DIY Air Filters

Erin suggested I write a follow-up post to Prepping for Asthma, explaining how to make a box fan air filter. This is useful when you deal with asthma, but also dealing with air quality issues, such as forest fires.

For most of you, an air filter is something you really don’t think of very much. Maybe you stick it in the furnace and change it once a year (if that often; even if people should, they usually don't), and most of you don’t think of it beyond that except to think “Oh, the air smells a little funny” once in a while. One of those great things about living in modern society is that often we live in an environment where we don't have to think about these things.

Unfortunately, if the SHTF scenario you will occasionally find that certain things don’t happen --  things like shipments of allergy drugs, or furnace filters, or similar, and you will find that sometimes you have to improvise something. The goal of this article is to show you how to make a do-it-yourself, HEPA type filter unit, with near HEPA grade air filtration. This should be usable in case of emergencies, and while not muscle driven, it draws a very low amount of power and should work for extended periods of time off of a simple battery bank or solar panel system.

Parts

Box Fan

A square fan that is commonly found in the summers across the US. Usually 20 inches by 20 inches, they tend to be very common in places that cannot afford air conditioning. During the summer they tend to run between $20-30, but you can find them in a great many discount/thrift stores in working order for as little as $2-3.

Hog Hair Filter

A coarse prefilter (usually made from a synthetic fiber, usually used as a simple particulate filter on swamp coolers) can be useful to extend the life of your other filters by catching things like large chunks of things and pet hair. Washable and reusable, it comes in very useful. Hog hair filters can be obtained at most hardware stores and Walmarts. They're meant to be cut down to the size needed, and usually cost between $5-10.

Mid-Grade Filter

Most furnace filters come with a rating system. Major retailers tend to have their own, but what it boils down to is that some of them are more efficient than others. More effective ones cost more money, and so having a cheaper one in front of it on the air intake saves your more expensive filter from having to be replaced as often. If you are willing to spend noticeably (8-10 times) more than the disposable filters cost, you can buy a cut-to-size washable mid-grade filter for $20-30. These are useful for filtering out as much as possible before you get to the pricey ones, to save you money on replacements.

(The listed Amazon link is an example. Amazon doesn't sell these in small quantities; I think it has to do with shipping.)

HEPA Filter

HEPA stands for High Efficiency Particulate Arrestance. Originally developed for use by the Manhattan Project to trap radioacative particles in the air before the scientists' lungs did, since then they have found many uses in hospitals, homes, manufacturing, and even agriculture. HEPA will filter down to a very small level, even taking out viruses and odors. For this project you want a HEPA furnace filter from someplace like Home Depot. They typically are rated to last around 3 months of continuous use, but if you use other filters in front of them they will last much longer. They cost upwards of $10.

(Again, multi pack.)


Assembly

Measurements

• A box fan is typically 20 inches by 20 inches. 
• Filters, which we will be adding to them, are measured by height, length, and depth. 
• You will want a 20 inch by 20 inch by 1 inch filter, for both the mid grade and HEPA filters.
• In the cases of washable and/or sizeable filters, simply find something that is around 1 inch deep, and that can be cut to size that is at least 20 inches by 20 inches. Excess material can be used for other projects, such as air filters for lawn mowers.

Procedure

1. Stack the filters in front of the air intake to the box fan from least fine on the outside to most fine on the inside.
• In this example, put the hog hair on the outside and the HEPA filter next to the fan, with the midgrade between them. 
2. Make sure that if there are any airflow direction arrows on the outside edges of the filters, that they are pointed the correct direction.
3. The easiest way to secure your filters to your fan is simply with suction. If you turn a fan onto high, usually it will keep a three stage filter (HEPA, mid grade, and hogs hair) in place while the fan is running. 
• You may want to put zip ties at the corners of the filter, punching through the filters and connecting to the plastic grid protecting the fan blades.
• Alternately, use duct tape or similar.
4. Sit in front of the filter, and enjoy.

