Does the human body have a relative humidity?

Okay, scoff if you must, but I have searched for this answer on the boards, and Google as well.
Does the body have a relative humidity? We’re made of so much liquid, it seems like we must. I’ve wondered this several times over the last year, nothing like the great wonders of the universe keeping you up at night!

I don’t entirely understand your question. Relative humidity is about the amount of water that’s found in aiir. We’re not made of air, so relative humidity doesn’t apply. Are you just asking what percentage of the human body is water?

I assume you mean *inside * the body?

If you fill a container half full of water, and put a lid on it, the air space above the water will eventually reach 100% RH. (An abrupt increase in temperature will make it go below 100% RH for a little while, but then it will become 100% RH again after it reaches steady state.) So I would suspect any air spaces inside the body would be at (or very close to) 100% RH.

i understand that to be the case, or close to it, for the air in our lungs. Biologists, doctors?

I am not sure I understand the question either but I got badly dehydrated a few weeks ago and had to go to the emergency room. A quick blood test confirmed how dehydrated I was and how much fluid I needed via IV. Maybe the question means some types of ratio between body solids and water.

I also don’t understand the quetsion. RH is a property of gas, the human body is a solid and so the answer to the question as stated is “No, the human bpdy doesn’t have a relative humidity”


Sorry, I wasn’t trying to be vague.
I wasn’t thinking about inside the body, but thanks crafter_man, that would make sense.
I was thinking about the air directly outside the body, and to further extend, would we be able to affect the humidity in a room, from one person to a group of people?

I imagine a person in a warm, humid room would have sweat a little, contributing to the humidity. A lot of sweaty guys in a hot room would, more than likely, effect the humidity of the room. This is just a guess, btw.

Eventually, sure. But that moisture is coming from your body, you would be dehydrating yourself.

People in a room do affect the relative (and absolute) humidity, because they sweat and they breathe out water vapor. But as I’ll explain, there could be situations in which a person in a room could lower the relative humidity, but raise the absolute humidity.

Following up on **Crafter_Man’s ** explanation, any gas will hold a certain amount of water vapor, and that amount varies with temperature. The absolute humidity is measured in mass of water vapor per volume of gas, and the relative humidity is the absolute humidity divided by the maximum absolute humidity. If the gas is at 100% absolute humidity, adding any additional water vapor results in water falling from the gas - rain.

The human body does not have a relative humidity - it does have a percent water content, which is related (because it’s a fraction) but different because the mechanisms of saturating the body with liquid water are very different from saturating a gas with water vapor.

So if you are in a sealed room, body moisture will evaporate into the air and your breath will add to the water vapor in the air as well. Most likely that will increase both the absolute and relative humidities. But your body heat will increase the air temperature as well. Warmer air can hold more water vapor than colder air, so if you increase the room temperature faster than you add water vapor, you could actually lower the relative humidity of the room air, even though you are adding water vapor and increasing the absolute humidity. At some point, though, you will reach a temperature equilibrium because you can’t increase the room temperature above 98.6 F (or whatever your normal body temperature is) with body heat alone. At that point, you’ll continue to add moisture, which will definitely increase the relative humidity. If this continues, you’ll reach 100% relative humidity, and most likely, the water vapor will condense on the walls. That’s assuming that you survive that long :stuck_out_tongue:

Is that confusing enough?

Well, yeah, when you size air conditioning units, you take into account not just the sensible cooling load (the actual heat generated) but also the latent cooling load which measures the amount of moisture generated by people, kitchens, plants, etc. A comfortable room temperature is a function of not just temperature but also humidity and one of the air conditionings primary jobs is to remove moisture from the air.

For sizing units, we used to use charts that told you how many BTUs you would need and it varied depending on building usage. Gyms wouold obviously need more than offices and very crowded places like auditoriums need a lot more than something like a residential house. Most of these calcs are done with computer programs nowdays but here’s one of the charts which shows sensible and latent heat gains for different activities (measured in watts).

Wow, thats a lot of info, thanks! I’m not as confused as when I first started, so consider ignorance fought on the subject.
Thanks to everyone who responded with answers, it’s helped me a lot.

>the relative humidity is the absolute humidity divided by the maximum absolute humidity

This isn’t correct. Relative humidity is the absolute humidity divided by the equilibrium humidity in contact with a flat surface of water. If the water surface is convex towards the air and all is equilibrated, the relative humidity will be over 100%. This is called the Kelvin effect and occurs, among other places, in clouds and in “cloud chambers” used in atomic research. If the water is concave towards the air likewise, the relative humidity will be below 100%, and this occurs in capillaries that are partly filled with liquids. This is the mechanism by which a physisorbent lowers humidity, at least in the regime where there is more than a monolayer of sorbate on the sorbent.

A little thought will suggest that the “divided by absolute maximum” definition would lead to another definition for “supersaturation” that says “…when the humidity is higher than it is possible for it to be”.