Monday, October 8, 2018

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.


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.)


  • 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.

  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.
  • 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.

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