Watch any television commercials or flip through a fashion magazine, and you are sure to find ads for mascara, guaranteed to make your lashes longer, fuller, and more desirable. But eyelashes are not just cosmetic. Your eyelash length may already be predetermined.

Researchers have found that not only humans but also mammals all have eyelashes one-third as long as the eye is wide. This isn’t merely a mathematical or genetic pattern, however, but an ideal proportion to keep eyes from drying out.

Professor David Hu at the Georgia Institute of Technology became enthralled with the eyelashes on his newborn daughter, so he and his graduate biomechanics students and colleagues investigated eyelash length in detail.

The scientists measured eye last length on 22 species of mammals at the American Museum of Natural History and found that eyelash length is always around one-third the width of the eye.

They then created an artificial eye with lashes and tested it inside a wind tunnel to create a mathematical formula for airflow over lashes.

All this testing and observation led them to believe that eyelash proportions are ideal for diverting airflow around the eye to prevent the eye from drying out. This length prevented water on the eye model from evaporating as well as preventing small particles in the air from landing in the water. If eyelashes on the model were lengthened, the air currents were channeled to the eye, and it would dry out. The findings were published in the Journal of the Royal Society Interface.

These findings differ from previous theories for the purpose of eyelashes, which had included protecting the eye from dust or triggering blinking to keep them lubricated. Some had even proposed that lashes were merely for mating purposes, an attractive way to encourage seduction. While each of these may be, in fact, roles the lashes play, the unchanging proportions of eye lashes across species of mammals and with humans indicates a precise purpose such as this airflow regulation.

“Now we know why eyelashes are the length they are,” said Steven Vogel, professor emeritus at Duke. Scientists are now considering if wearing false eyelashes should be a concern for eye health.

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