You’re in the shower, and everything’s splendid: The water’s warm, the scent’s uplifting and you’re belting out your favorite showertime tune. But then, a stray drop of shampoo lands in your eye, feeling like the red-hot soapy trident of Satan himself. What makes this stuff hurt so damn bad?
“It’s about the pH level,” the American Optometric Association told us in an email, referring to the scientific measure of how acidic or basic a substance is. “The normal eye pH is 7-7.4, which is in the neutral range. When the pH is above that, it’s basic; below is acidic. Shampoos tend to be acidic [since basic detergents, like industrial cleaners, would damage the hair], so soap components make it more uncomfortable and irritate the eye.”
More specifically, shampoo triggers the lacrimal and zygomatic nerves sitting beneath the cornea, which perceive the slightest variation in pH as painful. They then trigger our blink reflex and tears in an attempt to remove foreign bodies or chemical contaminants.
There are some shampoos that have a neutral pH: Baby shampoos, in addition to being more diluted, have a pH much closer to seven (the normal eye pH). As a result, they sting less, but they also don’t work as well and cause more damage to the hair.
All in all, unless you’re a baby (and if you are, congratulations on your ability to read this), you’re better off simply keeping your eyes closed when aggressively shampooing your hair. Don’t worry, there aren’t any monsters in there with you—trust us.