[decimon]

Something that's come up in the past is the effect of glass on UVB.

From Wikipedia: "Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.[6][7][8]

UVB is 280nm to 315 nm."

Even the cat on the windowsill will not get enough beneficial sunlight through glass.

[blam]

I subscribe to this magazine (Science News) and this weeks issue contained the subscribers comments to this article in the magazine.
I was a little suprised at the number that referenced an improvement in their depression symptoms from taking increased doses of vitamin D.