REFLECTORS
When Christopher Columbus first explored the Caribbean island of Hispaniola in 1492, he recorded that the natives "all paint themselves, some black, some other colors, but mostly red. I have learned that they do this on account of the sun, which does not injure them as much if they are painted." This account of Native American "painting" of minerals mixed with fats or tallow has been called the first printed reference to suntan lotions (1). Yet such formulations probably were not unknown in the Old World. Greek athletes preparing for the Olympic games about 400 B.C. trained outdoors and in the nude. Because even an oliveskinned Greek would suffer under the Aegean sun, athletes are said to have rubbed themselves with a mixture of sand and oil-sand to block the damaging light and oil to provide a nonvolatile base resistant to sweat
Such formulations are remarkably similar to the reflecting sunblocks of today. Ointments still use an oily base, although mineral oil, white wax, white petrolatum, or long-chain benzoic: acid esters have replaced olive oil and tallow. The mineral component is now zinc oxide or titanium dioxide, white powders highly reflective of UV light. However, the basic principle is little changed from the days of the early Olympics. Physical sunblocks remain the most effective chemical protection from UV rays and are favored by professionals such as lifeguards who face daily exposure to intense sunlight
The main drawback of physical sunscreens is that they are opaque. Whereas children may love face painting with some of the brightly colored formulations, few adults relish looking like a clown or a ghost. For sunscreens to achieve wide acceptance, the skills of the organic chemist were needed to make a product that blocked the sun's rays while bemig itself invisible-a difficult task indeed.
ABSORBERS
In principle, the mechanism of UV absorbers is simple. A certain range of the solar spectrum is damaging to human skin. So if we coat the skin with an organic chemical that strongly absorbs that part of the spectrum, we are protected from danger, even though the coating may be transparent to the rest of the range and therefore virtually invisible to sight Which compounds are the best UV absorbers? To answer this question, we need to look first at the characteristics of the UV spectrum.
A UV PRIMER
Ultraviolet light of interest in suntanning may be divided into three classes. UVA is the lowest energy component with a wavelength of 320-400 nm. Ordinarily, it causes little burning, but it penetrates deeply, beyond the surface layer of the skin (epidermis). Over long periods of exposure, UVA is believed to cause wrinkling and premature aging of the skin.
UVB is a more energetic component, with rays in the range of 290-320 rim. These are the chief burning and tanning rays and are a major factor in producing skin cancer. However, they do not penetrate beyond the epidermis.
UVC is the most energetic component with rays in the range of 200-290 nm. 'Mese are intensely burning rays. Fortunately, virtually all of them are absorbed by the earth's own sunscreen, the ozone layer. They are not a factor for those of us on the surface of the earth, at least not yet.
Thus, the ideal organic sunscreen should strongly absorb UV fight in the range of 290-400 nm, screening out both UVA and UVB. It should also be nontoxic, nonstaining, odorless, waterproof, sweatproof, and cheap. Combining all of these factors in a single compound is a tall order, and the history of sunscreen formulations shows how difficult it has proved to be.
EARLY ABSORBERS
The first UV absorber to be widely used was para-aminobenzoic acid (PABA) which was populat in sunscreens as early as the 1920s. PABA is a good absorber of UVI3 and is relatively cheap. However, it is a poor absorber of UVA and has significant solubility in water, making it useless for swimmers. Furthermore, it tended to be irritating to many people, parficularly those who had already been slightly sunburned; indeed some people whc needed it the mos could not use it
Sunscreen teclinology took a major step forward with the development of PABA esters in the mid-1970 in which the carboxylic acid group of tf molecule has been functionalized with long,chain aliphatic alcohol such as a C 8 unit. The esterification did,not affect the UV absorption spectrium significantly, but it rendered the compound virtually insoluble in water, making possible the first "waterpro sunscreens. PABA esters were also much less irritating to the skin than their carboxylic acid precursors and were among the most popular sunscreer gredients through the 1980s.
