Sunscreen UV Ultraviolet Light and SPF

UVA (ultraviolet-A)  Long wave solar rays of nanometer (billionths of a meter).  Although less likely than UVB to cause sunburn, UVA penetrates the skin more deeply, and is considered the chief culprit behind wrinkling, leathering, and other aspects of "photoaging."  The latest studies show that UVA not only increases UVB 's cancer-causing effects, but may directly cause some skin cancers, including melanomas.

UVB (ultraviolet-B)  Short-wave solar rays of nanometers.  More potent than UVA in producing sunburn, these rays are considered the main cause of basal and squamous cell carcinomas as well as a significant cause of melanoma.

UVC (Ultraviolet-C)  Short-wave solar rays of nanometers.  Completely absorbed by gases in the atmosphere before it reaches the ground.  The shorter the wavelength, the greater the energy level of light and the more damage it can do.

SPF- Sun Protection Factor  The effectiveness of sunscreen is indicated by the sun protection factor.  measures the length of time a product protects against skin reddening from UVB, compared to how long the skin takes to redden without protection.  If it takes 20 minutes without protection to begin reddening, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer -- about 5 hours. (Actually, it may take up to 24 hours after sun exposure for redness to become visible.) To maintain the SPF, reapply sunscreen every two hours and right after swimming.

Sunscreens and sun blocks  Sunscreens chemically absorb UV rays, sunblocks physically deflect them.  Sunscreen has long blocked UVB effectively, but until recently provided less UVA protection.  New ingredients such as octylcrylene and the benzophenones have improved sunscreen's defenses against shorter UVA rays, and the revolutionary chemical avobenzone (Parsol 1789) works against all UVA wavelengths.

Active Ingredients  Oxybenzone is a chemical used in sunscreen to block UVB. It is a derivative of Benzophenone. It is also known as 2-Hydroxy-4-methoxybenzophenone, [2-Hydroxy-4-methoxyphenyl]phenylmethanone and Benzophenone-3.sunscreenUVBBenzophenone

Avobenzone  4-t-butyl-4'-methoxy-dibenzoylmethane  C 20 H 22 O 3  protects against long wavelength UVA rays.  These active ingredients are photon absorbing agents and they function by absorbing UV radiation.

 Once the UV radiation has been absorbed, they undergo very rapid vibrational relaxation back to the ground state.  Once in the ground state, these molecules can absorb another photon to repeat the process.

Titanium Oxide  Titanium dioxide (TiO 2 ) does not absorb UV light at all, but rather blocks light from reaching your skin by reflecting or scattering it.  It is sometimes referred to as a “nonchemical” sunblock (as a chemistry student, you should find this silly because, of course, it’s a chemical). TiO 2 is different, however, from the organic compounds because the skin does not absorb it and it works by physically blocking the light.

 TiO 2 sunscreens are extremely effective and hypoallergenic.  Another consumer product also uses TiO 2 —white paint! TiO 2 sunblocks were not widely used in the past because they stayed white when applied.  The particle size of new “micronized” TiO 2 formulations is so small that TiO 2 sunblock is invisible on skin.

The photochemistry of Sunscreen  To understand how sunscreens work, we need to understand what happens when molecules interact with light energy.  In general, when a molecule absorbs a photon whose energy is high enough, an electron is promoted from a lower energy level to a higher one. The molecule is said to go from its ground state to an excited state.  Once in the excited state the molecule has several different pathways that it can take: The molecule can emit a photon and return to its ground state.

 The molecule can return to its ground state by emitting the energy thermally through a series of vibrational transitions.  The molecule can undergo some type of reaction from the excited state, which is generally termed photochemistry.  Finally, the molecule can convert to a lower energy state.  The favored path will be the one which is most rapid.

Natural protection- Melanin  Humans’ natural protection mechanism against sunlight’s (UV) damaging effect involves specialized cells called melanocytes.  When the skin is irritated by exposure to UV light, these specialized cells produce a black pigment called melanin and distribute it through the skin.  The presence of melanin results in a tanned looked.

 Melanin protects the skin by absorbing the UV radiation thus preventing the type of photochemical reactions that produce skin damage.

References   benzone.html benzone.html 