.

The first version of the shelter structures, “plug-and-play” ready are expected to retail for $39k. The structure would include the following components, with estimated cost:

• Above 14 log serial filters along with certified low-leakage ductwork - $3,900 for high-quality, industrial grade components
• Bubble - $10,300 for high quality ones that have been deployed without failure for years in climates as diverse as the Wadi Rum desert and Iceland (material and construction used in “bubble hotels” meaning they also comply with fire safety standards)
• Sterile water supply (includes heat sterilization but excludes ground well) - ~$4,000
• Waste system - ~$2,000

The difference between retail price and the sum of the component cost is for design, construction, company overheads, return to investors, etc. Note that the earlier $10k/person number does not include anything but material costs. This is because it is unclear how, in a “war time mobilization” by the government to make as many units as possible in the early days of a crisis, how the cost of manufacturing etc. will be accounted for. The design might even evolve to be simple enough for people to make such shelters by themselves out of commonly found and varied plastic materials and HEPA filters repurposed from other uses.

Beyond this, the following purchase prices (note that power and food can be consumed and as such might at least partially “pay for itself”):

• Power - In a sunny location with climate that does not require air conditioning during crisis use this is estimated to be ~$12,000
• Food is estimated to cost $1,200/person/year
• Additionally comes furniture, lighting and decoration

Lastly, in order to exit the shelter during low atmospheric concentrations, the following would be needed in additions:

• BSL 4 reusable suit - ~$2,000
• VHP generator - ~$5,000

It is when the material costs above are summed that one ends up around the $10k/person mark:

3900+10300+4000+2000+12000+(1200*6)+2000+5000=$46400 total or 46400/6=$7733/ person. Note that the inhabitant number here has been increased to 6. This is because an assumption, based on research into inflatable construction, is that these shelters can be made much less luxurious than bubble hotels (that people spend $200/night to stay in!). Therefore, less luxurious units can be made much larger for the same price and easily house several more people.

Lastly, as the currently designed units planned for immediate sale is based on comfortable bubble hotel construction and design, it is imagined that in certain jurisdictions, these units can even be used during “peace time”, when there is no imminent crisis. For example, they could be put up on a lawn to provide space for guests or teenagers. Or if one has a remote piece of land, as a weekend getaway. As such, the hope is that this will sufficiently increase the attractiveness of these units so that a number of them are actually deployed, marking real-world progress on an “end-to-end” x-risk intervention: If a sufficient number of these units are deployed, this might have already decreased existential risk by some amount, especially if we can get some distance beyond ~100 units over a not-too-large geographical area. And given the relatively modest philanthropic funding of this project to date, this effort might represent a cost effective, “end-to-end” x-risk reduction in and by itself. However, the ideal scenario is one where governments are ready to produce thousands of units so not only a minimum viable population survives, but enough people to carry on the most critical, welfare-generating parts of our societies.

A bit more context: Funding until now + future funding

The work described here has been funded from a number of sources, including the LTFF and the SFF.  We’ve been planning based on relatively limited philanthropic-scale funding, thus the decision to set up a for-profit company to see if private capital can be leveraged to make progress on mirror bio shelters.

If significantly more philanthropic funding were to become available, we don’t think we would advocate for more expensive fortified designs contemplated in the past:

• First, a cost-minimizing version, while appropriate for fewer scenarios, targets what we see as, unfortunately, a much more likely scenario. Simultaneously trying to solve other, perhaps less likely and often much less well-described scenarios introduces unnecessary complexity and uncertainty. Uncertainty in turn can lead to expensive over-engineering.
• Second, in a wildfire or stealth pandemic, which we see as the other two potentially globally catastrophic biorisk scenarios, effective protection does not look like shelters. It’s much more effective to target each scenario with a separate solution: Better to drive to an airport and then fly to another country rather than trying to make a combined car and airplane that solves “everything”.
• Third, even if there were temporarily more funding, this might not last. In the face of funding uncertainty,  better to progress in a direction that could continue under a variety of funding regimes. Moreover, it would be good to use more funding to more quickly bring mirror bio shelters to the point of a “complete” break-glass solution as soon as possible. If that can be achieved, one can essentially shelve that solution, reducing ongoing funding to only a minimal amount that ensures that (a) the plan is kept up to date as manufacturing technology changes and (b) the right people know about the plan and are prepared to use it should a threat become likely in the near term.  They could then take the plan down from the shelf and implement it with confidence.

