I have to assume this is in the event of chemical, biological or nuclear attack,
All of which is VERY unlikely...
In that case, you would want to seal off a room and have POSITIVE PRESSURE in the room to keep contaminants out.
This constitutes a 'Pressure Vessel', and all rules apply same as any pressure vessel.
---------------------------------------------
I scuba with a re-breather (Draeger Mk V), and the material there will only SCRUB the CO2,
There is NO ADDED OXYGEN to replace the depleted oxygen your body used...
The Re-breathers add oxygen to the mix as needed from an O2 tank on board the scuba frame.
The training to use one CORRECTLY IS EXTENSIVE!
If you are in a LARGE, SEALED SPACE,
Then there is enough extra oxygen to keep you going for a while, but there is NO WAY to seal a large space for positive pressure unless you have the shipyards build it for you,
And again, it will have to conform to pressure vessel standards.
But you WILL need to add O2 eventually or YOU WILL DIE...
-------------------------------
The chemical is 'Soda Lime', and medical grade soda lime should ONLY be used by someone that knows EXACTLY what they are doing...
Medical grade soda lime changes color as it absorbs CO2,
But Divers don't use the dyed version since it's suspected that the dye was poisoning divers when under pressure.
(Remember that positive pressure thing you need to keep contaminants out? If not, keep reading...)
A simple indicator is used to keep track of CO2 and Oxygen content.
(Field expedient version can be rigged from a vehicle O2 sensor and volt meter)
For a small room size containment "Vessel"
("Vessel" being proper usage for a sealed environmental chamber)
One should have a 'Chest' size container, around 30 pounds, of soda lime for a 5 hour containment.
44 Lbs. of life support grade soda lime will run you about $180 with a $45 ground shipping fee.
Medical grade that changes color is about $40 a pound.
CONSTANT checks on O2 and CO2 levels will be necessary,
And automatic alarms and/or automatic provisions for rescue in the event of unconsciousness should be in place!
(IE, dead man's switch to ventilate and rescue your butt if you loose consciousness!)
--------------------------------------------
Adding oxygen or other INERT gasses will create 'Positive Pressure' in your 'Vessel' and positive pressure keeps contaminants from entering.
Inert in regards to the effect they have on the human body, most of which are EXPENSIVE.
Doors/windows MUST be sealed and pressure rated,
And doors should be "Double Sealed'...
that means an external seal when the door is 'Dogged',
(Like a large 'O' ring)
And an internal seal between door and frame, which is usually greased/inflated to fill the gap completely between door and frame...
Windows,
Internal window needs to be sealed to the INSIDE of the vessel, with a good seal, and the window MUST be pressure resistant or it will blow out with just a few psi of pressure internally.
The outer window (should there be one) needs to be sealed to the OUTSIDE of the vessel.
Usually, alcohol or other liquids are used between windows to ensure a seal is kept,
Leaks will show up as air space between windows.
The vessel can't be built out of anything rigid!
Concrete will give way at about 3 psi no matter how thick.
Steel liner that DOES NOT touch walls is best,
But even some fiberglass tanks will work as 'Pressure Chambers' or 'Containment Vessels'
------------------------------------
Long exposure to even low pressure (1-5 psi),
Can give you the "Bends" when you decompress.
Decompression should be done VERY CAREFULLY to avoid bubbles forming in the blood stream that can attack organs, nervous system, even cause clots, strokes, ect.
-------------------------------------
There are a LOT of ugly things that can happen when using pressure vessels,
And if you are NOT using a pressure vessel,
Then there shouldn't be any need for re-breather...
Medical grade HEPA filters would be MUCH cheaper and more efficient for partial filtering and you would have positive flow that wouldn't be as claustrophobic.
(HEPA: High Efficiency Particle Arrestors, very common and easy to use)
