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.