my understanding standing is that the older you get the less that your body can metabolize from sunlight and it’s sunlight that occurs for a small period of time during the day based on the angle of the sun and the type of UV rays that are filtered. and you need to be naked
I've read a few paper on this subject you are absolutely correct, I don't know why you are have been downvoted.
Vitamin D is only produced with in UVB wavelength [1]. No vitamin D is produced over 318nm. I wonder if that's the reason they divided the range in UVA, UVB and UVC.
The solar ray incidation angle, pollution and altitude (higher, more UVB) affects the amount of UVB that hits the surface [2].
Therefore, the time of the day you are sunbathing is important to optimize Vitamin D synthesis. Midday is the best time. If you live bellow 25 degree latitude, the UVA/UVB ratio on winter is about the same on summer. Of course, you should take in consideration the weather and the UV index. I'm talking only about the ratio.
This paper [3] has guideline for sun exposure for Australian. It is possible to correlate the data for other places in the same latitude -- taking in consideration the Australian differences in UV index.
