Posted: 06/20/2011
http://www.medscape.org/viewarticle/743531There’s no “may” about it. Neither is there any “may” about sunlight causing “photo-aging,” the development of leathery, wrinkled skin with long-term sun exposure. And the damaging effects of sunlight are not restricted to farmers toiling in the fields or to sun-worshipping yuppies in quest of that alluring tan. Children and infants are also vulnerable. Luckily, chemical protection is readily available. Uncertainties do, however, emerge when it comes to deciding on which specific chemicals to use. There is also the question of whether children, because of their more sensitive skin, need special products.
Some activist organizations claim that certain sunscreen ingredients 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.
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 UV 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 “vehicle” that allows for easy spreading. Secure Food and Drug Administration (FDA) approval, which is required unlike for other cosmetics. Quite a list of demands.
The first commercial “sunscreens” appeared in the 1960s and were designed to filter out “UVB,” the shorter wavelengths of UV light (290–320 nm). 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 PABA, octocrylene, phenylbenzimidazole sulfonic acid, and various cinnamates and salicylates being up to the task.
Products with different concentrations of these ingredients were introduced for different skin types, each prominently featuring a “sun protection factor (SPF),” 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/cm2 of product to the skin of volunteers. Using a product with an SPF of 15 means that a person who normally begins to burn in 10 minutes can in theory stay in the sun for 150 minutes before experiencing any visible effect on the skin.
It didn’t 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/cm2and 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 UV 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 (“photo-aging”).
Unlike UVB, they can even pass through glass.
Furthermore, UVA also was found to be potentially carcinogenic.
This created 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 reflect all UV light. Titanium dioxide and zinc oxide, both mineral pigments, fit the bill but left a white residue on the skin. That was all right for lifeguards’ noses, but not for vain sunbathers. The search was on for cosmetically acceptable molecules capable of absorbing UVA. Oxybenzone and avobenzone (Parsol 1789) were up to this task, but as usual, there are some “buts.”
When oxybenzone absorbs UV light, it becomes energized and some of this energy is dissipated through the production of free radicals. These are very active molecular species that have been linked to cancer. Oxybenzone also undergoes a reaction in the presence of UV 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.
This is 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 behavior of estrogens, at least in fish exposed to high doses. It has therefore been labeled a potential "endocrine disruptor."
Concern has been raised, mostly by the EWG, an American activist organization, because surveys have shown that oxybenzone can be found in the blood of 97% of the population.
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. All of this goes to say that the risks of oxybenzone as implied by the EWG are overstated.
Avobenzone is cosmetically elegant and nonirritating but becomes unstable after a couple of hours of exposure to UV light. However, its stability is increased when combined with oxybenzone, especially if another stabilizing agent known as diethylhexyl-2,6-napthalene (DEHN) is added. This combination, developed by Neutrogena, is known as Helioplex. An important question arises here: what happens to the UV energy that these chemicals absorb? The energy has to go somewhere. Might it not have a damaging effect? The answer: DEHN takes the energy absorbed by avobenzone and transfers it to oxybenzone that then fluoresces it as harmless red light.
Another effective broad-spectrum sunscreen is tetraphalydine dicamphor sulfonic acid, which goes by the trade name Mexoryl. It is stable, absorbs UV light, and dissipates the energy as harmless heat. Mexoryl isn’t absorbed through the skin and so far there are no safety issues. And recently, excellent products using “micronized” titanium dioxide and zinc oxide have been developed, which do not leave a tell-tale white residue. Presently it is difficult to judge exactly how much protection a product affords against UVA, because no SPF-like system has yet been devised. But regulatory agencies are working on it.
Sun protection products also contain ingredients beyond the ones that reduce UV light exposure. However, the EWG has also taken aim at some of these, particularly retinyl palmitate, added to some sunscreens with the aim of reducing “photo-aging,” is a potential carcinogen. The criticism is based on an inconclusive study on rodents that has not been published in the peer-reviewed literature. Furthermore, the comparison was not between sunscreens that contained retinyl palmitate and ones that didn’t. A cream that contains only retinyl palmitate is not an appropriate model for a sunscreen preparation. And why not mention a recent 2009 study that examined the combined effect of UV light and retinyl palmitate on hamster ovary cells, a protocol that is consistent with the current recommendations for effective testing of photogenotoxicity? This published peer-reviewed study concluded that retinyl palmitate had no photogenotoxic potential! Admittedly, though, the evidence that retinyl palmitate actually prevents skin damage is pretty thin, so there is really no need to include it.
There is 1 more “may” about sunscreens that has been converted to fact. We no longer have to say that sunscreens may prevent skin cancer, we can say with authority that 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 4.5 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.
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’s next?
The general recommendation is to look for a product with SPF 30 containing avobenzone, Mexoryl, titanium oxide, or zinc oxide. Since titanium dioxide (5%) and zinc oxide (10%) are the least irritating to the skin, products that use only these ingredients are the most appropriate for children.
Infants should be protected with clothing, but there is no evidence to suggest that sunscreens cannot be used on exposed parts. Fragrance-free products are also available if there is a concern about allergies and sensitivities.
The sun protection products should be applied about 15 minutes before going out in the sun.
As a general guideline, a shot glass full is needed for the body and half a teaspoon for the face.
Reapply frequently. Forget terms like “waterproof,” “all day protection,” and “sweat-proof.” They don’t mean much. And if you are buying something that is “chemical-free,” you are not getting a good deal because you’re buying a vacuum.
Sunscreens should not be used to prolong sun exposure, but rather to protect the skin when exposure is unavoidable.
Above all, it is important to remember that unfortunately there is no such thing as a healthy tan.
No comments:
Post a Comment