Surface Methods – Liquid Penetrant Inspection (LPI) Exam 1 Practice

Synthesis of vitamin D in the skin is driven by UVB radiation. When UVB photons (roughly 290–315 nm) strike 7-dehydrocholesterol in the epidermis, they break a bond and form previtamin D3, which then thermally isomerizes to vitamin D3. From there, it’s processed by the liver and kidneys to become the active form.

UVA doesn’t efficiently trigger this photochemical step, and UVC is largely blocked by the atmosphere and would be harmful if it reached the skin. So UVB is the component of sunlight needed for vitamin D production.