I've never read a report about this but I have had this suspicion for a long time. On the one hand, we need to expose our skin to the sunlight, preferably on a daily basis, in order to let the body synthesize enough vitamin D. On the other, we don't want so much exposure as to cause skin cancer. All reports or articles I know simply advise moderation in exposure time. But I have a hypothesis that intensity, or rather, local intensity multipled by time of exposure, matters more. Suppose the UV light coming to your skin is x lux's in intensity and it lasts y seconds. The chance of you getting skin cancer on that spot is proportional to x*y. If this hypothesis is true, then we have a good strategy to achieve both goals at the same time, getting enough UV and avoiding skin cancer: alternate the part of your body exposed to the sun, and never let the sun light come to one small part of the skin for an extended period of time. That's exactly what I did after lunch today. I went to the back yard, facing the sun, topless (in mabu or horse stand position, practicing basic punches, but that's irrelevant). Then turned the bare back to the sun (practicing splits, again, irrelevant). Total time was about 10 minutes, split in two halves. I think it's good enough for an April sun in south Texas on a good sunny day.