Maintenance

• When the hogshair is visibly filled with debris (such as pet hair) or is noticeably darker than it was, wash it using dish soap and water, and put it back into use. 
• When the midgrade or HEPA filters become visibly darker (they start out as white or a very light grey), replace them. 
• I usually visually inspect my filters about twice a season, and depending on how much they have pulled out of the air, they may last the entire season (or longer).

This air filter is simple, effective, cheap. It clears a room of burned food smoke remarkably quickly; pollen, pet hair and dander, and even viruses get caught in the filter and can save you a lot of grief. And if the power goes out, just use the power module you built; the power draw for this is quite low.

Area Air Purification, Part 3: Monitoring

I've covered the basics of sealing off a “clean” room and filtering the air for it, and now comes the hard part: measuring air quality. It is a science, and the methods used vary by the pollutant that you're trying to measure. I'll try to break it down into major groups and give some simple examples of testing methods.

Oxygen Level

OSHA defines “safe” breathing air to have between 19.5 and 23.0% oxygen (O2) by volume. This band is narrow for regulatory reasons, and it can be stretched a bit lower if the gasses displacing the oxygen are not dangerous and a bit higher if you control sources of ignition.

I know all of you are trained in CPR and mouth-to-mouth resuscitation. Do you realize that the air you're exhaling into that unconscious person is about 16% O2? That falls below the OSHA standards, but is enough to keep a person alive. You'll likely pass out at around 10-15%, which is why I use 15% as a lower limit. Once you've passed out, you are no longer functional and are as good as dead in a crisis/emergency situation. Physical condition and various health issues are going to determine your personal threshold.

On the upper side, you can breathe 100% oxygen for short periods of time (hours) without permanent damage. The major problem with any concentration over 25% is the increased risk of fire. Oxygen itself doesn't burn, but it combines with other substances to create fire. Oxygen enriched environments require special attention to the choices of clothing, electrical equipment, and other things that may ignite or cause a spark.

So, how do you measure the oxygen level in a room? There are various detectors on the market that use electrochemical sensors to measure specific chemical concentrations in the air, but they're not cheap. A typical 4-gas detector will measure O2, Carbon Monoxide (CO), Hydrogen Sulfide (H2S), and combustible gasses, but will cost between $500-1000 and require frequent calibration. I found the test results from an experiment that gives a much cheaper method: a candle will go out if the O2 drops below about 18%.

For years, the US Coast Guard used “flame safety lamps” to check for sufficient O2 in holds and other shipboard spaces, since a flame can't be sustained with less than 16.5% O2. Miners used similar lamps when canaries got to be too expensive (just kidding, they needed a light that wouldn't cause an explosion). Simply put, if a candle won't burn, you can't breathe the air and expect to live... but remember that a flame is consuming oxygen as it burns.

Carbon Monoxide (CO)

CO is formed by the incomplete burning of organic material. CO is flammable (in the range from 12.5-74% by volume) as well as toxic (it binds to the part of your blood that normally carries oxygen, preventing your cells from getting sufficient oxygen). It is a colorless, odorless gas that will cause headaches at about 1-1.5% by volume, and death at about 4% after 30 minute's exposure. This is a nasty chemical that used to be piped into houses as “man-made gas” or “coal gas” for lighting and cooking, and was replaced by “natural gas” (methane). There are tons of battery operated CO detectors on the market, most of them are less than $30. Get one.

Carbon Dioxide (CO2)

CO2 makes up about 5% of what you exhale with each breath. It can also be formed by fire, decomposing organic material, dry ice, and leaks in food service soda machines. CO2 is heavier than air and will settle into low spaces, like basements and cellars, and displace the oxygen that was there. CO2 will cause unconsciousness and death at 10% by volume, and intoxication at 5%. The regulatory limit for exposure is 0.5%. CO2 detectors are available, but they're hard to find (search engines assume that you're an idiot and meant to type CO so they don't give proper results). They're also not cheap, running around $100-125. For local suppliers, ask around at bar and restaurant suppliers or greenhouses.

