An SPF 30 filters nearly 97% of UV rays, SPF 50 nearly 98%, while an SPF 70 filters 99%.
SPF stands for sun protection factor. Sunscreens are classified by an SPF number which refers to their ability to deflect UVB rays. The SPF rating is calculated by comparing the amount of time needed to burn sunscreen-protected skin vs. unprotected skin.
Use a broad spectrum SPF 30 or higher to protect not only against sunburn but reduce the risk of skin cancer and premature skin aging caused by the sun.
To ensure maximum SPF coverage, apply generously over exposed skin, usually about 1 oz for the full-body, or at least a quarter teaspoon for the full face.