Sunscreen and sunblock are both sunscreens. Within this broad category are chemical and physical sunscreens. The difference between the two is how the ingredients treat ultraviolet rays.
Chemical sunscreens absorb ultraviolet radiation before it penetrates the skin. These sunscreens often are colorless and maintain a thin visible film on the skin. They usually contain UVB absorbing chemicals and more recently contain UVA absorbers as well.
Physical sunscreens, sometimes referred to as sunblocks, are products containing ingredients such a titanium dioxide and zinc oxide which physically block ultraviolet radiation (UVR) by scattering or reflecting UVR before it can penetrate the skin.
Physical sunscreens or sunblocks provide broad protection against both UVB and UVA light. However, they can be cosmetically unacceptable to many people, because they are often messy, visible and do not easily wash off. However, some new zinc oxide products are available in brightly colored preparations which are popular with young people. Physical sunscreen is recommended for individuals who have unusual sensitivity to UVR.
All sunscreens contain an SPF or sun protection factor, a measurement of the amount of UVB protection. Currently, there is no standard rating system to measure UVA protection.
SPF is not an indication of how much time you can spend in the sun. For example, if you use a sunscreen with an SPF 30 rather than one with an SPF 15, it doesn’t mean you can stay in the sun twice as long. In reality, an SPF of 15 filters out about 93 percent of the UVB rays; SPF 30 filters about 97 percent of UVB rays. The beneficial effects of sunscreen decreases over time, so after a few hours the difference between the two may be even less.
UVA, UVB and Broad-Spectrum Sunscreen
Although there is currently no rating system that measures UVA protection, both UVA and UVB are dangerous.
Ultraviolet-A: UVA are long wave solar rays. Although less likely than UVB to cause sunburn, UVA penetrates the skin more deeply and is responsible for wrinkling, leathering and other aspects of photoaging. The latest studies show that UVA increases UVB’s cancer causing effects, but may directly cause some cancers, including melanoma.
Ultraviolet-B: UVB are short wave solar rays. They are more potent than UVA in causing sunburn and are the main cause of basal cell and squamous cell carcinomas as well as a significant cause of melanoma.
Broad spectrum sunscreens protect against both UVA and UVB rays. Look on the ingredient labels for oxybenzone, sulisobenzone, avobenzone (Parsol 1789), ecamsule, titanium dioxide or zinc oxide.
Most people use sunscreen too sparingly. A liberal application of sunscreen, recommended by the medical community, is about one shot glass or one ounce or 29 milliliters of sunscreen. If you have a four-ounce tube of sunscreen, you should use a quarter of it for every application.
Apply sunscreen about thirty minutes before going outside and reapply every two hours or more often if needed.
Be safe and have fun in the sun!