hi mel, in my view, two ways...
one, cells in the skin makes vitamin D (cholecalciferol) from cholesterol . exactly as we take in pill form which is usually derived from sheeps lanoline.
but this form is inactive..
Uvb sunlight also activates some special cells in the skin which make the active high test form of Vitamin D..calciferol..this is approx 1000 times more potent and can moderate or turn down the tcells by locking on to their VDR's or vitamin D receptors.
The second way...certain wavelengths of uvb radiation...310-320 nm..nanometers are very effect in destroying tcells...initially this dampens the immune response because, wherever the skin is exposed to these wavelengths tcells are destroyed.....To prevent this lowering of the immune system, the skin attempts to protect itself and the tcellswith melanocytes which migrate to the surface and are what we call a suntan.. The wavelength can't penetrate the tan or melanocytes and your immune system returns to normal. The plaques or spots , which are still white..and not protected by tanning cells leave the tcells exposed in those plaques and they are destroyed if the right conditions are met.
This 310-312 nm wavelength are what the narrowband bulbs are based on....and i believe the positive effects of the dead sea also...the haze acts as a filter blocking the most energetic and most fragile but the most dangerous levels of uvb..300nm and under.....as you go down in nanometer size..the filter is more effective. This is why there is less burning..
The high energy uvb wavelengths...260 -300 nm..are small energetic wavelengths..they cause burning and genetic changes within the cell which is why they are so dangerous..but they are also fragile..glass, dense clouds, atmosphere will stop then..As the wavelengths get longer..approaching UVA light 320nm and higher..they lose energy..a little less dangerous, but have more penetration. to activate tanning cells.melanocytes.. UVA can go through clouds, more atmosphere and deeper in the skin...so a combination of the two is critical for the effect. A small window exists where enough energy to destroy tcells and enough wavelength to penetrate the skin..results in hopefully ..less P!
Of course, caution must be used. All radiation does damage...to minimize that, one must be informed about risks, how to minimize them and to use a generous amount sunscreen after the desired effects are achieved.
at least thats the way i see it....
bj...just babbling...
