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.
I secured the filters in the box with more duct tape. This worked very well because I was able to add tape to the areas that had air leaking through some of the seams. Remember, YOU WANT THE MOST EXPENSIVE FILTER TO BE THE LAST ONE THAT IS IN THE AIRFLOW !
After I built the Homemade HEPA filter, I was thinking of another idea that might work as well. A friend of mine wanted a filter setup that was portable. He has a glove box and that works well for smaller tasks but he would have to spray tons of disinfectant every time he opened the glove box. We got a small but very efficient room air filter that was made by 3M. This filter was self contained and had a fan that forced air through the filter. We took the outlet side of the filter and mounted it in a hole that we cut in the back of the glove box. We then sealed the gap between the box and the filter housing. This is a good set up because it is portable and can be stored. He still needs to use disinfectant but the amounts are considerably less that before. After we built the filter for the glove box I also thought that someone could build a large enough box and just have the filter and housing just sit in the box while you are doing the inoculations. This would work well because you are just having to filter the clean air the remains in the box
Some other Ideas for sterile work:
When you are doing inoculations using the filter, you want to have the area around the filter clean. This means that if you are doing the work in a basement you may want to seal the area off with a plastic drop cloth. This is also effective in reducing the rate of infection because many times the corner of the basement that you use has corners that are full of little nasties that would just love a meal of grain or agar. The Plastic will seal off the contaminates and prevent them from spreading to your work space. This will reduce the area that needs to be filtered and thus, reduce the contamination that will occur. Another idea would be to do inoculations in a small walk in closet or pantry.
Another idea that you may want to use is a 10% bleach-water solution in a cool mist humidifier. This is only good if you have a small area that you can do this in without concern for damage to the area (e.g. don't use this in your bedroom or everything that you own will be bleached and mom and dad will wonder why there is a bleach odor coming from your room, kick the door down and find out that you have a rather large pan of mushrooms growing and call the cops to incarcerate you for 5-10 years with good time off for good behavior and you will never be able to be a plumber because you are not allowed to hold a license of any kind because of the felony conviction prohibits you from attaining such a license.)(I guess Mario and Luigi never got busted for all those shrooms then!)
This is good because it is cheap and very effective. I was able to get a cool mist humidifier at a Thrift store for about $3.00 and I use it in addition to the filter setup that I have running.