Posted by Larry Hoover on December 31, 2004, at 22:33:58
In reply to Re: Light box - Vitamin D?, posted by Dave001 on December 31, 2004, at 21:24:55
> > > > I'm not sure if I get Vit. D from this sunbox.
> > >
> > > If the light can't give you a sunburn, then it can't give you vitamin D. All lightboxes are shielded against UV light, so that also blocks vitamin D formation in the skin.
> > >
> > Why is UV light necessary for a sunburn? Is the skin unable to absorb energy from the spectrum of light with wavelengths below 400 nm?
> Oops. I meant to say above 400 nm.
It absorbs that >400 nm light, and turns it into heat. The lower the wavelength, the greater the energy. Bond-disrupting energies come roughly below 320 nanometers, about the limit of UV-A. UV-A is the tanning band. UV-B, the more intense of the two UV bands (down to 290 nm), is responsible for vitamin D formation, and about 90% of the injury we call sunburn. Shorter wavelengths than UV-B are pretty much absorbed by atmospheric gases.
Sunscreens that block UV-B do help reduce sunburn, but they reduce vitamin D formation just as much.