Reclaiming water lost from breathing
What can we do to bring water loss from breathing down to the absolute minimum possible?
Techniques or devices useful to adventurers with no frequent water access is what I am looking for, such as hiking or biking across a desert for longer than is reasonable to carry or find sufficient water.
Water does not necessarily need to be reclaimed by drinking. Any answer for which water that would be otherwise lost due to breathing but is in your body keeping you healthy instead is a good answer.
See also: "Is drinking urine safe?" (Short answer: No) Urine is another one of the biggest sources of bodily water loss, and you can distill it back into pure water with a small amount of resources and a sufficient heat source.
Food for thought: I recall that in the story "Dune" the people on that desert planet had access to wearable water reclamation equipment which reclaimed almost all of their water, not only from breathing but other sources as well. Dune is science-fiction, but it does make me wonder if such technology is actually available in reality; is there any easily portable equipment that can reclaim a significant portion of breath water in the same way?
This question is inspired from reading this answer to a question about water rationing and death from dehydration. From that question, and reading up on the topic elsewhere, it appears a reasonable portion of water lost per day can be due to water vapor lost in your breath. This can even be a significant portion of water loss in some environments, such as cold dry air.
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So with the disclaimer that this answer is not based on experience with any actual field-proven practices or tech, and posted in the hopes of contributing to idea generation by people who like to invent and test new gear:
Breath is saturated (100% humidity) and the moisture content of air at different temps varies (see https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html ) so there is reason to believe one could reduce moisture loss from the body (and thus reduce necessary water consumption) by either humidifying the air inhaled, or recapturing the air exhaled.
One method might be to figure out a way by which the moisture you breathe out migrates into the air you breathe in. I think this happens when breathing through a scarf in cold weather: the exhaled moisture condenses on the cold fibers of the scarf, and this condensate moistens the incoming air. One might conceivably optimize this via some membrane that wicks moisture from your exhaled breath into the path of inhaled air while preventing you from rebreathing the bulk of the exhaled carbon dioxide.
Another might be to use that moisture to dampen the skin—reducing sweat if not actually making it possible to reabsorb, although this would likely be undesirable in cold weather.
A third would be to trap it (condense it or absorb into some hydrophilic material) and release it later. If you assume available science-fiction energy source, this last method could work—you could capture all vapor with a tiny refrigerator coil, kill bacteria with ultraviolet or remove it with reverse osmosis, and eliminate volatile compounds (stank) with a carbon filter. The question is are their circumstances in which it’s practical: Cool desert-morning air flowing past a heat exchanger on a bike frame might be one.
This article on nih.gov pubmed indicates water loss ranges widely due to exertion and weather, from 7 to 70ml/hr. So your setup needs to be pretty light and/or your trip pretty long before it’s better than just carrying additional water. But the “heat exchanger” could literally be a crumpled beercan or length of thinwalled aluminum tube so I don’t think it’s out of the question.
Note also that the water loss calculation is based on moisture out minus moisture in, so humidifying the air you breathe in really would help.
I think humidifying breath for cold weather activity is the killer app for this. Stumbled on this 1957 report on arctic face masks for US military here. This suggests other design criteria for this device.
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In cold weather a simple thick scarf over face and nose is effective and reducing exhaled water by between a quarter and a half. On inhaling, the scarf is chilled. On exhaling, some of your breath moisture condenses. On your next inhale some of the wet scarf moistens and warms the incoming air.
Generally the scarf ends up a frozen mass from the water vapour frozen. Acrylic scarves are easier to beat the ice out of and dry.
In passing, in cold weather, about 1/4 of your energy is used to evaporate water to keep your lungs from turning into dust. (ok, hyperbole...) This calc based on effectively 0 absolute humidity in -10C air, and 100% humidity in your lungs.
The energy use is going to be similar in a hot desert, but in this case it is acting in your favour. You need to get rid of surplus heat. If you don't lose it from your lungs, you have to sweat instead. Water lost by dripping off your face onto the ground isn't going to cool you.
Ask instead, how you can reduce the heat you pick up from the environment.
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