However, in 1989 a report surh in the news media stating that a PABA ester, padimate-O, could decompose to, give an N-nitrosamine degradation product. Because some N-nitrosamines were known to be potent carcinogens this report caused a flurry of alarm among sunscreen users. The air of nitrosamines actually detecti padimate-O sunscreens was small, and these formulations had been used for fifteen years without ca any problems associated with skin cancer. Nevertheless, this issue hurt the reputation of PABA-based sunscreens and . hastened their departure from the field. Today, it is difficult to find a sunscreen that contains PABA or PABA esters.
FULL-SPECTRUM ABSORBERS
Many sunscreens currently are advertised as "full-spectrum" absorbers, meaning that they absorb both UVA and UVB. To absorb lower wavelength UVA light, a greater degree of conjugation was needed in a molecule compared with PABA derivatives. One of the first compounds to be used for broad-spectrum protection was oxybenzone, also known as benzophenone-3, which was included in sunscreens by the early 1980s. Also developed was octocrylene, regarded as one of the most "substantive" or persistent agents for waterproof formulations.
Oxybenzone and octocrylene are currently mainstays of full-spectrum formulations, although they are always combined with other agents that give overlapping absorption in the entire range of 290-400 nm. Some of these latter ingredients are derived from salicylic acid, for example, homosalate and octyl salicylate, and may be considered replacements for the PABA esters. Cinnamates, such as octyl methoxycinnamate, are also popular sunscreen ingredients.
Thanks to the efforts of chemists and formulators, the consumer now has the choice of dozens of sunscreen products. The most common oil-based lotions are those that contain moisturizers or even Vitamins A, C, or E, allegedly for extra skin care. For people who dislike the greasy feel of the oils, alcohol- and gel-based products are available, some of which claim to be "sand resistant" because they are less tacky than the oils and don't pick up sand as easily.
Whichever sunscreen is chosen, it is important to pick one strong enough for the job. Sunscreens are rated by a 4isun protection factor" (SPF), which is defined as follows:
SPF =
Time it takes to burn if wearing sunscreen/Time it takes to burn if not wearing sunscreen
Thus, if you apply an SPF 4 sunscreen, you can stay in the sun four times longer without burning than you could without wearing it. Unless you are trying to get a tan, your sunscreen should be a minimum of SPF 15. Proper application is important. If you 'Plan to do any swimming (or sweating), apply a copious amount of waterproof sunscreen half an hour beforehand. This will give it a chance to permeate the skin and provide maximum protection. To cover an entire body requires about two ounces of lotion. A standard eight-ounce bottle won't last long for someone whole' wears a bikini or a Speed
Just how dangerous is the sun for those who insist on getting a tan? 'Me majority of dermatologists argue that the tanning process is a reaction to skin damage and therefore any tan should be avoided: There is no such thing as a "healthy tan." Certainly no one who burns easily should try to get a tan, and people who tan readily should exercise moderation. Such people still need sunscreens, however, in order to keep their tans moderate. Even people who have naturally darker complexions need to use sunscreens. Nobody should get sunburned.
Some salons have advertised that they use "nonburning" ultraviolet fight (i.e., UVA light), implying that customers may tan safely. This, however, requires long exposure to radiation that may cause leathery or wrinkled skin later in life. Is a tan worth this risk?
Regardless of whether you want a tan, you no longer have to avoid the sun. With a little help from chemistry, activities in the sun have become a lot safer for everyone.
AcknowledgmentsCIENCIA/
The author would Eke to acknowledge Bill Durkin and Mike Davies of Chattem, Inc. (Chattanooga, TN) and Jim Tehan of Aloe Gator Suncare Products (Irving, TX) for their assistance with this article.
Reference
(1) The Log of Christopher Columbus; Fuson, R H., Translator; International Marine Pubfishing: Camden, ME, 1987. (Me description of Caribbean sunscreens is contained in the December 24, 1492, entry.)
Suggested reading
Gourley, P L; Gourley, G. M. Protect Your Life in
the Sun; Highlight Publishing: Albuquerque, NM, 1993.
Siegel, M-E. Safe in the Sun; Walker and Co.:
New York, NY, 1990.
WILLIAM H. STEVENSON III received a A.D. in organic chemistry the University of Ilinois, Urbana-Champaign in 1984. He lives in Huntsville, AL, where he develops new ingredients for rocket fuel. His freelance articles have appeared in The Toastmaster and Sail.