For humans, it’s well established that longer eyelashes are sexier. That’s especially true for women, and even men to a certain degree. However, when it comes to what eyelash length is best for our health, humans are in staunch agreement with the rest of the animal kingdom. According to scientists from Georgia Tech, the golden ratio for eyelashes is one-third the total width of the eye.
“Eyelashes form a barrier to control airflow and the rate of evaporation on the surface of the cornea,” said Guillermo Amador, a Georgia Tech Ph.D. candidate in the George W. Woodruff School of Mechanical Engineering who authored the study. “When eyelashes are shorter than the one-third ratio, they have only a slight effect on the flow. Their effect is more pronounced as they lengthen up until one-third. After that, they start funneling air and dust particles into the eye.”
This is, again, not unique to humans: In studying several species of animals, they found that every species (save for elephants) had eyelashes that obeyed this ratio.
To test why this might be the case, Amador and his colleagues built a small wind tunnel complete with an artificial eye made from an aluminum disk. Mesh was used to simulate eyelashes, and the scientists measure airflow around the eye as they altered the length of the “lashes.”
What they found was that when lashes approached the one-third ratio, they created an area of turbulence in front of the eye that kept it moist and protected from particles. Shorter eyelashes were less effective, and anything beyond the one-third ratio appeared to have the opposite affect, acting as a funnel for air and debris. Longer may be better, but only to a certain point.
As for why humans seem to prefer longer lashes (particularly in women), science isn’t 100% sure. One theory states that because our eyes remain roughly the same size while our faces grow with age, the eye-enlarging effect of long, dark lashes give the appearance of youth (and therefore reproductive fitness) in women. It’s also believed that the contrast of dark lashes makes the eyes appear whiter, similarly signaling health and vitality.
So while over the top fake eyelashes may be detrimental to health, they aren’t without their value. People with no or very short eyelashes, for instance, could benefit from using them.
“Even if they’re not the correct length, more eyelashes are always better than less,” said Alexander Alexeev, an associate professor in the School of Mechanical Engineering. “If fake eyelashes are dense enough, they may give the same overall effect in protecting the eye even if they are longer than one-third.”