SPF: Sun protection factor (SPF) only applies to protection from ultraviolet B (UVB) rays, not to ultraviolet A (UVA) rays. This is important as the UVB rays burn, however the UVA can cause sun damage and can increase the rate of melanoma (skin cancer) and photodermatitis.
Many sunscreens do not block UVA radiation, which does not primarily cause sunburn so the use of broad-spectrum (UVA/UVB) sunscreens can address this concern. Diligent use of sunscreen can also slow or temporarily prevent the development of wrinkles and sagging skin. Remember that it’s important to use an adequate amount and reapply after a number of hours or if your in water.
Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or chemical sunscreens (i.e., those that absorb UV light).
SPF is a reciprocal function: SPF 15 lets in 1/15 of the burning rays; SPF 30 lets in 1/30; and SPF 50 lets in 1/50. Therefore, the difference between SPF 30 and SPF 50 is only a difference of 97% effectiveness versus 98% effectiveness.