# What are the necessary components of air to sustain human or mammalian life? **URL:** \ **Category:** Factual Questions\ **Created:** [February 16, 2021, 6:20am UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009 "2021-02-16T06:20:03Z")\ **Posts on this page:** 6\ **Page:** 4
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [February 22, 2021, 7:28pm UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/61 "2021-02-22T19:28:59Z")
> [@Machine\_Elf](#): > > Based on math I did **[elsewhere](https://boards.straightdope.com/t/how-cold-does-it-have-to-be-before-bundling-up-does-no-good-and-you-still-freeze-to-death/933312/36)** , I’d estimate water loss due to complete humidification of 0%RH inhaled air as about 21 grams per hour, less than an ounce I can confirm that I repeated the calculation and got a number of 28 grams per hour (using your flow rate of dry air). This is comparable to your answer. However the watts needed per my calc was about 80. Not that it matters, but we can compare our calcs sometime. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [February 22, 2021, 8:05pm UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/62 "2021-02-22T20:05:39Z")
> [@LSLGuy](#): > > Part of the discomfort of long-haul air travel is that the atmosphere at altitude is so very low in humidity. Once that’s compressed to a decent pressure it has the same gas mix as ground-level air, but is still real low in water vapor. People exhale a LOT of water in that situation. > > Which creates a need to drink a lot of water too. > [@Machine\_Elf](#): > > Based on math I did **[elsewhere](https://boards.straightdope.com/t/how-cold-does-it-have-to-be-before-bundling-up-does-no-good-and-you-still-freeze-to-death/933312/36)**, I’d estimate water loss due to complete humidification of 0%RH inhaled air as about 21 grams per hour, less than an ounce. This assumes you’re at rest (e.g. stuck in an economy class seat). If you’re exerting yourself (e.g. you’re in a struggle with the Federal Air Marshal after having angrily demanded yet another glass of water)), then your tidal volume and breathing rate will be considerably higher, and so will your water loss rate. Even if you start with zero humidity air, the water that people are losing is going into the air. So as the passengers are getting dehydrated, the air in the plane is getting hydrated. Planes are sealed, so this water in the air isn’t being lost. So there must be an equilibrium point when people stop losing water into the air. Of course that point may be low enough that people die before reaching it. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [February 22, 2021, 9:50pm UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/63 "2021-02-22T21:50:28Z")
> [@Little\_Nemo](#): > > Planes are sealed, so this water in the air isn’t being lost. I am not sure this is true. I think air from the jet engines (after the compression stages) is added to keep the cabin pressurized. (After chilling of course). I have the distinct ear pain whenever the pilot “idles” the engine before landing, thereby reducing the cabin pressure. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 23, 2021, 2:34am UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/64 "2021-02-23T02:34:22Z")
ETA simulpost w @am77494 just above. > [@Little\_Nemo](#): > > Planes are sealed, so this water in the air isn’t being lost. So there must be an equilibrium point when people stop losing water into the air. Of course that point may be low enough that people die before reaching it. And that’s where you’re wrong. Planes are _not_ sealed. You get 100% all new air from the outside every 5 minutes or so. More like 3 minutes if the recirculation systems are off. Airplanes have vastly MORE fresh air than office buildings or stores do or your house has. The system is designed that way for many reasons, but smoke removal is one of the biggies. We need to keep you alive for 10-15 minutes to find a place to land despite having a smoky fire on board. That implies a need to flush a hurricane of new air into, through, and back out of the cabin. The flow of air in and out is carefully metered to ensure we can keep the tube several PSI above the outside atmosphere pressure. But that doesn’t mean no air goes out. It just means air comes in at the same rate it goes out. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 23, 2021, 11:03am UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/65 "2021-02-23T11:03:34Z")
[quote=“Little\_Nemo, post:62, topic:933009”]Of course that point may be low enough that people die before reaching it. [/quote] The very high air exchange rate on a passenger jet has already been addressed, but this point caught my interest. According to **[a psychrometric chart](https://www.youtube.com/watch?v=DhInwCMUjXQ)**, the moisture content of air at 20C and 100% RH is 14.76 grams of water per kilogram of dry air. This air has a density of 1.194 kg/m^3. If your bedroom is 4 meters x 4 meters x 3 meters, that’s 48 m^3 of air. You could fully humidify that air with just 834 grams of water, less than a liter; you’d be thirsty, but not dead. Based on earlier calcs, it would take one person about 24 hours of respiration in the room to achieve this (this disregards any water loss due to perspiration). I’m not sure how to estimate one passenger’s share of cabin volume in a jet, but I’ll wager it’s much less than 48 m^3; I suspect a sealed, full cabin would be extremely humid (and gross) in a relatively short time. You wouldn’t be dead, but you might wish you were. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [February 23, 2021, 12:38pm UTC](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009/66 "2021-02-23T12:38:41Z")
> [@LSLGuy](#): > > And that’s where you’re wrong. Planes are _not_ sealed. You get 100% all new air from the outside every 5 minutes or so. More like 3 minutes if the recirculation systems are off. Airplanes have vastly MORE fresh air than office buildings or stores do or your house has. Okay, I stand corrected. I had always thought airliners were a relatively sealed environment. [Previous page](https://boards.straightdope.com/t/what-are-the-necessary-components-of-air-to-sustain-human-or-mammalian-life/933009.md?page=3)