Nearly all Glass and Plastics block UV-B which is needed to make Vit D. UV-A found in tanning salons oxidises the melanin pigment aka the sun tan and melanin co

Nearly all Glass and Plastics block UV-B which is needed to make Vit D. UV-A found in tanning salons oxidises the melanin pigment aka the sun tan and melanin converts 99.9% of UV radiation and converts it into heat, so the suntan regulates how much pre-vitD can be made by the skin. There is a corrugated plastic sheet (2mx1m) which allows UV-B through, but it needs replacing every few years as it breaks down and its expensive!