Sunscreen

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Sunscreen

• UV Ultraviolet Light and SPF

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UVA (ultraviolet-A)

• Long wave solar rays of 320-400 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.

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UVB (ultraviolet-B)

• Short-wave solar rays of 290-320 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.

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UVC (Ultraviolet-C)

• Short-wave solar rays of 200-290 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.

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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.

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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.

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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-methoxyphenylphenylmethanone and
  Benzophenone-3.

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Avobenzone

• 4-t-butyl-4'-methoxy-dibenzoylmethane
• C20H22O3
• protects against long wavelength UVA rays.
• These active ingredients are photon absorbing
  agents and they function by absorbing UV
  radiation.

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• 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.

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Titanium Oxide

• Titanium dioxide (TiO2) 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, its a chemical).
  TiO2 is different, however, from the organic
  compounds because the skin does not absorb it and
  it works by physically blocking the light.

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• TiO2 sunscreens are extremely effective and
  hypoallergenic.
• Another consumer product also uses TiO2white
  paint! TiO2 sunblocks were not widely used in the
  past because they stayed white when applied .
• The particle size of new micronized TiO2
  formulations is so small that TiO2 sunblock is
  invisible on skin.

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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 takeThe
  molecule can emit a photon and return to its
  ground state.

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• 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.

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Natural protection- Melanin

• Humans natural protection mechanism against
  sunlights (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.

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

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References

• http//jaxmed.com/articles/wellness/spf.htm
• http//physchem.ox.ac.uk/MSDS/AV/avobenzone.html
• www.acs.org