The Sun and Your Skin

Perhaps the photochemical reactions with which we have the most direct sensory experience are those that cause skin to tan and burn.

Most of the sun's radiation that reaches the surface of the earth is in the visible and parts of the ultraviolet and infrared regions of the EM spectrum. Most of the other wavelengths are filtered out by the earth's atmosphere. This is fortunate because if much ionizing radiation reached the surface of the earth, life as we know it would not exist.

The infrared radiation from the sun that strikes your body is felt as warmth. The ultraviolet radiation that strikes your body causes photochemical reactions to occur. One such reaction which is beneficial converts a compound within your body, 7-dehydrocholesterol, to vitamin D, the "sunshine" vitamin. Too much sun in a short amount of time however causes the skin to literally burn. In lower doses, the uv radiation results in a defensive reaction - tanning of the skin, which protects the skin from further burning.

The uv radiation activates specific enzymes (certain proteins that have specific biochemical tasks) to photochemically convert tyrosine, an amino acid component of skin proteins, into the compound melanin. Melanin is brown in color and is efficient at absorbing uv radiation, so once a tan develops, the melanin sacrificially absorbs the uv radiation, thereby protecting the skin somewhat from further uv damage such as burning or activation of skin cancer formation.

The structure of melanin

Although some in our culture consider suntans to be a desirable characteristic, those in the medical community warn that excessive exposure to the sun can result in premature aging of the skin and skin cancer. Even a good dark tan does not protect the skin completely.

Sunscreens contain chemicals, the structures of which have been designed by chemists to sacrificially absorb uv radiation, in the same way as melanin, and thus protect the skin against uv damage. Uv light also produces photochemical reactions within the eye that produce cataracts and retinal damage. Dark sunglasses do not necessarily filter out uv radiation. Those that are made specifically to filter out the damaging uv radiation should always be used.

2-ethylhexyl-p-methoxycinnamate	oxybenzone
Two compounds present in many sunscreens

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