The Basic Rules of Math Don't Apply to SPF Numbers.

SPF 100 does not offer twice the protection of SPF 50.
 

SPF 15 blocks approximately 93% of UVB rays, SPF 30 blocks about 97% and SPF 50 blocks about 98%. There is hardly a detectable increase in protection over 50 SPF.
 

TIP: Regardless of claims, the SPF benefit is only achieved when you use the right amount. Use a one-ounce shot glass worth of sunscreen to cover your body and a nickel-size amount to protect your face.Reapply as needed.

With each sun exposure - whether or not sunburn forms - damaged DNA triggers a cascade of detect/respond events. Repeated sun exposure lessens the skin's ability to repair itself.
 

Studies have shown that repeated UV exposure:

+ Breaks down collagen
+ Impairs synthesis of new collagen
+ Causes DNA damage accumulation
 

After decades of similar detect/respond cycles, discolorations, wrinkles and other signs of photodamage develop. Protection, repair and removal of sun-induced DNA damage is key for defense.
 

Photoaging Occurs Over a Period of Years

When skin is hit by UVA and UVB rays, damaged cells send out stress signals that begin a series of wounding responses.
 

Freckles, age spots, spider veins on the face, rough and leathery skin, fine loose and dry skin, a blotchy complexion, actinic keratosis and skin cancer can all be attributed to sun exposure. DNA damage must be repaired properly and quickly; otherwise,the visible damage to the supporting structure of the skin appears as premature aging.