I think the point is that you can get the same vitamin D dose from a gentle exposure of a lot of skin, or a massive and damaging over-exposure of a small part o

I think the point is that you can get the same vitamin D dose from a gentle exposure of a lot of skin, or a massive and damaging over-exposure of a small part of your skin, or a spectrum in between. The same general idea that you could light a room with a single LED die overdriven to thermal death in minutes, or many adequately-cooled underdriven ones virtually indefinitely. Or maybe more similarly, cooking with a 10kW cutting laser rather than an electric stovetop would make a huge mess of your cookware.