When it comes to health matters, scientists rarely make statements that do not begin with 鈥渕ay.鈥� But here is one. Excessive exposure to sunlight causes skin cancer! There鈥檚 no 鈥渕ay鈥� about it. And here is another one. Chemical protection can effectively reduce exposure. Uncertainties do, however, emerge when it comes to deciding on which specific chemicals to use. Activists claim that some sunscreens are unsafe and blame regulatory agencies for not looking after the welfare of the public, while manufacturers profess that their products have been thoroughly tested for safety and efficacy. As usual, the public is left confused. Actually, when you blow away the superfluous blather emanating both from the alarmists and from industry, there is some simple advice to offer. Let鈥檚 work it out.
The challenge is clear. Find a chemical or mixture of chemicals that can be applied to the skin to reduce exposure to the full spectrum of ultraviolet light. Then make sure these chemicals do not degrade upon exposure to light, have no topical or systemic toxicity, are minimally absorbed into the body, are resistant to water, do not have a greasy feel, are cosmetically acceptable, do not stain clothing and can be incorporated into a 鈥渧ehicle鈥� that allows for easy spreading. Quite a list of demands.聽
The first commercial 鈥渟unscreens鈥� appeared in the 1960s and were designed to filter out 鈥淯VB,鈥� the shorter wavelengths of ultraviolet light (290-320 nanometers). These are the rays that cause sunburn, which was the main concern at the time. Slightly longer waves, those responsible for tanning, were deemed safe. Finding chemicals that absorb the nasty UVB rays was not particularly difficult, with para-aminobenzoic acid (PABA), octocrylene, phenylbenzimidazole sulfonic acid and various cinnamates and salicylates being up to the task.聽
Products with different concentrations of these ingredients were introduced for different skin types, each prominently featuring a 鈥淪un Protection Factor (SPF),鈥� basically a measure of the time it takes for skin to redden compared with having no protection. The SPF value is determined in the laboratory by applying 2 mg of product per square centimeter to the skin of volunteers. Using a product with an SPF of 15 means that a person who normally begins to burn in ten minutes can in theory stay in the sun for a hundred and fifty minutes before experiencing any visible effect on the skin.
It didn鈥檛 take long for this scenario to prove to be too simplistic. As a clear link between skin cancer and UVB emerged, the focus shifted from preventing sunburn to preventing skin cancer, resulting in an industry frenzy of products with higher and higher SPF values. In truth, an SPF of 15 already blocks 94% of UVB, only 3% less than one labeled as SPF 30. In any case, these numbers are only meaningful if the product is applied the same way as in the lab studies, which turns out not to be the case. Most people were applying far less than 2 mg per square centimeter and were not getting the protection they thought they were getting. What many were getting, though, were various skin reactions. And something else became apparent as well. The longer wavelengths of ultraviolet light, 320-400 nm, known as UVA, previously thought to be innocuous, were found to be more deeply penetrating than UVB and responsible for premature wrinkling and aging of the skin (鈥減hotoaging鈥�). Unlike UVB, they can even pass through glass. Furthermore, UVA also was found to be potentially carcinogenic.
Now there was a need for a novel class of products that would protect the skin both from UVB and UVA. Ideally, not one that would just absorb some wavelengths, but one that would reduce the effects of ultraviolet light by a combination of absorption and reflection. Titanium dioxide and zinc oxide, both mineral pigments, fit the bill, but left a white residue on the skin. That was alright for lifeguards鈥� noses, but not for vane sunbathers. Recently, excellent products using 鈥渕icronized鈥� titanium dioxide and zinc oxide have been developed which do not leave a tell-tale white residue. Some concerns have been raised about titanium dioxide that is micronized to the extent that it contains particles smaller than 100 nanometers, known as 鈥渘anoparticles.鈥� Ingested titanium dioxide nanoparticles can damage the genetic material in cells, but there is no evidence that such particles are absorbed through the skin. However, if they are present in spray products, inhalation could be a problem. Another issue is that on exposure to sunlight titanium dioxide reacts with water to produce tissue damaging hydroxyl radicals. Manufacturers have approached this problem by adding chemicals such as 2-(acetoacetoxy) ethyl methacrylate that coat the titanium dioxide particles and protect them from sunlight. The sunscreen needs a sunscreen!
Some organic compounds such as oxybenzone and avobenzone (Parsol 1789) can also absorb UVA but with oxybenzone there are some 鈥渂uts. When oxybenzone absorbs ultraviolet light, it becomes energized, and this energy is dissipated mostly through heat. But the interaction of ultraviolet light and oxybenzone can also produce free radicals that have been linked to cancer. Oxybenzone also undergoes a reaction in the presence of ultraviolet light to form a compound called a semiquinone which in turn can inactivate some of the naturally occurring antioxidants in the skin, such as reduced glutathione. Not a good thing since antioxidants offer protection against free radicals. And if that weren't enough, it turns out that oxybenzone can also mimic the behaviour of estrogens, at least in fish exposed to high doses. It has therefore been labeled a potential "endocrine disruptor." Another concern about oxybenzone is that it can disrupt coral reefs.
But, and a big but it is, there is no evidence reported in the scientific literature of oxybenzone being linked to any human health problem, except for photodermatitis, a skin reaction triggered by exposure to sunlight. There are hundreds and hundreds of compounds, both natural and synthetic, that if scrutinized the same way as oxybenzone, could be linked to problems. Phthalates, bisphenol A, soy extracts and various pesticides are estrogenic. We live in a world full of hormone-like substances and a complete analysis of our blood would reveal hundreds of these. Nevertheless, producers now shy away from including oxybenzone in their products.
No single ingredient can protect the skin from all wavelengths of UV light. However, a combination of avobenzone, homosalate, octosalate and octocrylene is up to the task. Inclusion of tetraphalydine dicamphor sulphonic acid, which goes by the trade name 鈥淢exoryl鈥� increases protection from UVA. It is stable, absorbs UV light and dissipates the energy as harmless heat.
There is one more 鈥渕ay鈥� about sunscreens that has been converted to fact. We no longer have to say that sunscreens may prevent skin cancer, we can say they do. A study in Australia, where skin cancer is a huge concern, involved 1600 subjects who were given sunscreen to use every day for four and a half years. They developed 40% fewer squamous cell cancers than a control group who just maintained normal skin care without being given specific instructions about the use of sunscreens.
So, there it is. Sunscreens can prevent skin cancer, which is not a rare disease. The World Health Organization estimates 48,000 deaths a year from melanoma (likely sun related but not conclusively proven) and 12,000 from other forms of skin cancer. What to do?
Look for a product with an SPF of at least 30. Rated most highly by Consumer Reports as affording the most protection is a combination of avobenzene, homosalate, octosulfate and octocrylene as found in products such as Coppertone Water Babies Lotion, Walmart鈥檚 Equate Ultra Lotion or Eucerin Advanced Hydration Lotion. Apply fifteen minutes before going out in the sun, use a shot glass full for the body and half a teaspoon for the face. Reapply frequently. Forget terms like 鈥渨aterproof,鈥� 鈥渁ll day protection鈥� and 鈥渟weatproof.鈥� They鈥檙e meaningless. And if you are buying something that is 鈥渃hemical-free,鈥� you are not getting a good deal because you鈥檙e buying a vacuum. Sunscreens should not be used to prolong sun exposure, but rather to protect the skin when exposure is unavoidable. Above all, remember that unfortunately there is no such thing as a healthy tan.
