Why does shampoo make your eyes sting?
The eyes are a vulnerable entry point for bacteria, dirt, and... shampoo. But what is it in particular that makes us tear up?
Asked by: Anonymous
Vertebrate eyes need to focus a sharp image onto the retina so the surface needs to be optically very smooth. A dry eyeball would quickly get scratched and dusty and surface cells would die without soluble nutrients, causing them to slough away and break up the image quality even more.
So instead, we have evolved a wet eyeball that sits in a lubricated socket. But this presents a vulnerable point of entry for bacteria, foreign bodies and chemical contaminants. The blink reflex, and our tears, are able to wash these things away, but only if we can adequately sense the hazard in the first place.
For this reason we have the lacrimal and zygomatic nerves, which branch just beneath the cornea and make it acutely sensitive to the slightest disturbance. In particular, our eyes are very sensitive to variations in pH. Normal tears have a pH of seven, but shampoo is intended to be slightly acidic (because alkaline detergents, such as soap, damage the disulfide bonds in the keratin protein of our hair).
Baby shampoo, as well as being more dilute, has a pH much closer to seven. This makes it less stingy, at the expense of slightly reduced cleaning efficiency and somewhat more hair damage.