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So the human body temperature is much lower than that of the cave environment. That means that the air you're breathing is hotter than the inside of your lungs. In these conditions, the humidity from the hot air will start condensing in your lungs.

In the Naica Crystal Cave, conditions are such that, without also having breathing apparatus, a human will eventually die due to the high humidity because the water vapour condenses in the lungs.

I am wondering how to have an atmosphere that would cause a human to eventually develop pulmonary edema due to high water vapour and humidity. Two questions:

  • Is there any way I can have this effect happen without people dying from heat stroke far quicker?

  • What kind of gravity, atmospheric pressure, density, content, etc. would be the best way to replicate this effect and have humans die due to the water vapour condensing in their lungs? Assume that all other gases in this atmosphere are breathable and at a suitable partial pressure.

  • 1
    $\begingroup$ It isn't obviously possible to drown someone this way... your lungs already have mechanisms to remove small quantities of fluid after all, and there's only so much water you can inhale from the air. Unless you have some active cooling system though, being in an evironment that's hotter than body temperature and too humid to sweat is going to incapacitate and kill you in the near future. I don't think you can get both thine things you want at the same time. $\endgroup$ Oct 13, 2023 at 7:58
  • $\begingroup$ I asked some vaguely similar questions a while back that you might find of interest: worldbuilding.stackexchange.com/questions/235219/… worldbuilding.stackexchange.com/questions/235220/… worldbuilding.stackexchange.com/questions/235307/… worldbuilding.stackexchange.com/questions/235617/… $\endgroup$
    – Slarty
    Oct 13, 2023 at 11:03
  • 4
    $\begingroup$ The humidity level is absolutely critical. At just a few percent relative humidity humans can survive for perhaps an hour or more at 100 degrees C air temperature. But at 100% humidity at just 40 degrees C they would be overcome much faster perhaps less than half an hour. $\endgroup$
    – Slarty
    Oct 13, 2023 at 11:07
  • $\begingroup$ All very good answers! I think I now have an idea of what I could go with. An environment 45°c+ at 100% humidity should have a water partial pressure higher than 0.062 atm (and also a lot of water mist could be present). The character has a cooling suit but the respirator doesn’t work properly and slowly dies from pulmonary damage. Not sure how long this would take though. Perhaps 1-2 hours? $\endgroup$ Oct 13, 2023 at 19:25
  • $\begingroup$ @casualworldbuilder you can't have a large area at 100% humidity constantly, night and day will cause it to rain lowering the humidity, it works in a cave not outside. $\endgroup$
    – John
    Oct 13, 2023 at 21:09

5 Answers 5


The key is in the vapor pressure of water. If you can lower the vapor pressure (by lowering the temperature) to below the partial pressure of water vapor in the air, then water will condense out of the air and form a liquid.

For water to condense inside a human's lungs, then the partial pressure of water vapor in the air must be greater than the vapor pressure of water at the temperature of a human's lungs.

Human body temperature is about 37˚C. Wikipedia has a nice table showing the vapor pressure of water at various temperatures; and for a more precise number (since 37˚C falls between entries on Wikipedia's table) we can ask WolframAlpha, which reports 0.062 atmospheres.

So, you'll need to have more than 0.062 atm of water vapor in the air, and in order for all that water to be in the gas phase (and not already condensing on the walls and floor), the temperature will need to be higher than 37˚C.

Which are pretty much exactly describes conditions in Naica Crystal Cave.

Humans won't be able to shed heat by sweating (in fact, water will condense on their skin as well, heating them up even faster), so they'll develop heatstroke pretty quickly without using some technology to keep themselves cool.

But can the pulmonary edema set in before the heatstroke? I have no idea.

You may be able to make pulmonary edema occur faster by increasing the temperature and partial pressure even farther above 37˚C and 0.062 atm. This will speed up the heatstroke as well, but it'll still take some time for that heat to spread throughout the mass of the body, so the heatstroke may not speed up as much... maybe. I'm guessing wildly here. I'm not a doctor.

But, at some point, your ill-fated explorers will basically be walking into an autoclave, at which point they may die to some other condition entirely, which I do not have the words to name.

Is there some sweet spot in the middle where pulmonary edema wins? Maybe. I don't even know where to look that up.

What I can say is that gravity, density, and partial pressures of gases other than water aren't likely to make any difference at all for your question. A lack of oxygen, say, or the presence of any number of toxic gases may cause death before either heatstroke or pulmonary edema; but barring effects like that, gases other than water vapor can fairly safely be ignored.

  • 2
    $\begingroup$ There was a question several months ago, that had a graph of working capability of humans based on temperature and humidity. The question was about 50C and 90%RH, somewhat higher than this cave's conditions, however that didn't cover reasons of incapacitation, naming them "overheat". So I assume there is no such window where pulmonary edema is the primary cause of human death than plain brain overheat. $\endgroup$
    – Vesper
    Oct 13, 2023 at 5:18
  • $\begingroup$ I think the words your looking for are "boiled alive" $\endgroup$
    – No Name
    Oct 14, 2023 at 2:05

Is there any way I can have this effect happen without people dying from heat stroke far quicker?

Highly unlikely.

Wikipedia's handy water vapour pressure page says that at 50 °C, water's vapour pressure is about 12.3 kilopascals. A typical breath moves about half a liter of air through the lungs (the tidal volume). Using the ideal gas law and water's molecular weight (18 g/mol) and density (1 mL/g), this gets you at most 0.04 mL of liquid water condensing out of each breath (Google Calculator) -- this ignores both the time that full condensation would take and the water vapour that would remain in air at a lower temperature.

