The epidermis replaces all its skin cells every 28-30 days.
Skin cells contain melanin, and as a result of UV exposure, rise to the surface and flake off.
Therefore, a tan can be maintained only by repeated exposure to UV light. The SPF value is the length of time a person can be exposed to UV light without burning, when a sunscreen is applied compared to when it is not. For example, an SPF of 15 means you can be exposed 15 times longer without burning. Acrylic shields should be replaced according to the manufacture s guidelines. Unfortunately, both of these statements, without further qualification, are terribly misleading.