The road ahead

At this point it might be worth revisiting the epistemic status of the topic of how these shelters would actually be used in a mirror biology catastrophe. Put succinctly, the epistemic certainty drops significantly when speculating on the road ahead. So far, these units seem to physically offer significant protection and they might be tolerable from an inhabitant well-being perspective although larger units would be desired. But both because there is inherent uncertainty about exactly which mirror pathogen would be the concern, as well as how any mirror pathogen would interact with the environment it is really hard to say what surviving such a catastrophe looks like. For example, might there actually be periods with sufficiently low atmospheric concentrations so that people can be outside with only 2-3 log protective PPE? Also, much more work would be needed on trying to give any survivors more long-term strategies such as where to replenish supplies of essentials such as food and disinfectant. But one step seems clear: We need to take these shelter plans from paper to reality, and start producing, testing and improving on these shelters.

On this latter, more imminent point, I will continue working on shelters in the following way, if things go well:

1. The first unit of these shelters is planned for construction in early 2025. It will be subjected to basic testing such as “can we maintain a stable pressure in this space while in use?” and “how easy is it to integrate an “industrial”, cleanroom-type ventilation system with a basic inflatable plastic structure”, etc.
2. I will then judge interest from various groups (gov’t, philanthropic, private) in these shelters. If there is enough interest it might be feasible to run a company that constructs and delivers these shelters. Simply having such an organization provides protection: There would then be centralized knowledge, expertise and a network of suppliers and contractors that could react to changes in the threat landscape to produce more units. The organization would also act as a blueprint for other similar organizations to be set up which would further increase production capacity and also use market competition to improve quality and drive down costs.
3. Work with engineers and researchers to give input on how these shelters could fit in a more general plan for responding to a mirror biology crisis. For example, if researchers or risk analysis are developing a plan that relies on manufacturing several such structures quickly at scale, this organization would be able to supply information about the feasibility of various approaches (e.g. would it be possible to use a wide range of sizes and shapes of HEPA filters to create a sufficient seal with the ventilation duct they would be installed in? Or would it be possible to use already-in-use HEPA filters?

There are also some “binary” thresholds in terms of the number of units deployed in a crisis:

• At a deployment that reasonably assures the survival of a Minimum Viable Population (MVP), there is at least some chance that a small deployment (likely 100s of units) will be the difference between extinction and survival of intelligent life in the universe. However, with such small numbers it is unclear what the post-catastrophe game plan looks like, how habitable the planet will be, etc. Most likely, this would work in the subset of scenarios where for some reason there is a high number of mirror microbes for the first few months but that after this the levels subside to something more manageable (e.g. requiring “only” 2-5 log protection during small periods of the year).
• At a deployment level that includes enough skilled labor of the right types, not only will a higher chance of long-term survival be possible, but one might even have made possible the continuation of high welfare communities even if the biosphere is permanently altered to something significantly more hostile to human life. This could enable the continuation of liberal democracies or similarly welfare-supporting forms of society.
• Lastly, at deployment numbers in between the two scenarios above, there is a slowly rising chance of survival and the continuation of societies that support human flourishing. But this is far less than linear: For the first additional units beyond a few hundred - things do not look much better at all. E.g. doubling the number above what is required for MVP survival does not at all double the chances of “rebuild”, especially of something like liberal democracies. But at 10% units less than the deployment level that right away supports continuation of liberal democracies, one probably has almost the same chance of rebuilding as with “all” the units.

