People on the orange website were talking about some article about conference-room air making people stupid because it’s full of CO₂, sometimes several thousand ppm, while the outside air is only 400 ppm. Other high-CO₂ environments discussed included full facial motorcycle helmets at a stop light or after a crash, bedrooms at night, and so on.
It turns out that a breathing mask to eliminate carbon dioxide from input air is eminently feasible at a technical level, requiring only a few dozen grams of lithium carbonate or soda lime per day.
(This is related to House scrubber and Notes on a possible household air filter.)
One of the major reasons I’m growing aloe vera plants is to eventually be able to remedy this situation in my own house by using crassulacean acid metabolism, and in the 19th century, according to The Book of Useful Knowledge, one reason for painting buildings with whitewash was that it purified the air (p. 249):
DISINFECTANTS. Agents which destroy miasmata. … Quicklime rapidly absorbs carbonic acid [CO₂], sulphureted hydrogen [H₂S], and several other noxious gases, and is therefore commonly used as a wash for the walls of buildings.
But of course it isn’t practical to go around whitewashing every new conference room you want to attend a meeting in, although submarines do something similar by hanging up curtains containing lithium hydroxide.
Could a person sufficiently unconcerned about social acceptance work around this with a mask containing some carbon dioxide sorbent, like a rebreather, but for input air? Or would the reaction take place too slowly, or saturate the CO₂ sorbent too fast? You’d want to arrange the mask to only filter the inhaled air, not the exhaled air, perhaps using one-way valves like those used in snorkeling and scuba diving to bypass the sorbent for exhalation. A different possibility is to use nasal tubes to infuse filtered air continuously into the nasal cavity.
Spacecraft and rebreathers, like submarine emergency scrubber curtains, also typically use lithium hydroxide, but “soda lime” (75% slaked lime, Ca(OH)₂, with a catalyst mix of 20% water, 3% NaOH, 1% KOH) is a common alternative used in environments where weight is less critical, like anesthesia.
Wikipedia’s lithium hydroxide article helpfully explains:
one gram of anhydrous lithium hydroxide can remove 450 cm³ of carbon dioxide gas.
The reaction consumes 2LiOH + CO₂, while the corresponding slaked lime reaction consumes Ca(OH)₂ + CO₂, since calcium has two valence electrons to sacrifice to the hydroxyl gods. 2LiOH weighs 47.9 daltons, while Ca(OH)₂ weighs 74.093 daltons, so although soda lime might be a little heavier, the difference is not as great as you would expect from just thinking about the relative atomic numbers of lithium and calcium. There’s a corresponding difference in density so that the number of hydroxyls per unit volume is almost the same for both compounds.
So one gram of lithium hydroxide, or a gram and a half of calcium hydroxide, gives us 450 cm³ of CO₂ absorption; but how many cm³ do we need to absorb to get through the day?
Typical human breath tidal volume is 500 mℓ. Of that, normally 400 ppm is CO₂ on the way in, but perhaps more like 2000 ppm in bad circumstances, which works out to 1 mℓ of CO₂ per breath. So 450 cm³ is 450 breaths, or maybe five times that if the levels of CO₂ are low enough that you should just shut the mask off anyway. Typical adult respiratory rate is 10–20 breaths per minute, so 450 breaths is 22½–45 minutes. A whole 24-hour day, then, would be 32–64 grams of lithium carbonate.
Normal rebreathers, like those used for anesthesia and closed-circuit scuba diving, need to absorb something like 4% CO₂ from the exhalation — 40000 ppm! Moreover, they need to absorb nearly all of the CO₂ (>95%), while in this case it would probably be adequate to absorb half of it. So sorbent used here could probably have about 100× smaller surface area to take up CO₂ from the gas.