Flammable/Combustible Gasses

This is a wide category that covers everything from hydrogen (H) to volatile organic compounds (VOC). Any gas or vapor that will burn in air is a flammable or combustible gas, and is one of the main things that commercial gas monitors check for. Something as simple as an idiot getting too liberal with a spray can of penetrating oil can ruin your whole day (been there, seen that) if there is an ignition source present. If you're going to be working around or expect to see combustible gasses, get a monitor/detector. Broken gas lines after an earthquake or tornado are common, but natural gas and liquid petroleum, both colorless, odorless gasses, have an additive that stinks (methyl mercaptan) just to make them easier to notice.

Dust

Normal household dust is an annoyance, made up mostly of dirt, dead skin cells, and minerals that are not a hazard. If you live in an older building with the potential of having asbestos insulation, dust caused by a natural disaster could be a long-term health hazard. Unfortunately, there is no simple test for asbestos, so if in doubt, get out the respirator. Wood dust, concrete dust, volcanic ash, and silica (fine sand) dust all have health risks similar to asbestos and are visible in the air before they reach dangerous concentrations.

Other dusts can be an explosion hazard. Any vegetable-based dust will burn rapidly. If suspended in the air and ignited, the flame will spread (propagate) fast enough to be classified as an explosion. Visual testing is the easiest: if you can see it in the air it is too much. At your own risk, toss a handful of flour into the flames of a campfire some time for a demonstration or do a search on “Cremora pots” for some interesting videos. We're not responsible for your lack of eyebrows, arm hair, or any other injury if you try these.

Chemical Warfare Agents

There are people on this planet who have, and will use, chemical warfare. Terrorists have been known to use some of the simpler nerve agents, usually in subways or other enclosed spaces. Without the proper testing gear (which is normally only found in military units and is in the “if you have to ask you can't afford it” price range), the only method I can suggest is the miner's canary. Birds breathe differently than mammals, and are more susceptible to anything in the air. Canaries, chickens, geese, or even sky rats (commonly called pigeons) can be used as a warning method. If you're sitting inside watching the birds through the window and they start to fall over, it's time to take some action. Caged sky rats placed within sight can give a few seconds or minutes of warning, if this is a potential threat you want to prepare for.

If there are any questions or you would like me to expand on any of these, feel free to leave a comment here or on ourFaceBook page. I will try to answer as well as I can and I enjoy getting the feedback.

Area Air Purification, Part 2: Filtration

In Part 1, I explained the basics of how to seal off a room or other small area to keep contaminated air out. Before I start on Part 2, "How to Clean the Air," I need to answer a few questions that arose from Part 1. 

What kind of plastic should I use?

Any solid plastic sheeting will work; the thicker they are, the more durable they are. Plastic painter's drop cloths are cheap and don't take up much room on a shelf, but a roll of 4 or 6 mil (thousandths of an inch) “clear” plastic is easier to work with. Be aware that “clear” often means “translucent” instead of “see-through”. Black plastic may work better if you're trying to provide blackout curtains on exterior windows.

What kind of expanding foam should I use? 
Where do I find it?

I like the Great Stuff brand. It comes in two flavors, normal (for cracks under 1 inch wide) and widegap filler (for larger cracks). Most hardware and home supply stores carry it, as well as Amazon.com.

If you have a lot of cracks to seal, there are semi-pro systems out there that use a wand and a disposable cartridge.

What kind of tape do I use?

I keep blue painter's tape on hand for business reasons, and I know for a fact that it works to hold up plastic over a broken window in an Iowa Autumn. Duct tape varies in quality by brand, and I have seen some of the cheaper duct tape fall off of a vertical seam after less than a day. “Gaffer's”tape, 100mph tape, and Gorilla brand have all worked well for me in the past. Packaging tape, normally used to seal cardboard boxes, sticks to plastic very well and is easy to apply if you have a tape gun. Basically, any tape that will stick and is at around 2 inches wide is what you're looking for.

Part 2: How to Clean the Air

Once you have your area sealed off, you need to have a way to pump filtered air into it. That means you'll need an air mover and a filter.

