1. Diminishing Marginal Sunblock Utility: SPF 50 blocks 98% of UVB rays; SPF 100 blocks 99%. SPF 30 to 50 will offer adequate sunburn protection, even for people most sensitive to sunburn.
2. Lackluster UVA Protection: UVA rays penetrate deeper into your skin, cause free radical damage, suppresses the immune system, and is associated with higher risk of developing melanoma. Most US sunscreens are designed and oriented to block UVB rays which cause sunburn. Because of the ingredients used to focus on blocking UVB rays, higher SPF sunscreens may actually block UVA rays less effectively.
3. SPF Labs Are Not Real World: Sunscreens are tested in labs under unreal and various conditions. What one company calculates to be 100 SPF another company may calculate to be 30 SPF with only the slightest changes to thickness applied, or light intensity. P&G asked the h FDA to cap SPF at 50+ because it is misleading and inappropriately influences purchase decisions.
4. False Security Leads to Bad Behavior: When sunbathers apply high SPF sunscreen it was found that they stayed in the sun longer. The effect is increased sun exposure, and sun worshippers tend not to apply sunscreen regularly because of perceived protection. In both cases, there is increased risk of skin damage.
5. Toxic Ingredients: Sun-filtering chemicals required for high SPF sunscreen can penetrate the skin causing tissue damage, hormone disruption, and allergic reaction. Without posting gains in the area of proven extra protection from skin damage, it is not worth the additional risk of such ingredients.