Sunscreen works by combining organic and inorganic active ingredients. Inorganic ingredients like zinc oxide or titanium oxide reflect or scatter ultraviolet (UV) radiation. Organic ingredients like octyl methoxycinnamate (OMC) or oxybenzone absorb UV radiation, dissipating it as heat. Some sunscreens protect us from the two types of damaging UV radiation: UV-A and UV-B. Both UV-A and UV-B cause sunburns and damaging effects such as skin cancer.

Ultraviolet radiation is broken into three types of wavelengths:

• UV-A: This is the longest wavelength and is not absorbed by the ozone. It penetrates the skin deeper than UV-B.
• UV-B: Responsible for sunburns. It is partially blocked by the ozone layer.
• UV-C: This is totally absorbed by the earth's atmosphere; we encounter it only from artificial radiation sources.

The principle sunscreen is ethylhexyl p-methoxycinnamate, which provides excellent UVB and quite limited UVA absorbance. To achieve SPF 30, the sunscreen must absorb all UVB (which is the reason for the inclusion of the 2-ethylhexyl salicylate and the homosalate).

Oxybenzone provides UVB absorption, but more importantly, provides some absorption into the short UVA II (320-340 nm) region, which is essential to achieve SPF30.

Often, octadecene/MA copolymer is added as a waterproofing agent and may also thicken the sunscreen film on the skin to increase the SPF.

The C12-15 alkyl benzoate is often seen in sunscreen products. It acts as a good solubilizer for oxybenzone in addition to providing a good skin feel. The preservative system is a mixture of iodopropynyl butylcarbamate/diazolidinyl urea with some added disodium EDTA

To be able to be protected from ultraviolet rays, choose a sunscreen with a SPF 15 or higher. SPF numbers indicate the length of time one can spend in the sun without risk of burning. When using a SPF 15 sunscreen, a fair-skinned person who normally sunburns in 20 minutes of midday sun exposure may tolerate 15 times 20 minutes (300 minutes) without burning. Apply as much sunscreen as you would a lotion for dry skin. Spread it evenly over all uncovered skin, including ears and lips, but avoiding eyelids. Apply sunscreen about 30 minutes before sun exposure and reapply it every two hours even on cloudy days. If swimming or participating in intense physical activity, sunscreen may need to be applied more often.

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