The purpose of eyelashes is to block, deflect and collect debris. Working with the blink reflex, they help keep debris out of the eye.
It seems there is another function that the eyelashes serve. Researchers at the Georgia Institute of Technology looked at 20 different mammal specials and found that the eyelash length was the same for all them and that same length matches humans. The magic length of a proper set of eyelashes was found to be one third the width of the eyelid. This length allows the creation of a pocket of slowly moving air in front of the eye. This microenvironment helps to keep dust and debris out as well as minimize moisture lost to evaporation.
If eyelashes are longer than this, for example in the case of fake eyelash extensions, wind tunnel models show the lashes actually funnel more air to the eye. It makes the eye more vulnerable, and possibly drier. It is another example of how most things in the human body tend to be designed to work just so.
The team found that shorter eyelashes had a similar effect, but this was not as bad as having long lashes because of the lack of the wind tunnel effect. This research has implications on one medication that is prescribed specifically to increase eyelash length for cosmetic purposes. It seems to be prescribed mostly by dermatologists. It will remain to be seen if this research affects the number of prescriptions written for this product.