The sun emits light at all different wavelengths, including x rays and radio waves, but 99% of its output is in the form of ultraviolet (UV), visible and infrared (IR) light. Of this 99%, approximately 46% is visible, 49% IR and 5% UV. The UV range is responsible for skin changes and aging.
The ABC’s of UV
UV C rays are the shortest, highest energy ultra violet light and are the closest to x rays on the scale (moving left). These are sterilizing rays that kill small organisms and are the most carcinogenic. Sunscreens provide no protection.
UV B rays are the burning rays, which penetrate to the epidermis and are present from 10 AM to 4PM. They are 1000 times stronger than UVA, stimulate melanin biosynthesis, and are linked to squamous cell carcinomas. They cause all the nasty symptoms of sunburn; edema, redness, and itching. They can also contribute to cataracts.
UVA, the long wave, low energy waves, are present from sunrise to sunset, enter the dermis and are 1000 times more prevalent than UVB rays. For low-energy waves they have a long list of inimical effects: they cross-link collagen and elastin, damage DNA, destroy langerhans cells and immune function, turn melanin darker and cause hyper, hypo-pigmentation and broken capillaries.
A good mnemonic is UVA = aging, UVB = burning and UVC = carcinoma
Three ingredients commonly used by sunscreen manufacturers provide protection for the longer UV wave-lengths: Avobenzone, 310-400 nm, Titanium dioxide, 290 to 360 nm, and Zinc oxide, 290-400 nm. Avobenzone looks like a good bet, but unfortunately it degrades after 30 minutes in the sun, so sunscreens including avobenzone as an active ingredient must be applied very frequently if one is to receive adequate protection.
Many companies have turned to nanotechnology for a solution. “Micronized” (micronization is a process whereby particles are reduced to 50 microns or less in size, and are referred to as nanoparticles) zinc oxide and titanium dioxide particles are absorbed into the skin, leaving no trace.
In the event that there are potential risks associated with nanoparticle use, Friends of the Earth has recently begun a campaign advising the public to avoid using products that contain them. Their argument is that since the physics of nanoparticles is different we can’t predict their behaviour. They quote a 2004 report by the United Kingdom’s Royal Society, which recommends that “ingredients in the form of nanoparticles should undergo a full safety assessment by the relevant scientific advisory body before they are permitted for use in products.” FOE points out that many companies continue to use nanoparticles, in the absence of independent safety testing, and advise a moratorium on their use until studies can demonstrate their safety.
Many companies are adding micronized mineral particles to their sunscreens, and FOE adds a caveat about this practice: “Nanoparticles of titanium dioxide and zinc oxide—used in large numbers of cosmetics, sunscreens and personal care products—have been shown to be photoactive, producing free radicals and causing DNA damage to skin cells when exposed to UV light.”
Sources for this information
Graphs and sunscreen and UV information from:
Ingredient Technology 2002: Fact vs Fiction by Dr. Diana L. Howard, VP Technical development, The International Dermal Institute
Strong: Nanotechnology: Nano-ingredients Pose Big Risks in Beauty Products : Friends of the Earth press release, May 16, 2006. http://www.foe.org/new/releases/may2006/nanorelease5162006.html
Seite S, Moyal D, Richard S, et al. Mexoryl SX: a broad absorption UVA filter protects human skin from the effects of repeated suberythemal doses of UVA. J Photochem Photobiol B. 1998;44:69-76.
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