How Sunburns and Sun Tans Work

How Vitamin D Works
All human beings need Vitamin D in order for their bodies to grow and function properly. Vitamin D helps the body metabolize calcium. When children lack vitamin D, their bodies cannot use calcium properly and they develop rickets. Rickets is marked by “soft bones” and things like bowleggedness and knock-knees.Cod liver oil supplies Vitamin D, and it was commonly prescribed before the advent of Vitamin D fortified milk. The other way to get Vitamin D is to expose your skin to ultraviolet light. Ultraviolet light converts 7-dehydrocholesterol flowing in your bloodstream into Vitamin D. (See How Vitamin D Works for details.)

Sunscreens block or absorb ultraviolet light. You can block UV light with opaque creams like the white zinc oxide cream that you see lifeguards putting on their noses. You can also absorb UV radiation in much the same way that melanin does. The first and most common of the absorption chemicals is PABA (para-aminobenzoic acid). It absorbs UVB. If you have sensitive skin, take care when using sunscreen with PABA as it may cause irritation or an allergic reaction in some individuals. Other sunscreen chemicals include:

  • Cinnamates absorb UVB.
  • Benzophenones absorb UVA.
  • Anthranilates absorb UVA and UVB.
  • Ecamsules absorb UVA.

All sunscreens are labeled with an SPF, or Sun Protection Factor. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply an SPF 10 sunscreen, you will be OK in the sun for 100 minutes. In order for the sunscreen to work, however, you have to apply plenty and it has to stay on. You should apply it about half an hour before going out in the sun (or the water) so it can bind to your skin — if you don’t, then it is very easy for the sunscreen to wash off.

The SPF rating, by the way, applies only to UVB radiation.

In July 2006, the FDA approved Mexoryl SX (ecamsule) for sale in the United States. Mexoryl SX has been sold by L’Oreal as Anthelios SX in Europe and Canada for more than 10 years, and it does a better job of blocking UVA rays than other sunscreens on the market. Dermatologists are calling it the best sunscreen in the world.

There is something about this whole discussion that is fascinating. On your body is an organ — the skin — and it responds in all of these interesting ways to sunlight.

For more information on sunburns, sun tans, sunscreens and related topics, check out the links on the following page.