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I am wondering why do eyes become red after getting soap in them. What is the cause behind it?

another 'Homo sapien'
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AksaK
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2 Answers2

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This is because of the soaps chemical nature. They are alkaline agents and human eyes have a very low tolerance for alkalinity. Human eyes are roughly neutral with a pH of around 7, and soaps are around 7.5 to 9.

The eye can't handle the alkaline object as it is potentially harmful to the eye so it will start watering to get rid of the alien substance. Blood vessels then enlarge, coming closer to the surface of the eye, making them appear red.

Most soaps are tested before released to make sure if they come into contact with eyes they are non-toxic. Baby soaps have a lower pH as well to make sure that the baby doesn't come to harm.

Either way, its probably best not to put it in your eyes.

See here for more information.

Beastly Gerbil
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  • Seriously, the page you link looks (in general) as realiable as yahoo answers. – Tomáš Zato Nov 29 '16 at 13:07
  • @TomášZato looks can be deceiving – Beastly Gerbil Nov 29 '16 at 13:12
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    This does not answer the question. Yes, soap/shampoo has different pH values from the standard eye environment, but that is not a really surprising answer. Why do different pH values cause redness? If we could make a "soap" that contains almost the same ingredients as actual soap but has its pH fixed to 7 still cause red eyes? Will any other substance that is wildly different from water but has ph=7 also cause red eyes? What is the function of the redness (if it has any except to tell us that we rather not put that stuff into our eyes). Etc. – AnoE Nov 29 '16 at 13:52
  • @AnoE I don't think its a function, more a reaction – Beastly Gerbil Nov 29 '16 at 13:56
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    Yeah... take my longer comment up there as an incentive to enlarge the answer to the (rather interesting) question. But if it is just a nonfunctional, arbitrary reaction, it would be at least nice to see details about the actual way it comes to pass (i.e., why does the eye get red, why does it not just start watering profusely to dilute the substance down to usual pH values). – AnoE Nov 29 '16 at 14:54
  • @AnoE if I understand right, the red is simply blood vessels having grown larger and come closer to the surface, which is why we can see the red colour. – Beastly Gerbil Nov 29 '16 at 14:57
  • Anecdotally, "tear free" shampoo still hurts my eyes. – Wayne Werner Nov 29 '16 at 20:27
  • Basically, the eye would react to anything that is not similar (w.r.t osmolarity and pH) to the solution that covers it. Eye will react to neutral substances too. Tap water (without chlorine or other such chemicals) would also hurt in the beginning. – WYSIWYG Nov 30 '16 at 07:14
  • So how do red eyes help to get rid of that pH change? – AksaK Nov 30 '16 at 07:34
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When soap gets into the eye, Our immune system reacts to the foreign substance, it releases a chemical known as histamine as a part of the inflammatory response.

Now as result of this, blood vessels in the eyes become enlarged, dilated or swollen.

It leads to redness in eyes.

Source

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