Air Movers
I've read of hand-operated air pumps in prepper novels, but I'm not sold on them. The amount of air that you can pump through a bellows or similar system is tiny compared to a powered fan. Unless you're thinking of building a very small, totally sealed underground bunker, plan on using some form of electricity to move the air for you.

Most home heating/cooling systems use a squirrel-cage fan to move the air through the unit. They're small, quiet, and move a large volume of air at relatively low pressure. If you can access your furnace and isolate the input or return air ducts (hint: they'll be on the side with the filters) and you have a way to power the blower, you're already set. Finding a small squirrel-cage fan isn't that hard, and they aren't terribly expensive.

In-line duct fans are another option, and are easy to fit into a lot of DIY projects. I've used them to boost air flow into rooms that didn't get enough air flow when the AC was running and they last about ten years if you keep your filters and ductwork clean. Dust will kill them within a couple of years.

Whichever method you choose, you need to be able to move enough air through your filters to maintain a slightly positive pressure inside your clean space. This will force your plastic sheeting against any holes, and it will keep contaminated air out if your seams aren't perfectly taped.

You'll also need to figure out how much air you're going to need to move and size your blower accordingly. A good rule of thumb is to calculate the volume of your clean space (length x width x height) in cubic feet and expect to pump that much air at least five times every hour. You could probably get by on less, but it is going to depend on how many people you have breathing the air and how active they are. As a point of interest, when doing confined space entries we often use 20 ACH (Air Changes per Hour) when calculating air movement.

• Example 1: a small bedroom of 10 x 12 x 8 feet = 960 cubic feet (cf) of air. 5 ACH (Air Changes per Hour) would be 960 x 5 = 4800 cf per hour. 4800 cf / 60 minutes per hour = 80 cf per minute (cfm). That's not a very big blower when you look at the ratings of most fans. 
• Example 2: an enclosure around a child's playpen, roughly 4 x 4 x 3 ft = 48 cf. 5ACH x 48 cf = 240 cf per hour. 240 cf / 60 minutes per hour = 4 cfm. That's possible with a battery operated 12V fan.

Filters
Okay, you've got a sealed area and have a way to push air into it. How are you going to filter the air before you move it? I can't recommend a set of filters that will remove everything possible, but I may be able to point you towards something that will meet your needs.

• Household filters that you'll find in the hardware stores are ranked by Minimum Efficiency Reporting Value (MERV) going from 1 to 16, where the filters that stop smaller particles earn higher numbers. Follow the link for a good explanation of the different ranks and example of what they'll stop. 
• HEPA (High Efficiency Particulate Air) filters are the standard and correspond to a MERV of at least 16. A proper HEPA filter will remove at least 99.97% of all particles larger than 0.3 micron. 0.3 micron means that a HEPA filter will stop most bacteria and all pollen or dust, but won't catch all viruses or smoke. 
• Hospitals use HEPA filters backed up with strong ultraviolet (UV) light to clean the air in operating rooms and infectious disease wards. The UV lamps are placed in the ductwork and kill off viruses, molds, and yeasts that might be floating around. They are placed inside the ducts to prevent damage to human eyes, since UV light is mostly invisible and very destructive to living tissue. 
• Electrostatic filters work by passing the air through a series of electrically charged grids, something like a very fine mesh bug zapper, that causes particles and chemicals to cling to the grid due to the difference in electrical charge. They are quite efficient at removing pollutants like smoke, but needs to be cleaned , usually by running them through the dishwasher or something similar. Some of the small room “air purifiers” work on the same principle but I have no experience with them. 
• Ozone generators work by using high voltage electricity to create small amounts of ozone, a form of ionized oxygen that chemically attacks volatile pollutants and breaks them down into less dangerous compounds. Think of it as the air filtration version of using bleach to clean up water. Like bleach, ozone in large quantities is corrosive. 
• Carbon filters (activated charcoal) work to clean air the same way it does water, by trapping contamination in the microscopic pores of the carbon. Good for removing chemicals and odors. 
• Potassium permangenate is another water treatment that is used to clean air. Useful in removing hydrogen sulfide, but......
• Chemical filtration is a technical field that is beyond the scope of this humble blog. If you're expecting to be dealing with nerve agents or other airborne chemical weapons, you need to consult with experts in that field.

