If you have ever thought about building a fallout shelter/safe room you know one of the major expenses is the ventilation system. Without it you would suffocate in a concrete tomb. The heart of the system is a good quality HEPA filter to keep out the radioactive dust.
Since these filters are not cheap, I wanted to find something that would work just as well for a low cost. I see 2 options. The 1st one is to use the air filter assembly off of an automobile engine. These filters are made to keep small particulants out of your engine.
If you take 2 or more of these filters and glue them together with some silicone, you can make a vary large filter quite easily. Large V8 engines will breathe over 650cfm, which should be more than adequate for most smaller shelters, so choose a filter for one of these. Anything over 350ci should be sufficient.
If you build a box to house it in, you can create a prefilter out of furnace filters. Build it fancy or not, you can staple, glue, slide fit; whatever it takes to attach them. Personally, I would build it so they could be replaced easily in case they plug up.
The next best option would be an oil bath air cleaner from an old diesel engine. You can get these from old farm equipment such as combines, balers and tractors.
These types of filters were also used on emergency vehicles when Mt. St. Helens blew it's top. They did just fine filtering out the ash, protecting the engines from the grit.
Choosing a proper fan is critical as not just any fan will do. You need one that is designed to work against a static pressure as they are built differently than fan motors that just blow air, like your common box fan. Here is one that will work for this purpose:
With a little bit of ingenuity this can be incorporated into your ventilation system and you will have an inexpensive solution to filtering your shelters air.
This could be also used to filter air for a safe room, but for that you may need the HEPA filter. I looked on FRAM's website but could find no details on how small of particles their air cleaner will filter. The oil bath method should work fine though they are a little harder to come by.
For extra protection against Biological and Chemical attack, create a prefilter of activated charcoal to absorb these agents before they hit your main filter.
Use furnace filters and hardware cloth to create a "cage" to hold the charcoal in. Be sure that it will fit around your auto filters, or create a prefilter box through which your system will draw air and fill it with the charcoal.
AlphaRubicon has some great info on this which was some of my inspiration to come up with this solution. Much of their info is for members only, but they have some valuable stuff available to the public at large.