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There is this forest in my world with the vegetation feeding off the humidity of the air. Because the plants keep draining humidity, the air becomes dry -- near 0% of humidity. Precipitation happens, and rivers flow throughout the forest, therefore it is not that hard to find water to survive. Plenty of berry bushes, mushrooms, and fruit trees populate the area, making it rich in food for herbivores. But the air is void of water. The air is harsher than a desert in nature.

Basically, this is a normal forest with no humidity in the air. I know that 100% humidity is extremely harsh on humans because they can't regulate temperature, but I have no idea of the consequence of 0% humidity. So, I was asking myself:

Can humans or other animals even breathe air that dry? For a limited/extended period of time? What sorts of effect would it have on their bodies?

Essentially: What are the consequences of living in an area with VERY dry air?

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    $\begingroup$ While some plants are able to collect humidity from the air, others (the vast majority) lose water from their leaves. Similarly there is a lot of evaporation from the rivers, the ground after a rain, etc. Additionally, your humidity-capturing plants can be only efficient at high humidity levels, and get rather rubbish (maybe even losing it!) when going seriously low. So while a drier-than-normal forest could exist if you have lots of these plants in there, it's not going to go nowhere near 0, maybe down to 20% or so like in deserts... $\endgroup$ – BIOStheZerg Jul 10 at 11:34
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    $\begingroup$ People live well enough at close to 0% relative humidity. There are many well populated places on Earth where humidity is very low in the dry season; for example, Riyah has more seven million inhabitants and the relative humidity is around 10% from June to August. On the other hand, plants don't do well at all at low relative humidity; and on the third hand, if there are rivers then relative humidity cannot be all that close to zero; on the fourth hand, the circulatory systems of plants work by transpiration and evaporation, so the presence of plants will guarantee some humidity. $\endgroup$ – AlexP Jul 10 at 13:01
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    $\begingroup$ Well, for one thing, your car won't rust as fast. $\endgroup$ – Paul TIKI Jul 10 at 14:00
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    $\begingroup$ "The air is harsher than a desert in nature." You just described winter; the humidity where I live, and probably anywhere that "winter" usually means "snow", can easily drop to 15% or lower. A great many humans experience extremely low humidity for at least several months of the year. Some of us may have issues with dry skin, but extremely low humidity is hardly fatal. $\endgroup$ – Matthew Jul 10 at 15:00
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    $\begingroup$ @Matthew: And then there's winter in the desert :-) $\endgroup$ – jamesqf Jul 10 at 17:26
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It might be pretty nice.

https://weatherspark.com/y/3513/Average-Weather-in-Taos-New-Mexico-United-States-Year-Round

taos humidity

Taos New Mexico is a great place to visit. It is beautiful high desert and stays at 0% humidity. Maybe your woods are the woods outside Taos?
Very low humidity also is a frequent occurrence when the temperature is below freezing because air capacity to hold water drops.

Humans are not laundry on a line. We are big bloody sacks of juice. We can keep surfaces moist with our internal water. Dutch is right that people get dehydrated quicker under conditions of low humidity so people would need to drink up.

