They are always exposed but rarely protected.
The skin of your lips is very fine and therefore vulnerable to sunburn.
When exposed to the sun, apply a protective balm containing sun screen (SPF 30 or 50+) several times a day.
In case of sunburn, apply honey, which has anti bacterial and curative properties. It also reduces the pain and inflammation which accompany swollen lips.
Ladies, always chose lipsticks with a minimum SPF 15.