Researchers from Georgia Institute of Technology have discovered that 22 species of mammals, including humans, have eyelash length one-third the width of their eye that keeps the eyes healthy.
Anything shorter or longer, including the fake eyelashes that are popular in Hollywood and make-up aisles, increases airflow around the eye and leads to more dust hitting the surface.
"Eyelashes form a barrier to control airflow and the rate of evaporation on the surface of the cornea," said Guillermo Amador from 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," he cautioned.
To reach this conclusion, Amador and the research team sent a student to the American Museum of Natural History in New York to measure eyes and eyelashes of various animals.
Aside from an elephant, which has extremely long eyelashes, every species studied had evolved to the same ratio of lash length to eye width.
The team then built the wind tunnel to re-create air flows on a mimic of an adult, human eye.
A four-millimetre deep, 20-millimetre diameter aluminium dish served as the cornea.
Mesh surrounded the dish to replicate the eyelashes.
They discovered the ideal ratio while varying the mesh length during evaporation and particle deposition studies.
"As short lashes grew longer, they reduced air flow, creating a layer of slow-moving air above the cornea," study co-author assistant professor David Hu noted.
This kept the eye moist for a longer time and kept particles away. The majority of air essentially hit the eyelashes and rolled away from the eye.
"This is why long, elegant, fake eyelashes are not ideal. They may look good, but they are not the best thing for the health of your eyes," the authors wrote.
The team also says the findings could be used to create eyelash-inspired filaments to protect solar panels, photographic sensors or autonomous robots in dusty environments.
The study was published in the Journal of the Royal Society Interface.