Eyelashes are seen, in many cultures as more of a fashion accessory than anything else. People use mascara to change their tint, purchase fake lashes to make them longer and in some cases even have them permanently tinted or extended.

However, eyelashes are there for purely decorative purposes. They serve as a warning system that let you know that something is getting too close to your eye and, more importantly, they protect your eyes from dust and debris.

According to a team of researchers from Georgia Institute of Technology, there is an ideal length for eyelashes to perform this function and longer is not better.

The team began by studying animals in the American Museum of Natural History. The researchers discovered that 22 species of mammals, all of the subjects they studied with the exception of elephants, had evolved eyelashes to a specific proportion.

All of the specimens lashes were one-third the width of the animal’s eye. The team found that shorter or longer lashes increased the airflow around the eyes, leading to increased dryness and more dust hitting the eye.

“Eyelashes form a barrier to control airflow and the rate of evaporation on the surface of the cornea. 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,” said Guillermo Amador in a statement.

Amador is a Georgia Tech Ph.D. candidate in the George W. Woodruff School of Mechanical Engineering and the author of a study published in the Journal of the Royal Society Interface.

The research team tested the response of eyes and lashes by building a wind tunnel to mimmic air flows. An aluminum dish, 20-millimeters in diameter and 4 millimeters deep served as the cornea. It was set on top of an acrylic plate which represented the rest of the face. Mesh was used as a replacement for eyelashes.

The length of the mesh was varied to test the impact of lash lengths on evaporation and dust deposits.

“As short lashes grew longer, they reduced air flow, creating a layer of slow-moving air above the cornea. 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,” said Assistant Professor David Hu, the leader of the team.

With longer lashes, the opposite was true. Long lashes extended further into the airflow, creating a cylinder. This increased particle accumulation on the eye and increased evaporation.

“This is why long, elegant, fake eyelashes aren’t ideal. They may look good, but they’re not the best thing for the health of your eyes,” said Amador.

The team also noted that fake lashes are not always a bad thing. In cases where people can’t grow eyelashes properly, fake lashes are better than no lashes especially if they can achieve the ideal ration.

“Even if they’re not the correct length, more eyelashes are always better than less. 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,” said Alexander Alexeev, an associate professor in the School of Mechanical Engineering.

Beyond simple human eye health, the researchers believe that the findings could have other useful applications. Fake lashes or “eyelash-inspired filaments” could be used to protect solar panels, photographic sensors and other sensitive electronic equipment.