HEPA (High Efficiency Particulate Air) filters are the highest efficiency air filters available for the filtration of small particles. Defined by the Institute of Environmental Science, a Certified HEPA filter must capture a minimum of 99.97% of contaminants at 0.3 microns in size.

The first HEPA filters were developed in the 1940's by the USA Atomic Energy Commission to full-fill a top-secret need for an efficient, effective way to filter radioactive particulate contaminants. They were needed as part of the Manhattan Project, which was the development of the atomic bomb. The first HEPA air filters were very bulky compared to the HEPA air filters that are produced today. HEPA filter technology was declassified after World War 2 and then allowed for commercial and residential use.

HEPA air filters have been traditionally used in hospital operating and isolation rooms, pharmaceutical and computer chip manufacturing, as well as in other applications requiring "Absolute" Filtration. Today HEPA air cleaners, vacuum cleaners and air filters are used in a wide variety of critical filtration applications in the nuclear, electronic, aerospace, pharmaceutical and medical fields. HEPA air cleaners, vacuum cleaners and air filters are required by law to be used in all equipment for asbestos, lead, toxic chemical and mold abatement. These HEPA filtered products must meet the strict Military Standard 282 HEPA filtration efficiency test.

TODAY: HEPA filtered air cleaners, air purifiers and vacuum cleaners are highly recommended for all allergy and asthma sufferers.

Building  Homemade HEPA Filter System

I use a filter system that allows me to do some sterile transfers without having to worry about some of the problems associated with infections. However, for me to build an actual HEPA filter system would have been too expensive because the HEPA filters themselves are very expensive. I have built what I call a Homemade HEPA filter system that cost less than $100.00 everything that I needed to get. This document will list all the things that you will need to build entire system.
What you need:

4 Standard Furnace Filters (about a buck each)
1 3M Filtrete Furnace Filter (about $15.00)
1 HEPA Furnace Filter (about $25.00)
1 Furnace Fan with a plug
1 4'x8' piece of 3/4" plywood
100 1 1/2" sheet rock screws
1 tube Silicone sealer
1 Roll Duct tape

Most of these things you can get at a local home improvement store. The furnace fan you may have to get at a salvage yard. I have seen the fans new but they are rather expensive and if you look you can get one for cheap.

Preparing the Filters:

When you purchase the filters, make sure that you get them all the same outside dimension. I was able to get all the filters 20"x25". Tape the filters so they are sandwiched together starting with the El Cheapo filters first, followed by the 3M Filtrete, then the home HEPA filter. Seal the edges well with the duct tape because you want no air to get by them. There are arrows pointing the direction that the air is supposed to flow. You want the arrows pointing the same direction. Here is a simple way to remember the order that you need to put the filters in: Cheapest to most expensive.

Construction of the box:

I constructed the box from a piece of plywood that I had on the garage. This was the cheapest that I could find. First you want to measure the final outside dimension of the filter. This may not be the printed dimension of the filter because they are manufactured smaller than what is printed on the filter. My filters were about 1/2 inch smaller than what was printed on the filter. The opening of the box was 24 1/2"x19 1/2" to accommodate the filter. I decided that the depth of the box should be 18 inches. This would allow for 6 inches of filter and 12 inches of plenum. The plenum is the space that the fan will force the air into.

Then seal the joints of the box with the silicone sealer. This will ensure that more air will be going through the filter and not through any holes.

After I constructed the box, I made a hole for the fan. It was on the top of the box and the outlet of the fan forced the air parallel to the filter. This is a recommended setup for a professional HEPA design. The air that is forced into the plenum will have equal pressure out of the filter.

The basic home or office air filter is simply a panel that fits over the intake of the buildings furnace or air vents. Simple ones are made from fibreglass or polyester, and allow for the passage of air but trap large particles of dust.

These filters are often placed in front of fans because the large particles of dust can accumulate over time and eventually clog up the machinery. These basic filters serve no real protection, but they do protect the machinery from burning out.

Some types of filter can be washed and reused, but the majority of them are meant to be thrown away after they become too dirty to use anymore. Then they are simply replaced with a new one.

The problem with these basic filters is, of course, that they are meant to mainly protect the furnace or air conditioner that they are used on. They may collect dust and smoke, and pollen (also good if you want to live an allergy-free lifestyle), but they do nothing against harmful moulds and bacteria. They are equally useless against airborne viruses.

Generally, basic filters are not recommended for someone in the market for a serious home air filter