At 20-30 breaths a minute, that would get you about 1 mL of condensate per minute (if not much less), and over an hour that would give 60 mL of condensate. I'm sure it would be unpleasant, but given this is only one tenth of a breath's volume and 1 percent of usual total lung capacity, I don't think the pulmonary condensation will be an acute medical emergency -- not in comparison to the heatstroke one would suffer being out at 50 °C in 100% relative humidity for an hour.

What kind of gravity, atmospheric pressure, density, content, etc. would be the best way to replicate this effect and have humans die due to the water vapour condensing in their lungs?

None of these variables are relevant because water's vapour pressure is largely independent of other gases (Dalton's Law). Indeed, if there were some handwavium gas that kept water in the atmosphere at a higher vapour pressure, it would, well, keep water in the atmosphere at a higher vapour pressure and hinder condensation in your poor characters' lungs.

  • 1
    $\begingroup$ pulmonary odema is not caused by volume -- it is caused by damage to the surfactant and disruption of the alveolar capillary membrane. I don't know what the numbers are, but surface area of the lung is important, not volue. $\endgroup$
    – david
    Oct 13, 2023 at 22:18
  • $\begingroup$ In an experiment in the 60s on dogs, dogs mostly survived aspirating 2-10 mL/kg of chlorinated freshwater, albeit all showing profound acute metabolic changes. Upon extrapolation I don't think aspirating 60 mL of water (while certainly harmful and painful) will be generally lethal to a healthy adult, especially weighed against heatstroke. $\endgroup$ Oct 14, 2023 at 1:48

As the other answers point out, it's effectively impossible to conjure a situation in which a person inhales humid air and dies from pulmonary edema before they would die to heat stroke or some other immediate condition.

I would, however, propose a frame challenge. Instead of restricting yourself to water vapor, you could also consider water mist. Misted water can remain airborne for a short period of time before settling and can be easily inhaled at much colder temperatures than water vapor would require to condensate. Given enough time, a person could inhale enough mist that it would lead to pulmonary edema.

Granted, it would probably take a while, and the person in question would detect the warning signs long before it happens (like constantly coughing, for instance). A cave environment with such a scenario would also likely have a host of other problems for a spelunker as well; if there is mist being sprayed all over the place, erosion would be a significant factor in the cave's traversability and a thick blanket of slippery moss would cover every surface. But it's possible.


In fact, the inhaled air does not go directly in the lungs. If first goes through the nose and most of .the temperature change is expected to happen there. If you think about what happens in a hammam (temperature between 40 and 50°C and water vapour close to 100%), you just have to breathe slowly. If you try to breath too fast, it becomes uncomfortable because you feel burned deeply (down to the throat) but if you manage to keep calm and breath slowly you do not feel burned inside. I assume that the water vapor condenses in the nose and the upper respiratory tract, but very few liquid should reach the lung.

As said by others, if the temperature is higher, the water vapor in the lung will be a minor question, because you will have no way to control the body temperature and above 40°C you will feel really bad...


I am wondering how to have an atmosphere that would cause a human to eventually develop pulmonary edema due to high water vapour and humidity.

I may be going on a tangent here, but this is easily achieved when you have intelligent and resourceful humans... as long as they don't know how to properly manage excess heat.

Serge'sanswer has a good example of something that can happen if you don't know how to behave in a Moroccan bath. Now check this article from the US National Library of Medicine:

The military physician diagnosed a 19-year-old military recruit from an elite unit to have exertional heat stroke. Immediate treatment in the field with rapid ice water cooling and vigorous fluid administration resulted in pulmonary edema.

Dude's peers were just trying to help, but they almost killed their friend faster instead. But I expect people who live in hot areas to know better.

I also found this in MedScape:

Pulmonary edema is a common complication of heat stroke and may be due to a number of factors, including fluid overload from aggressive rehydration, renal failure, congestive heart failure, and ARDS.

You know what? The Urban Heat Island Effect is a thing. Some cities just have places that get freakingly hottter than the average temperature for the region, due to poor urban planning. And the modern, western urban dweller is a creature that is addicted to table salt, leading to renal and coronary problems being quite popular in developed countries. Going back to the National Library of Medicine:

More than 1 million patients are admitted each year with a diagnosis of pulmonary edema secondary to cardiac causes (heart failure).[4] An estimated 190,000 patients are diagnosed with acute lung injury each year. About 1.5 to 3.5 cases/100,000 population are diagnosed with ARDS.

That article is from April 2023. One million people there is 1 in every 331 americans.

Now that doesn't mean every single case of pulmonary edema followed a heat stroke, but according to USA Facts (disclaimer, I don't know how reputable a source this is, but let's assume it's accurate):

In April 2023, 69.7 million Americans lived in counties experiencing higher temperatures than the 20th-century normal.

That's almost one in five americans. Now let's elaborate:

In 2023, Americans are increasingly grappling with prolonged heat waves — sometimes dubbed “heat domes” or “heat islands” by the Environmental Protection Agency — even in areas accustomed to particularly high heat. Phoenix, Arizona, for example, registered an average daily high temperature of 114.7 degrees Fahrenheit in July.

114.7 F is about 46 Celsius.

And finally:

There were 1,714 heat-related fatalities in 2022. 2022 data is provisional. Heat-related deaths were identified using ICD codes P81.0, T67, and X30. Deaths with underlying cause W92 were excluded.

So I am not saying that those million edemas were majoritarily caused by heat, but I believe there is good cause here to justify funding research to find the degree of correlation (if this hasn't been done already, but I couldn't find anything quickly). But you know what? It may be that the conditions you need for pulmonary edemas due to heat are found on most big cities nowadays.


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