Unlike the rest of your skin, your lips have an extremely thin outer layer, which allows moisture to evaporate more quickly. Your lips also have none or just a little melanin, the natural pigment that screens out the sun. Lips also have few sebaceous glands and produce little natural oil. And, finally, lips are almost always exposed to the elements.

Although lipstick and lip gloss do provide some protection, they can't give you the same protection as a product made with an SPF rating. Don't lick your lips, because saliva acts like a lens and intensifies sun exposure.

The collagen, which gives lips body and resilience, can be altered over time. Fine lines and wrinkles can develop as well. Prevention is the best defense to keep your lips healthy, because it can take up to 15 years to see the effects of overexposure. Sun exposure can actually trigger an outbreak of cold sores, not clear them up. Lips will not tan, because they have little or no melanin.

It's important to protect your lips from the sun at all times. Although the level of UVB rays—those that contribute most to skin cancer—is greater in warmer months, the level of harmful UVA rays throughout the year is relatively constant. Clouds do not protect you from UVB rays. Snow does not absorb the sun's rays; it reflects about 90 percent of them.

Lip balm will not cause cold sores to break out, but can help prevent an outbreak by keeping lips protected from the sun. The start of a woman's menstrual cycle can also trigger cold sores, and occasionally, they will appear spontaneously.

Over time, excessive sun exposure can cause skin cancer to develop. Sun exposure can also trigger cold sores or cheilitis, a persistent inflammation of the lips.

Sensitivities or true allergies to certain types of food, cosmetics, or cigarette smoke can develop at any time. While wearing lip balm cannot prevent an allergic reaction, dry lips are more susceptible to irritation.

Lips require more moisturizing and conditioning as we age. The outer layer of skin thins even more, and natural hydration decreases.

Many prescription-strength antihistamines, antibiotics, and acne medications cause the lips to become very dry. Another side effect associated with many medications is photosensitivity. Most doctors will recommend using a strong sunscreen over all areas of the skin, including the lips, when taking these medications.