The human eye’s upper eyelids contain approximately 70 to 150 eyelashes each, and the lower eyelids contain between 60 and 80 eyelashes. Eyelashes form in layers of three to five on both sets of eyelids.
Eyelashes on the upper lid typically curve up, while lower eyelashes curve down to help keep dust, sweat and foreign particles out of the eyes. Small muscles in the eye contract automatically to provide protection, which is why humans blink. Eyelashes are composed of 90 percent proteins, such as melanin and keratin, and 10 percent water.
Follicles under the skin influence eyelash growth. The growth phase continues for about 45 days, while the dormant phase lasts about three weeks before the shedding phase occurs. When an eyelash falls out, it generally takes seven or eight weeks to grow back.
Eyelash diseases include trichiasis, which is when eyelashes grow inward instead of outward. Another disease is blepharitis, which is a chronic inflammation caused by allergies, infections, trauma or seborrhea. Parasitic infestations can also develop in the eyelash follicle due to mites, and this can affect eyelash growth.
Disorders include the common stye, which is caused by infected sebaceous glands or sweat glands. Two psychological disorders associated with eyelashes are trichotillomania, which compels sufferers to pull out their hair, including eyelashes, and trichophagia, which is a condition that causes individuals with trichotillomania to eat their hair.