Acknowledgements

Input from others have been absolutely essential, this has very much been a team effort. I am just highlight some examples in which the following people have contributed, those examples are far from exhaustive:

• I want to especially call out the late Sebastian Lodemann - the two of us were in a continuous dialogue from the beginning of my shelter work until his much too early passing away. I want to honor his legacy by calling out especially the following direct ways he has influenced my work (far from exhaustive):
  • He was a key person in making the SHELTER Weekend happen, an event with numerous important impacts and that might have been essential in securing the first round of funding of this line of shelter work
  • He did more detailed work on how to harden medical countermeasures production - an idea that still plays an important role in how I think about the current mirror bio shelters (they should be possible to deploy in and around facilities that can be used to develop countermeasures against a mirror bio outbreak)
  • Sebastian challenged my ideas around the values and organizational culture in a shelter organization alerting me to a contradiction in my initial thinking on the topic
• I am grateful to many people associated with Ambitious Impact - As I nearly got into one of their cohorts, I took this as a signal I should try to start something new. They also helped in many more ways and have been extremely generous with their time.
• Kayla Kim provided seasoned fundraising and communications advice in the early days of this work and was the first sign that I might be able to recruit high-caliber talent to contribute to my work.
• Ville Skoglund and I took a nice walk with me during my parental leave and when I suggested that EA Sweden incubates a shelter project (he was then the manager of EA Sweden) he was more or less like “let’s go”! That moment feels seminal!
• Joel Becker has perhaps dedicated more time to my work than most others and probably played a significant role in helping me secure funding, in addition to helping me see more clearly the best course of action in the project’s early and uncertain, but direction-setting stages.
• Aron Lajko has made numerous contributions, but one that stood out was identifying a book on BSL and cleanroom all-in construction costs which was critical for costing a previously suggested, highly complex design for a shelter.
• The LTFF made possible the start of my work on shelters and I am grateful for them and all their donors for placing the bet on me and giving me the benefit of the doubt.
• An unnamed contributor showed me military Collective Protection units in October 2022 before I had even considered something smaller than the previous, large, comprehensive design concepts for civilizational shelters. This contributor also excellently posed the following question in Spring of 2023 which was the starting point of the current affordable shelter concept: “How cheap can you possibly make a 13-log reducing space? Don’t hold back - think outside the box. We need this now.”
• Tereza Flidrova has done essential work in the architectural considerations for long-term isolation (check out her thesis on her LinkedIn profile - it is extremely useful and relevant), given me sound advice all along and made several specific contributions in terms of the livability as well as the aesthetics of these shelters.
• James Odene at User Friendly is such pleasure a to work with. And the most remarkable part is that such a friendly (not only user friendly!) person can still let me know quite clearly when he thinks I am headed in the wrong direction. At least on one occasion has James prevented me from making a potentially enormous blunder, as evidenced by subsequent events I managed to avoid getting tangled up in thanks to his friendly but stark warnings.
• I am grateful to the grantmakers at the Survival and Flourishing Fund and to Jaan Taalinn for making my current work possible and placing trust in me being able to carry this work out (one reviewer wrote “The plan is complex but apparently well-considered, and the applicant has the right background to pull it off.”)
• Critical voices have told me I was about to make a mistake several times. Had they not, I would have actually gone ahead and made those mistakes - I am deeply grateful for them pulling me back from the “the brink” (brink for the project, not sentient life that is!).
• Alex D has been a champion all along and is a person I am grateful for being a trusted “industry colleague” that I can ask those thorny questions I don’t know who else it would be appropriate to ask.
• Johan Täng who runs the NBG is always on the lookout for relevant connections in the Nordic region and has made relevant introductions.
• I am grateful to my current board members who are extremely generous with their time, sharing their significant expertise. Especially Maria Khimulya (who was recommended to me by her brother “Alvea”-Grigory) has given actionable, tactical advice on marketing and Jeff Kaufman has after only a few days in his board role given lots of good input on this post, as well as how to communicate effectively in the wake of the public release of the threat from mirror bio.
• Kiryl Shantyka, Emil Wasteson and the EA Sweden team has made my life infinitely easier by providing both tactical advice as well as fiscal sponsorship. They have been infinitely flexible and patient when I have come to them after months of silence with requests such as “can we make this happen next week?”. Another member of EA Sweden also very significantly told me something to the tune of “Grantmakers say you should just apply for funding - don’t overcook it. Just apply.” And I did and subsequently received support from the LTFF.
• JueYan has been generous in sharing relevant connections from his network and giving me fundraising strategy tips.
• Michael Andregg has been a great help in thinking through what minimal set of features users of these shelters might want.
• There are so many others to which I am deeply grateful in a myriad of ways and their exclusion from the list above does not mean their contributions were less significant. This is truly a team effort despite me being the one who did most of the grunt work on this. People ask me if I should get a co-founder and I usually respond: “With the amount of support from the biosec, EA and longtermist communities I feel like I have more than 10 co-founders rooting for me, always available for a call when I am in doubt and who actually does some heavy lifting to bring the project forward.” If I have missed mentioning your contributions please nudge me and I will be more than happy to mention you!