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    $\begingroup$ I grew up in Albuquerque, NM not all that far from Taos (less than a day's drive and the skiing there is amazing) The humidity in ABQ made us joke that anything above 25% was muggy. An odd thing in that area is that when days reach above 100 degrees Farenheit (38 degrees C), the subsequent temperature drop after sundown would cause hypothermia cases among the homeless population. Sundown often brought a temperature drop of thirty degrees and when it's that dry, sweat cools a body down really well. $\endgroup$ – Paul TIKI Jul 10 at 13:48
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    $\begingroup$ I am pretty sure 0% in the example does not refer to relative humidity. It never drops that low under natural conditions. $\endgroup$ – Alexander Jul 10 at 16:35
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    $\begingroup$ @Alexander Actually, in the centers of large deserts (such as the Gobi or Sahara), you can get that low in natural conditions, provided there hasn't been a rainstorm recently, though it's essentially physically impossible to be truly 0% RH. $\endgroup$ – Austin Hemmelgarn Jul 10 at 20:15
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    $\begingroup$ There is a huge difference between 0% and 0.1%. At work we run a 'dry-box' that is very near 0% inside. So low that we can measure the diffusion of water vapor through a piece of silicone. Sometimes insects and spiders have gotten inside. They die within hours. Wood becomes incredibly brittle and useless. It is interesting how many materials are fine all the way up until you get right down to that 0% and then all hell breaks loose. $\endgroup$ – evildemonic Jul 10 at 20:41
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    $\begingroup$ That chart's axis is not humidity (it isn't saying it is 0% humidity in Taos). According to your link it is showing "percentage of time in which the humidity comfort level is muggy, oppressive, or miserable". 0% humidity would be miserable for a human being. $\endgroup$ – evildemonic Jul 10 at 20:47
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Short permanence in dry air gives immediate discomfort, I speak out of personal experience.

I once was naïve enough to use the dehumidifier for drying up the laundry while I was in the room. Ten minutes after switching it on (so still far from 0% humidity) were enough to have irritated eyes, itchy nose and difficult to breath.

Moreover I am sometimes required to work in environments with controlled low humidity, and the advice is to drink often and to take frequent breaks, in order to reintegrate the lost water.

Completely dry air would dry up all your surfaces exposed to air: skin, eyes, nose and respiratory system, mouth, in many cases hampering their correct functionality. I am pretty sure it is not long term livable.

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    $\begingroup$ It's pretty livable when you get used to it. I prefer really dry environments. Places like New Mexico, USA, tend to have really low humidity. So much so that you can find $250k houses up there that use Swamp coolers in the summer and those are perfectly adequate for the high summer heat. Something like a 20 to 30 degree f difference from the outside temp in a building constructed in the 1950s $\endgroup$ – Paul TIKI Jul 10 at 13:53
  • $\begingroup$ @PaulTIKI I live in southern Idaho, and agree, low humidity feels great once you get used to it. That said, there is a big difference between 20% and 0% humidity. I also run a dry box at work that is near 0%...under 5% is painful. Near zero, I am almost certain is lethal (at least it is to any bugs that get inside the dry box). $\endgroup$ – evildemonic Jul 10 at 20:51
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    $\begingroup$ @evildemonic, bugs have a surface-area-to-volume problem that causes them to dry out a lot quicker than humans do. In a 0%-humidity environment, you'll need to drink a lot more than normal, but otherwise you should do fine. $\endgroup$ – Mark Jul 11 at 1:56
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When I lived in Phoenix, AZ the humidity would hit 7% on occasion. You would generate static electricity like in the winter in the northern climates. But other than that it made the higher temperatures more tolerable as your sweat evaporates almost instantly. At around 20% humidity I could feel it as between my fingers would feel sticky. I didn't live there long enough to completely acclimate to it but a dry heat is great. On the flip side, your gulf states will hit 100% humidity on occasion and it sucks. 95 degrees Fahrenheit and 100% humidity mean you sweat just standing there. So some personal experiences with low and high humidity

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    $\begingroup$ I live on the Gulf Coast, and I personally find it much more tolerable than when I lived where the air was drier. Like low humidity, high humidity is something you can acclimate too over time. $\endgroup$ – Nosajimiki Jul 10 at 21:41
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Can be uncomfortable but definitely not deadly

Apartment Therapy summarizes the effects of extremely low humidity:

Low humidity causes static electricity, dry skin and hair, increased susceptibility to colds and respiratory illness, and can allow viruses and germs to thrive. Wood floors, furniture and millwork will split and crack, paint will chip, and electronics can be damaged because of low humidity levels.

So your people will get sick more often and their wood furniture and buildings will require more maintenance. Plus they'll feel very dry.

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I would like to show you a flaw in your concept. If plants feed on moisture from the air, how will they feed when none of it would remain? It is like wolves feeding on rabbits which well eat all the game and will therefore all die out because of starvation.

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