What If? Rejects #2.2: Tears

Credit: Miika Silfverberg.

Credit: Miika Silfverberg.

Today’s question for What If? Rejects:

Q: Is it possible to cry so much you dehydrate yourself?

Randall’s response: “…Karl [submitter], is everything OK?”

My response: Surprisingly yes…if we’re loose about how we define it.

The human body is constantly losing water through the skin as sweat, through the lungs by evaporation, and through the kidneys as urine. Whether you’re crying or not, these processes will lead to dehydration and death within a few days (depending on the climate) if you do not consume any water.

So let’s modify Karl’s question to something more concrete: Is it possible for tears to become the largest source of water loss from the body?

Surprisingly, there’s not much literature about how quickly tears are produced while crying (at least that’s readily apparent). However, we can make a rough estimate from here. While tears normally drain from the eyes to the nose, more than 10 microliters of tears will spill out of the eye. If someone is crying a lot, I could perhaps believe that this spillage might reach 100 microliters per minute per eye.

What about losses from the nose and mouth? If you’re familiar with little kids, you know they don’t just cry with their eyes. a significant share of tears drains through the nose and probably washes some mucus out along with it. However, even in the most extreme case, I can’t imagine crying and losing more than 1 milliliter per minute of water, and probably a lot less.

Now let’s compare the other sources of water loss. In healthy humans, urine production is 1-2 milliliters per minute, but in the case of disease or dehydration, this number can be much lower, so it wouldn’t be hard for it to be smaller than the tear production rate.

Perspiration can vary wildly. A table in this book suggests that while sleeping, the body produces 250-500 microliters per minute of sweat, and this presumably wouldn’t be much more if you’re awake and crying in bed. Additionally, the medical condition of hypohidrosis can lead to negligible perspiration, so this can also be lower than the tear production rate.

How much water evaporates from the lungs while breathing? In warm and humid conditions, it can be as little as 117 microliters per minute. However, this rapidly increases with physical activity or less favorable weather.

So yes, with the right set of circumstances, it is possible to cry so much that it dehydrates you faster than any of the processes in your body. It might only happen if you have certain medical conditions and are living in a tropical rainforest, but it’s possible. Whether you could cry long enough for this to be a problem is another matter.

On the other hand, if you live on Arrakis, where all other forms of water loss can be eliminated with advanced technology, this is entirely plausible.

Just watch out for sandworms, Karl.

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About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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