1. ^{^}

   There are at least 2 different common ways of measuring the concentration of microorganisms in air - CFUs and DNA copies. CFU stands for Colony Forming Units and is a method of approximating viable microorganism counts by collecting them, putting them in growth media and visually counting the growing patches of microorganisms, the idea being that each patch originates from one CFU. DNA copies on the other hand ignore viability and look only for signature sections of species’ DNA (meaning it will also count a broken DNA molecule as long as the section it is looking for is unbroken).

2. ^{^}

   It might also be worthwhile to consider urine recycling as is done on the International Space Station. However, note that using 9 parts recycled water to 1 part outside water only results in a 1 log protection. Thus, urine recycling is more likely to be the result of cost optimization - if heat sterilization to the required log level results in unacceptably high power requirements (power might be expensive if generated on-site) it might be overall more cost effective to use urine recycling as much as possible.

3. ^{^}

   While air filters are used in many settings, cleanroom manufacturing as well as nuclear air cleaning stand out as two applications that push particle removal performance to the edge. Cleanrooms could be argued to be using “serial” filtration: They have air intakes inside the cleanroom itself which then brings the cleanroom air to the top of the room where the air is passed through a ULPA (much higher efficiency than HEPA) filter. This is done at a rate of up to, and perhaps sometimes beyond 50 full room air changes per hour. As such, the already clean cleanroom air is constantly and serially passed through this high-grade filter, again and again. While much of this is done to remove particles that have originated within the cleanroom (e.g. from furniture, clothing, etc.) it demonstrates that extreme levels of cleanliness is possible to achieve. However, it is unclear if cleanrooms achieve 14 log reduction in particles from the outside, as this is not the goal of cleanrooms (they instead place much more emphasis on particles originating from inside the cleanroom).

4. ^{^}

   I have done preliminary calculations showing that for anything to diffuse against the flow through small cracks in the envelope, the region of sufficiently low flow near the “walls” of this crack is smaller than the particles we are concerned with and therefore impossible. While this has not properly taken into account turbulence, given the indicative calculations, along with expert commentary, it looks highly improbable that particles can diffuse against the flow via small openings in the bubble envelope. Also, it should be noted that even though one could make a “near-wall-corridor” wide enough for a particle to go upstream, another challenge is that the stochastic motion of this particle would have to result in “only moves along the corridor” - the moment the particle “tries to move” sideways and into higher velocity flow, it will quickly be carried a long way towards the outside. For example, if the corridor is 2mm long, 1 micron wide, the particle 0.1 micron and each “move” is on average 10 microns, it would have to on net make 200 moves towards the inside while never making a single move “into the stream” where this probability is probably extremely small.