A Red and Green Compass Rose, logo for the Compass DeRose Guide Series.The Compass DeRose Guide to Air Supply

This page was written by Steven J. DeRose early in 2003, and was last updated on 2005-10-07.

(see disclaimer and warnings here)

Air is always your first survival priority. You'll only last a few minutes without it. This page discusses how to calculate air volume requirements for a shelter, and some ways to obtain that air and keep it clean and safe.

According to USA Today's preparedness article on 2003-02-13, a 10*10 foot room holds enough room for one person for 5 hours (presumably the room is the usual USA 8 foot height; I'm guessing they didn't subtract for furnishings). This may be overly optimistic.

I'm glad they pointed out that air is an issue; I'm disappointed they gave no information about how to get good air, or to keep potentially bad air away. I will discuss both parts here.

Your basic options

If you're hiding out from a nuclear, radiological, biological, or chemical attack or disaster, you don't want to breathe contaminated air. You have three basic safety options:

  1. Get out of range -- by far the best choice if you can do it in time
  2. Use a gas mask (makeshift ones may help if there's no choice)
  3. Get into an air-controlled shelter

1: Get out of range

Your first and best defense is to be out of the area of effect before the danger gets there, or at least before it gets there in lethal amounts. You can start this defense years ahead of time by living in unlikely target areas, or you can get out fast when you first learn of danger (hopefully not by literally smelling it!). With the nastier agents, if you smell it or see nearby people falling, and you're not falling down yourself yet, you've got a decent chance if you simply run away (not downwind, please!). One reason soldiers need all the fancy gear is that they don't have the option of running away.

It should be encouraging to remember that almost everyone in range of the Sarin attacks in the Tokyo subway survived, simply by running out into fresh air.

The range you have to go can vary widely. By type of danger:

Big. Say, a bare minimum of 10 miles, up to the entire earth (if enough nukes go off -- though fortunately big nuclear wars have gotten much less likely). One advantage you have is that the dust starts out so high, you have some time to run (or take another option such as those explained later); and the stuff doesn't effect you instantly as some chemical agents do.
Articles I've read estimate a downwind plume of under a mile, and upwind more like a few hundred yards. As with nuclear fallout dust, time of exposure is critical. So if you suspect a radiological bomb has gone off near you, go the other way, fast, and minimize your exposure time. If you're straight downwind from it, run to the side because the plume may well be narrow.
The largest recommended evacuation distance I recall for chemical spills is several miles downwind, and far shorter upwind. A derailed tank car can release far more chemicals than a typical planned attack, but even the worst chemicals would probably be diluted enough to be non-lethal well before those several miles. Still, a sophisticated attacker might find a way to spread chemicals over a wide area, making it impossible for as many people to just get out of the way. Remember to avoid low areas, because most chemical agents are heavier than air.
As with chemical dangers, biological ones have a pretty small initial area of effect unless effectively spread out. On the other hand, people can carry lethal doses of biotoxins around, without dieing immediately. They could get spores on their clothing or get a disease and become contagious (though many lethal diseases are not very contagious). In a full-fledged epidemic of a contagious disease, getting out of the area of effect may be impractical.

Protect your skin and eyes from direct contact with any of these agents -- this is easy with liquids (like VX is at normal temperatures), moderately hard with fine dusts, and harder with vapors. On the other hand, even agents that can go throught skin, usually have to be very concentrated to succeed that way.

In a pinch, you could get some protection from skin contact by a poncho made out of a plastic garbage bag, or a raincoat, or one of those pocket-size rain ponchos, or a mylar "space blanket" and some tape. But in almost any situation I think you'll be better off running instead, and just getting to clean air. Standing there taping plastic over yourself (or climbing into a real "MOPP" suit) while the cloud approaches is probably not a good move, and probably won't help anyway since most agents are not skin-absorbable, but almost all are inhalable.

