Sunscreen vs. sunblock

     Sunscreens can be classified into two categories depending on how they prevent the ultraviolet rays from reaching the skin. Chemical sunscreens contain ingredients that absorb the ultraviolet rays and prevent them from penetrating the skin. Each ingredient in chemical sunscreens can only filter a portion of the ultraviolet spectrum so most chemical sunscreens contain more than one active ingredient. Most chemical sunscreens break down after several hours of exposure to sunlight and must therefore, be reapplied frequently. Chemical sunscreens are often colorless and leave an invisible film on the skin after application. Chemical sunscreens are more likely to elicit an allergic reaction than physical blocks.

     Physical sunscreens, often referred to as sunblocks (although the FDA no longer allows this term on sunscreen labels), are products that reflect and scatter the ultraviolet light. These products contain titanium dioxide and zinc oxide, which physically block ultraviolet radiation (UVR). Physical sunblocks are considered broad-spectrum sunscreens, as they protect against UVA and UVB rays. Physical sunblocks can leave a whitish appearing film on the skin after application but they are much less likely to cause allergic reactions.