Putting It All Together

• I recommend placing your filters and air mover outside of your clean space. This may mean you'll have to wear a respirator while servicing it (a wise precaution anyway), but it will keep the concentrated filtrate (whatever gets caught in the filters) out of your clean space. There's no use going to all of the trouble of filtering it out of the air and then keeping it where you live. 
• Place the filters on the intake side of the blower. This will keep the blower fan cleaner, ensuring a longer life. 
• Make sure your source of air has enough oxygen in it to be worth filtering. No filter will provide more oxygen, so you need to make sure your source isn't deficient. Avoid drawing from underground spaces, since CO2 is heavier than air and will displace it in stagnant spaces.
• Keep your ductwork as straight as possible. It doesn't take many sharp turns to slow down air flow. 
• If you don't have metal ducting to connect your filter to your blower, improvise. Cardboard and duct tape with a layer of plastic over it will suffice for several days. The same goes with getting air into your clean space -- use flexible drier vent pipe or whatever else you can find to make it work.

As always, I will try to answer specific questions and help you find the information you need. Feel free to comment here or on our Facebook Page.

How to monitor for contamination is Part 3, since it gets fairly technical.

Area Air Purification, Part 1: Making a Clean Room

Since there was interest in an article covering air purification for more than just personal protection, I'll do my best to cover filtering air for a small area. This is a fairly deep subject, so I'm going to break it into at least two parts.

Filtering air isn't all that much different than filtering water. If you think of a personal respirator as analogous to a personal water filter, then it isn't too much of a stretch to think of a room air purifier as a small water filtration plant or a large water filter (like a Big Berkey).

If you have to shelter in place through a disaster, it's worth considering having to filter the air you'll breathe:

• Claustrophobia is a real issue for some folks, and a mask is just not an option for them. 
• Having a filtered enclosure that you can drop over a crib or playpen is worth looking into if you have ankle-biters crawling around. 
• Setting up filtration for small children, the elderly, and the infirm is also much easier physically and emotionally if they have space in which to move.

There are some steps to go through, I'll cover the first two in this post:

1. Identify what is in the air that needs to be filtered out (contamination).
2. Keep the contamination out of your air supply.
3. Filter the air coming in.
4. Monitor for contamination.

Identify the Contamination 
This is the step where you figure out what you need to remove from the air you want to breathe.

• If your main concern is the eruption of the super-volcano under Yellowstone National Park, the main contaminant will be fine ash or dust. 
• Forest fires a few hundred miles upwind can make life miserable for people with breathing problems; the smoke and ash in the air travel a long way. 
• If you live near a chemical plant that you fear may catch fire, the contaminants will be varied and considerably harder to remove. 
• Chemical warfare attack is the worst-case scenario, requiring filters designed to remove the specific chemicals used by the military. 
• Biohazards are another possibility that can be filtered out of the air with the right equipment. Anthrax has been used to contaminate buildings before, and may be used again. Airborne diseases are a medical research field all by themselves, but the filtration is fairly standard.

Keep the Contamination Out
Sealing a room or two is about as much as most of us will be capable of doing without spending more on filters than on the house itself.

The choice of which room to use as a "clean" space will take some thought and planning. Ideally you'll want a room with as few openings and as little exposure to the outside air as possible, like a basement room. Dirt makes a good seal against the foundation and will keep out more air than most common house walls. Basement windows are usually smaller than those above ground, and entrances often have a "mudroom" or stairway leading outside which can be turned into an "airlock" for entering and exiting the clean area. An airlock is a small sealed area with two doorways, one leading to the clean area and the other leading outside. This arrangement minimizes the loss of clean air when you have to go in and out of the clean area and also gives you a place to leave dirty or contaminated clothes before entering the clean area. The idea is to never have both doors open at the same time unless there is an emergency.

