Re: Sunscreen and UV

[RuthWeiner@AOL.COM]

In a message dated 8/3/02 1:10:48 PM Mountain Daylight Time, WesVanPelt@att.net writes:



> I am wondering exactly how sunscreen blocks UV radiation. When you put it on the skin, it "disappears" so it is transparent to visible light. And the layer of sunscreen is quite thin. I read recently that there are 5 or 6 different chemical compounds that can be used as sunscreen.
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> So, what is the mechanism that protects the skin from the effects of UV? Does the mechanism include some reaction with the skin? Would sunscreen spread on window glass to the same thickness screen out the same fraction of UV as on the skin?


The active ingredient in most sunscreens some derivative of para-aminobenzoic acid (PABA).  The sunscreen I use contains methyl p-amino benzoate.  These compounds absorb in the UV, not in the visible range, which is why they "disappear".  Window glass absorbs UV also, by the way.  The UV absorption of any compound depends on the amount of the compound present; in a colloidal suspension like sunscreen, it depends on the concentration. So yes, a layer of sunscreen on anything blocks UV to the same extent.


In 1974 I judged a science fair in which we gave a prize in the senior division to a girl who had smeared various sunscreens with different active ingredient concentrations on plant leaves, and had measured the extent to which the sunscreen blocked photosynthesis.  She didn't get it QUITE right -- photosynthesis used light at the red end of the visible spectrum also -- but she had very much the right experimental idea, and her results were pretty dramatic.  (She also got the prize because the whole idea and execution was hers.  All too often the entry reflects the work of some scientist or engineer Mommy or Daddy.)


Ruth Weiner, Ph. D.

ruthweiner@aol.com