2: Use a gas mask

A good gas mask will protect you against inhaling almost any NRBC agent. This is a very good thing. After 9/11 I heard many people saying a gas mask is useless because you can't wear it all the time; I still hear this from reputable sources. I don't buy that argument at all. True, it's not practical to wear a mask all the time. It's also not fashion-conscious, though there are some who believe masks are terribly comfortable, and think everyone'll be wearing them in the future. And true, there could be cases where you wouldn't even know you needed your mask on until it was too late (such as a quietly-executed biological attack, or a chemical attack that goes off right next to you). And true, it's unlikely you'll always have your mask with you. But all that does is show that having a gas mask won't give you 100% protection. That shouldn't be surprising.

Still, there are many possibilities where a mask would help: Say you see or hear about an attack or accidental spill nearby. Whatever it is won't arrive instantly, so you have time to put on your mask if you've got it; probably even a couple minutes to go get it. If you keep your mask in your car, you will likely be within a few minutes running distance of it a lot of the time, including when you're home (commuter parking lots are the biggest exception I can think of). If you're indoors and there's a release outdoors, or vice versa, it will take time for the agent to get to you.

If the attack happens right next to you, you'll be depending on spiritual realities, not gas masks. But it is much more likely that when an attack becomes known you won't have been affected yet -- you may be miles away, you may have happened not to be coughed on by the first round of victims, etc. Putting on a gas mask then will keep you far safer until you get out of the area or to a shelter with a clean air supply.

I've also heard people say that gas masks won't protect you anyway, since poisons are absorbed througgh the skin. Some are, and those require a complete coverage suit ("MOPP gear"). MOPP suits are cumbersome, and trained soldiers take about 8 minutes to get into one -- 8 minutes probably better spent running. Fortunately, relatively few poisons can be absorbed through the skin (though some can affect you through broken skin). I haven't heard of any biological weapons, radiological, or nuclear dusts that are, and only a few chemical weapons (mainly nerve agents such as Ricin, Sarin, VX, and Tabun). So in fact a mask can save your life. MayoClinic.com provide useful information on a variety of bio and chemical hazards.

There are many kinds of masks. They vary in price, quality, fit, and just what they'll protect against. It's also worth mentioning that if you happen to own scuba gear and happen to be near it when a problem arises, using it would be a fine idea. In a pinch, you can make an "expedient" mask from readily available materials -- whether it helps "all depends". Don't count on it. If you want to experience the difference between a real NRBC gas mask and an approximation, buy a top-quality dust/vapor hardware-store mask. These are great to use during messy or smelly household jobs. Then get a dull drill or saw blade, and cut through some wood with it to produce some smoke (or just stand next to a campfire). You'll quickly notice that the mask is totally useless even for plain wood smoke.

I've devoted a separate page to masks, ranging from the "wet washcloth over your face" all the way up to the best NBC-protective ones I know of.

3: Get into an air-controlled shelter

You can provide a safe air supply to just a shelter area, or to your entire house. The latter is obviously more expensive, but it amounts to a 24-hour-a-day gas mask for everyone in your house. It will also make your home healthier overall. I suggest that you consider protecting your entire home or apartment from airborne toxins. It is practical, though not cheap, to install whole-house protection against a wide range of toxins (cost: $2,000-3,000).

Duct tape around the windows won't do it; it may slow down airflow, but there will be a lot of leaks left in places you can't even find, much less tape (I just found a gap that took 3 full tubes of caulk to seal, where one bay window meets the foundation at my house -- never would have noticed without a systematic survey). Even if you do succeed in a complete seal, to last long you still must bring in air from outside.

I think the smartest approach is to do what hospitals and clean-rooms do: bring in fresh air on purpose, and clean it thoroughly on the way in. Seal all the leaks you can, so that inside air can't leak out as fast as you're bringing in fresh clean air. At that point your house is at "positive pressure", and no raw air is coming in because air is busy leaking out.