If you don't have a basement (perhaps you live in an apartment building), you'll want to pick an interior room with enough space to be comfortable in and as little contact with the outside walls as possible. Use as many existing barriers between you and the outside as you can, because each one is going to slow down or stop a portion of the contamination. Think of it as allowing that muddy water to settle out before running it through your LifeStraw.

Creating a Clean Room
Plastic sheeting and duct tape will take care of sealing up large openings like doors and windows, and expanding spray foam will work around cracks and gaps. (If you've ever winterized a house, you'll know where to start.)

1. Close the windows and place a layer of plastic over them, holding it in place with duct tape. Tape along the whole border to get a good seal around the frame.
2. Find and seal (plastic and/or duct tape) all vents coming into the room. This covers heater vents, floor drains (most of which have a P-trap to prevent sewer gasses from coming up through the pipe), drier vents, sink drains (the S-trap under the sink will stop more than a P-trap but isn't perfect), showers/tubs, and toilets. If you're in a hurry, a quick shot of expanding foam into a drain will seal it in a few minutes and the foam is easy to dissolve (acetone, gasoline, etc.) or remove later, when the crisis has passed.
3. Electrical outlets and switches on outside walls should have the covers removed and expanding foam sprayed in the cavity around the wiring box. If you don't have any foam, duct tape over the switches or outlets to seal them as much as you possibly can.
4. Doorways that are not going to be used should be sealed the same as windows. The doorways you are going to use to enter and exit the clean area (assuming you're not sealing yourself inside for the duration) will need to be closed off with overlapping sheets of plastic that will allow you to pass through without creating a 20 square-foot hole in your sealed area. 
1. In an open doorway, tape a sheet of plastic -- wide enough to cover the doorway and long enough to hang all the way to the floor -- to the top of the frame, leaving the sides free. 
2. Tape a second sheet of plastic to the top of the frame over the first one, but tape the sides of the plastic sheet to the sides of the frame and then carefully slice the second sheet from top to bottom in the center of the doorway. The idea is to make a plastic "valve" that blocks most of the airflow but will still allow a person to squeeze through. 
3. Two or more doorways like that will help keep dirty air out and clean air in. 
4. If you want to be able to close the actual door, you can nail wood to the frame of the doorway and attach the plastic sheets to that.
5. If you live in a house or apartment with unsealed wood floors (polyurethane is great), you'll need to either get underneath the floor and tape plastic to the joists, or lay plastic down on the floor of the room and cover it with rugs after taping securely. 
6. Ceilings are usually sealed better than floors, but look up and decide if there is anything that should be covered in plastic. Light fixtures and ceiling fans may need a layer of  plastic and tape if the wiring goes through to an unsealed attic or crawl space. Acoustic tiles or a drop ceiling are an indication that you'll need to get a ladder and investigate what's above your head.

Positive Pressure
Positive pressure is keeping the air pressure inside your clean room slightly higher than the pressure outside. Positive pressure systems are commonly used in hospitals for patients with failing immune systems, since they do a good job of keeping isolation rooms clean. Maintaining positive pressure will take energy to drive a blower, and sizing the blower gets difficult without knowing how well the room is sealed. More on this when I get to filters in part 2.

You don't need a lot of pressure -- typically not more than 0.05- 0.10 inches of water column*. This will force your sealing materials against any openings (providing a better seal) and prevent outside air from seeping in through cracks or openings that you may have missed.

The Next Step
Once you have a clean area established, seal up windows and other openings in the building as time and materials allow. All forms of protection are best done in layers and this is no different.

Part 2 will cover the expensive parts -- filters and monitors.


*inches of water column (iwc) is a low-pressure scale of measurement. One atmosphere of air pressure at sea level is about 14.7 pounds per square inch (psi), a common scale of measuring air pressure. 1 psi = 27.7 iwc, and 1 iwc = 0.036 psi. For reference, a 25mph wind exerts about 0.30 iwc of pressure on your skin.