Area Air Purification, Part 1: Making a Clean Room

Since there was interest in an article covering air purification for more than just personal protection, I'll do my best to cover filtering air for a small area. This is a fairly deep subject, so I'm going to break it into at least two parts.

Filtering air isn't all that much different than filtering water. If you think of a personal respirator as analogous to a personal water filter, then it isn't too much of a stretch to think of a room air purifier as a small water filtration plant or a large water filter (like a Big Berkey).

If you have to shelter in place through a disaster, it's worth considering having to filter the air you'll breathe:

• Claustrophobia is a real issue for some folks, and a mask is just not an option for them. 
• Having a filtered enclosure that you can drop over a crib or playpen is worth looking into if you have ankle-biters crawling around. 
• Setting up filtration for small children, the elderly, and the infirm is also much easier physically and emotionally if they have space in which to move.

There are some steps to go through, I'll cover the first two in this post:

1. Identify what is in the air that needs to be filtered out (contamination).
2. Keep the contamination out of your air supply.
3. Filter the air coming in.
4. Monitor for contamination.

Identify the Contamination 
This is the step where you figure out what you need to remove from the air you want to breathe.

• If your main concern is the eruption of the super-volcano under Yellowstone National Park, the main contaminant will be fine ash or dust. 
• Forest fires a few hundred miles upwind can make life miserable for people with breathing problems; the smoke and ash in the air travel a long way. 
• If you live near a chemical plant that you fear may catch fire, the contaminants will be varied and considerably harder to remove. 
• Chemical warfare attack is the worst-case scenario, requiring filters designed to remove the specific chemicals used by the military. 
• Biohazards are another possibility that can be filtered out of the air with the right equipment. Anthrax has been used to contaminate buildings before, and may be used again. Airborne diseases are a medical research field all by themselves, but the filtration is fairly standard.

Keep the Contamination Out
Sealing a room or two is about as much as most of us will be capable of doing without spending more on filters than on the house itself.

The choice of which room to use as a "clean" space will take some thought and planning. Ideally you'll want a room with as few openings and as little exposure to the outside air as possible, like a basement room. Dirt makes a good seal against the foundation and will keep out more air than most common house walls. Basement windows are usually smaller than those above ground, and entrances often have a "mudroom" or stairway leading outside which can be turned into an "airlock" for entering and exiting the clean area. An airlock is a small sealed area with two doorways, one leading to the clean area and the other leading outside. This arrangement minimizes the loss of clean air when you have to go in and out of the clean area and also gives you a place to leave dirty or contaminated clothes before entering the clean area. The idea is to never have both doors open at the same time unless there is an emergency.

If you don't have a basement (perhaps you live in an apartment building), you'll want to pick an interior room with enough space to be comfortable in and as little contact with the outside walls as possible. Use as many existing barriers between you and the outside as you can, because each one is going to slow down or stop a portion of the contamination. Think of it as allowing that muddy water to settle out before running it through your LifeStraw.

Creating a Clean Room
Plastic sheeting and duct tape will take care of sealing up large openings like doors and windows, and expanding spray foam will work around cracks and gaps. (If you've ever winterized a house, you'll know where to start.)

1. Close the windows and place a layer of plastic over them, holding it in place with duct tape. Tape along the whole border to get a good seal around the frame.
2. Find and seal (plastic and/or duct tape) all vents coming into the room. This covers heater vents, floor drains (most of which have a P-trap to prevent sewer gasses from coming up through the pipe), drier vents, sink drains (the S-trap under the sink will stop more than a P-trap but isn't perfect), showers/tubs, and toilets. If you're in a hurry, a quick shot of expanding foam into a drain will seal it in a few minutes and the foam is easy to dissolve (acetone, gasoline, etc.) or remove later, when the crisis has passed.
3. Electrical outlets and switches on outside walls should have the covers removed and expanding foam sprayed in the cavity around the wiring box. If you don't have any foam, duct tape over the switches or outlets to seal them as much as you possibly can.
4. Doorways that are not going to be used should be sealed the same as windows. The doorways you are going to use to enter and exit the clean area (assuming you're not sealing yourself inside for the duration) will need to be closed off with overlapping sheets of plastic that will allow you to pass through without creating a 20 square-foot hole in your sealed area. 
1. In an open doorway, tape a sheet of plastic -- wide enough to cover the doorway and long enough to hang all the way to the floor -- to the top of the frame, leaving the sides free. 
2. Tape a second sheet of plastic to the top of the frame over the first one, but tape the sides of the plastic sheet to the sides of the frame and then carefully slice the second sheet from top to bottom in the center of the doorway. The idea is to make a plastic "valve" that blocks most of the airflow but will still allow a person to squeeze through. 
3. Two or more doorways like that will help keep dirty air out and clean air in. 
4. If you want to be able to close the actual door, you can nail wood to the frame of the doorway and attach the plastic sheets to that.
5. If you live in a house or apartment with unsealed wood floors (polyurethane is great), you'll need to either get underneath the floor and tape plastic to the joists, or lay plastic down on the floor of the room and cover it with rugs after taping securely. 
6. Ceilings are usually sealed better than floors, but look up and decide if there is anything that should be covered in plastic. Light fixtures and ceiling fans may need a layer of  plastic and tape if the wiring goes through to an unsealed attic or crawl space. Acoustic tiles or a drop ceiling are an indication that you'll need to get a ladder and investigate what's above your head.

Positive Pressure
Positive pressure is keeping the air pressure inside your clean room slightly higher than the pressure outside. Positive pressure systems are commonly used in hospitals for patients with failing immune systems, since they do a good job of keeping isolation rooms clean. Maintaining positive pressure will take energy to drive a blower, and sizing the blower gets difficult without knowing how well the room is sealed. More on this when I get to filters in part 2.

You don't need a lot of pressure -- typically not more than 0.05- 0.10 inches of water column*. This will force your sealing materials against any openings (providing a better seal) and prevent outside air from seeping in through cracks or openings that you may have missed.

The Next Step
Once you have a clean area established, seal up windows and other openings in the building as time and materials allow. All forms of protection are best done in layers and this is no different.

Part 2 will cover the expensive parts -- filters and monitors.


*inches of water column (iwc) is a low-pressure scale of measurement. One atmosphere of air pressure at sea level is about 14.7 pounds per square inch (psi), a common scale of measuring air pressure. 1 psi = 27.7 iwc, and 1 iwc = 0.036 psi. For reference, a 25mph wind exerts about 0.30 iwc of pressure on your skin.