Of course, you first have to add a duct to bring in the freash air (to breathe, and perhaps for your furnace to use too), and a fan to maintain inward pressure in that duct. The three main approaches to cleaning that incoming air are:

HEPA filtration

This simply removes small particles of all kinds. HEPA (not "HEPA-type") filters have become commonplace: you can buy them for room air cleaners, vacuum cleaners, shop vacs, etc. If you have any of those, it's easy to stock a couple extra HEPA cartridges for them (see for example here). Figure out a way to use them to clean the incoming air (say, by being able to close off the main discharge of the incoming air duct, and direct the air into your shop vac instead). You can also buy fancy duct-mounted whole-house air cleaners.

Make sure you end up with "true HEPA" rather than "HEPA-type": they must remove 99.97% of particles at 0.3 micron size. Viruses are smaller than that, but a decent % of them will still be caught. To do better than HEPA, you could go to "ULPA" filters, which are rated for 99.999% of particles down to 0.12 micron.

Ultraviolet germ-killers
These mount in your furnace ducts, and fry the DNA of any microorganisms passing by. Good for reducing allergies. Honeywell UV-100A seems to be the device of choice. The downside is the cost of the yearly replacement bulb, something like $100.
Activated charcoal and other chemical filtration
Activated charcoal (not just any charcoal) latches onto an enormous range of nasty chemicals and vapors that can't be caught by HEPA filters or killed by UV germ-killers. Some of the high-end duct-mounted air cleaners include this (at least as an option); or you can buy standalone filters.

Sadly, for really good protection you need all of these.

Some nice all-in-one systems are available, for example from here. Be sure you have a way to run the fan even if you lose electric power -- that's not at all unlikely to happen.

How much air do I need in a shelter?

The USA today figures listed above work out to 10*10*8 = 800 cubic feet of air in the room, or 2.67 cubic feet per minute (CFM), or 75 litres per minute (LPM). This may be based on an old FEMA guideline for shelters, of 3 CFM per person. This is a lot more air per minute than people typically breathe, but it's barely enough to survive on given all the other variables involved.

With humidity, comfortable breathing, air circulation, and possible warm weather figured in, 10 CFM per person is a much better plan. One important variable that you can control, is that if you're hiding out from any kind of emergency situation you're likely to be stressed out and using up your air way faster. Don't do that. Find a way to calm down and your air will last a whole lot longer.

You can help move air by installing a fan right in the relevant duct. For example, to pull outside air in to your filter. "Duct fans" are readily available (for example, see here), and one for a 6" duct typically moves up to 250CFM; 8" 500CFM; 10" 650 CFM; and 12" 800CFM. These figure decrease significantly for any bends in the duct, or for long runs, or for pushing/pulling air through a filter.

A 4-person shelter should have a bare minimum of one 3-4" air intake if it's powered; and every bend in the pipe reduces the amount of airflow. There has to be a reliable way to power the fan. For manual ventilation, the "Kearny Air Pump" is a typical recommendation (for example, see Kearny's book online, or this book via Amazon): this is a hinged door/panel in a duct or entryway, with openings covered by flexible covers. As you swing it outward, the covers open and air comes through; as you pull it back the covers close and the air is forced into the shelter. Efficiency can be improved by making sure there are good seals around all 4 sides.

I will eventually be adding a section to help calculate duct and intake sizes and the number of people they can support. In the meantime see my page on airflow here.

Sealing a shelter room

Sealing any room is a tedious, detail-oriented task. But before even starting it, make sure that you don't seal it too well: under normal circumstances you need air circulation, and sealing your house too tightly can be unhealthy in itself. If you seal up tight, set up your system to bring in fresh air actively.

Having figured that out, you can look here for more details on how to go about hardening a shelter, including air handling.

First inspect every little corner and joint. For example:

Close the gaps. Start with the big ones, but don't neglect the many little ones. A few key materials to use:

Try to stay aware of how your seals will hold up to weather, time, and the possibility of blast pressure in either direction.

Finally, test your work and improve it as needed. Some things you can do to test:

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