As anyone who has ever spent too much time outdoors knows, rays from the sun are a potential source of injury. Overexposure to sunlight can lead to skin irritation, painful burns, and over time, even cancer. Sunscreen uses principles of chemistry to block or absorb the sun's harmful rays.
Significance
The purpose of sunscreen is to prevent specific rays of light from the sun from penetrating the skin. What is typically called "light" is actually just a small portion of the solar radiation spectrum. In addition to visible light, the sun also emits ultraviolet (UV) light, which is the kind that leads to sunburn and cancer. UV light can lead to burns and cancer because it breaks bonds in molecules, causing them to fall apart and malfunction. If the molecules involved happen to be genetic material---DNA---the malfunction can result in cancer.
Types
There are two major types of sunscreen. The first type involves physically blocking the sun's rays and preventing them from reaching the surface of the skin, much in the way that an umbrella or long-sleeved shirt would serve the purpose of offering protection. These types of sunscreens are opaque---either bright-white or colored---even when applied to the skin. A second type of sunscreen uses UV light-absorbing molecules. "The Chemistry of Everything" lists octyl methoxycinnamate and benzophenone as common ingredients in absorption-type sunscreen.
Function
While the mechanism for the function of opaque sunscreens is not chemically complex---any opaque surface or layer will simply prevent the penetration of light---the absorption-type sunscreens depend upon a very particular set of chemicals to function. In particular, sunscreen molecules must contain lots of carbon atoms, and the carbon atoms must be arranged into long chains or rings. While the most common kind of chemical bond is a "single" bond, many of the carbon atoms in sunscreen molecules share shorter, stronger "double" bonds. It is these double bonds that absorb UV light.
Expert Insight
Absorption-type sunscreens are actually formulated to act like melanin, the chemical responsible for making skin tan or dark. Like sunscreen, melanin has lots of carbons with double bonds, and it is able to absorb UV light. Either naturally in darker-skinned individuals, or in response to sun exposure in lighter-skinned individuals, the skin cells produce melanin to protect themselves and their genetic material from overexposure to the sun. In essence, sunscreen is designed to function just like a normal biological molecule made by the body.
Warning
Because melanin protects the skin from the sun's UV rays and is, at its heart, a natural sunscreen, many individuals assume that building a "base tan" is healthy. Unfortunately, there's a big difference between melanin produced as a result of tanning and sunscreen from a bottle---the body only increases its melanin production in response to damage. While anyone can apply sunscreen at any time to prevent sun damage, only after damage has already occurred do the cells increase melanin production, giving bottled sunscreen a distinct advantage over the body's natural mechanisms.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "The Chemistry of Everything"; Kimberley Waldron, Ph.D.; 2007
