Although Tears Are Salty Like Salt Water, Why Does Only One of Them Burns Your Eyes?


The salt concentration in sea water is about 3.5%. The concentration that would be isotonic to your body is 0.9%. So sea water is 4x the concentration.


Apparently the salt concentration in sweat is hypotonic. The concentration of NaCl in isotonic saline is 154mEq/L. Sweat is somewhere around 60-90mEq/L. Interesting study from the 50’s. But for sweat, you also have to consider what is on your skin that could get carried by the sweat dripping across your skin – lotions, oils, sun screen, makeup, etc. Those things could be irritating to your eyes as well.

There’s other stuff, like potassium, in sweat as well but my understanding is the total concentration is still hypotonic to cellular concentrations.

I was reminded that evaporation is a thing. Salt concentrations in the citation above I believe are initial concentrations and are estimates of what is actually secreted from sweat glands. So evaporation plus picking up additional solutes from the skin might actually make the sweat hypertonic. As mentioned by a couple commenters, any solution that is not isotonic could potentially cause burning sensation. This doesn’t happen to your skin because your skin has a layer of dead skin cells and keratin in addition to oils that insulate living cells from the environment. You also don’t get burning in your mouth from straight water because you have a layer of mucous protecting the living cells somewhat.


water likes to move from places with less water to places with more water. In salt solution, that means water moves from the less salty solution to the more salty solution. (the same thing happens with heat. that’s the second law of thermodynamics and what is often called entropy).

your tears have about 0.9% salt, same as your body. So when they’re in your eyes, everything is fine.sea water is about 3.5% salt. when sea water gets in your eye, the water molecules in your eye cells try to rush out to dilute the sea water. Ouch.

When plain water gets in your eye, the water molecules in the plain water try to get into your eye’s cells. Also ouch, but slightly less ouch because the difference in concentration isn’t as big.Eye-drops actually have a little bit of salt in them, enough to mimic your body’s concentration. that’s why you can bathe your eyes in eye-drops and it doesn’t hurt.your tears are your body’s own natural eye-drops.


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