Sunscreen and sunblock contain a combination of organic and inorganic chemicals that block or absorb ultraviolet light, lessening the amount that reaches the deepest layers of your skin. Since over 90% of skin cancers are caused by sun exposure, sunscreen and sunblock are essential weapons against damaging ultraviolet radiation.

Although the terms sunblock and sunscreen are often used interchangeably, there is actually a difference between the two products.

Sunscreen works like a screen door in that it only partially filters sunlight while sunblock reflects and scatters light away so it doesn’t penetrate your skin at all. Sunscreen typically contains sun-blocking chemicals as active ingredients. Sunblock is an opaque cream that contains zinc oxide or titanium oxide, which both consist of reflective particles. In the past, it was very obvious when someone wore sunblock becaus.e it left a white coating on the skin, but nowadays the oxide particles are smaller and less visible. Sometimes you may see lifeguards wearing white zinc oxide-based sunblock on their noses.

Sunscreen and sunblock filter or block ultraviolet radiation from sunlight. The three types of ultraviolet radiation are UVA, UVB, and UVC. UVA causes premature aging and skin cancer and UVB causes tanning and sunburn. UVC is completely absorbed by the earth’s atmosphere.

The most common type of chemical used in sunscreen and sunblock is PABA (para-aminobenzoic acid), which absorbs UVB. People with sensitive skin should avoid using sunscreen and sunblock that contains PABA because it can cause allergic reactions in some individuals. Cinnamates also absorb UVB while anthranilates absorb both UVB and UVA. Benzophenones and ecamsules absorb UVA.

The Meaning of SPF

All sunscreen and sunblock products have an SPF label. SPF stands for Sun Protection Factor and indicates the length of time a sunscreen can protect you from sunburn. SPF indicates how much you are protected from UVB but not UVA since UVB is what causes sunburn. The effectiveness of SPF can be measured through multiplication. For example, if you can usually stay in the sun for 10 minutes before burning, applying sunscreen with an SPF of 10 would allow you to stay in the sun 10 times longer, or for 100 minutes.

For maximum effectiveness, apply sunblock or sunscreen 30 minutes prior to going out into the sun so it is absorbed by your skin. After sweating profusely or going swimming, reapply your sunblock or sunscreen. If you plan to be outdoors all day, reapply your sunblock or sunscreen throughout the entire day.

Prior to purchasing sunscreen or sunblock, check to see if it offers broad spectrum protection (i.e. protection from both UVA and UVB).