A sun tan (see how sun tans work at www.howstuffworks.com) is the body's response to an excess of the sun's ultraviolet rays. In response to the sun's ultraviolet rays melanocytes in the skin (dermis) produce melanin pigment, called a tan. Melanin protects cells from damage from the UV rays by absorbing the radiation. Melanin production takes a fair amount of time - that is why most people cannot get a tan in one day. Too much sun exposure causes sunburn. Sunscreens protect against ultraviolet rays.

Sunscreens [local] are chemicals that block ultraviolet radiation and protect against the sun. They work [local] either by absorbing or reflecting the radiation. Choosing sunscreens involves many factors.

There are 2 types of sunscreens: organic and inorganic. Organic sunscreens are relatively complex hydrocarbon molecules that are to some degree absorbed into the skin. The most famous sunscreen is PABA (para amino benzoic acid). PABA's notoriety is due to the common allergic reaction to its presence. Most organic sunscreens block only UVB.