The skin of the lips is unique and differs from ordinary skin.
Looking after your lips is particularly important because:

• Unlike other skin, the outer layer of the lip (stratum corneum) is extremely
  thin and may not even be present in some people.
• Lips do not have the oil and sweat glands present in other skin. Sweat
  glands add moisture to the skin, but the lips only source of moisture is
  saliva inside the mouth.
• Lips have no melanin, the natural pigmentation in skin which helps screen
  out the sun’s harmful rays. As a result, lips can’t tan but they can easily burn.
• Lips are constantly exposed to the sun’s rays, even when other parts of the body are clothed.
• Harsh winter wind, cold and dryness – indoors and outdoors – make lips a vulnerable target
  for chapping.