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Light Energy and Sunscreens

By: Peter T. Pugliese, MD
Posted: March 26, 2009, from the April 2009 issue of Skin Inc. magazine.

page 6 of 13

The dermis contains two proteins that turn over very slowly. Collagen has a half-life of 15–17 years, and elastin is even longer at 70 years. What does half-life mean? This term is best explained by an example. Let’s say that in one area of the skin there are 1,000 molecules of collagen (actually you have millions upon millions of molecules of collagen). At age 17, 50% of these molecules will have either disappeared, or have been replaced, but 500 original molecules are still there. At age 34, another 250 molecules will disappear or have been replaced, but 250 of the original molecules are still there. At age 51, there are 125 of the original molecules left and at age 68 there are 6.25 original molecules left. That means these molecules have been subjected to UV radiation continuously for 68 years. No wonder a person gets worn out with time. Do not underestimate the ravages of UV light on these long-lived connective tissue proteins. Only by reducing the insult from high-energy UV radiation can this damage be prevented or attenuated.


Products that are designated as sunscreens are designed to prevent or markedly attenuate high-energy UV from entering the skin. The formulation, manufacturing and marketing of sunscreens is regulated by the U.S. Food and Drug Administration (FDA) since sunscreens are designated as over-the-counter (OTC) drugs. They can be inorganic physical UV blocking agents, or organic chemicals that absorb UV light at specific wavelengths.

Physical, inorganic blocking agents are usually finely powdered minerals, the most common ones are zinc oxide and titanium dioxide. They act to block UV radiation by reflecting or scattering the rays, although some absorption of UV does occur. They are not normally absorbed, so they tend to produce a white color on the skin. To prevent this, the particles are often reduced in size so they no longer reflect light; particles smaller than 200 nm generally will not be seen on the skin. It is thought that such small particles may be able to penetrate the skin, but one study shows this is not the case.8 Also, in 2006, the Australian government found that zinc oxide and titanium dioxide do remain on the surface and in the outer dead layer of the skin. Bottom line, zinc oxide and titanium dioxide are safe in nanoparticle sizes. They are used alone, in combination or combined with organic chemical sunscreens, which are designed to cover absorption of either UVA or UVB, although some will cover the spectrum of both, though not completely.

UVB and sunscreens. See UVB Sunscreens for the types and amounts of each required to be in a product to provide a given sun protection factor (SPF). When buying a sunscreen, advise clients to look on the label to see how much protection they will get from the sunscreen and help them locate the SPF designation. Keep in mind that this only results in protection from UVB radiation and understand that an SPF rating is only a relative protection from skin-reddening due to the sun’s UVB rays. Remember that no sunscreen will stop 100% of UVB, but if it is applied properly, you can depend on an SPF 15 filtering out 93% of UVB, while a product with an SPF 30 filters out 97%, but even a high SPF 50 will filter out only 98%. In some individuals, such as blue-eyed blonds and green-eyed redheads, even 2–4 % of noontime UVB can induce a sunburn.

UVA and sunscreens. Remember that an SPF value does not indicate any protection against UVA. Currently the United States does not have a requirement to test the effectiveness of UVA sunscreens, although four other countries do. See UVA Sunscreens for a full list of approved sunscreens. It does have pending a recommended method to test UVA sunscreens. The methods used are both in vivo, that is, on a live human, or in vitro, instrument type, (literally, in glass). The in vivo test is called the Persistent Pigment Darkening (PPD) method. A PPD score of 10+ is considered good. This test has flaws since it is difficult to relate to real-world sun exposure, and it only measures photon energy below 320 nm.