Real science is never settled, and anyone who has certainty on such things is not qualified to discuss it.
My own body tells me that in mid-day in the summer, about 45 minutes to an hour is about right, conservatively half an hour minimum. When lightly tanned. In fall/winter, much more. I have fair skin that browns somewhat with exposure, so cut that in half if I were untanned. BTW, am I less White when I am tan, since racist simpletons these days judge by skin color?
For a long time now, I must admit that I’ve had a disdain for dermatologists who see skin cancer as something that should be dealt with by avoiding sun exposure as much as possible—as if sunlight were devoid of benefits. With COVID at its worst, I saw plenty of people completely covered up in sunlight as before—starving their bodies of what is surely a health benefit.
The truth as I predict/assert it is that science will get to someday is that not only do we need sunlight for Vitamin D, sunlight has many other beneficial effects including how infrared influences mitochondria. The body has several types photo receptors and photo biomodulation is a 'thing'.
Of course, a body weakened by nutrient deficiencies and stress is not going to be able to fend of cancers of any kind. Skin cancer and other cancers IMO frequently a disease of poor healt in general that dos not allow the body to deal with the cancer. As with all things, there is just bad luck too.
Previously published solar exposure guidelines for optimal vitamin D synthesis based on a study of skin samples may need to be revised, according to new research published today in PNAS.
A study by researchers from King's College London, with support from the NIHR Guy's and St Thomas' Biomedical Research Centre, has tested the optimum ultraviolet radiation (UVR) wavelengths for human skin production of vitamin D in sunlight.
UVR from sunlight can cause sunburn and skin cancer, however, it is the most important source of vitamin D that is essential for healthy bone development and maintenance.
Public health advice on sunlight exposure takes both risk and benefits into account. Calculating the potential risks and benefits from sunlight exposure is not simple because the health outcomes from UVR exposure vary considerably with wavelength within the sun's UVR spectrum. For example, the sun's UVR contains less than 5% short wavelength UVB radiation but this is responsible for over 80% of the sunburn response. Each health outcome from solar exposure has its own unique wavelength dependency.
Professor Antony Young said: "Our study shows that risk versus benefit calculations from solar exposure may need to be re-evaluated. The results from the study are timely because the global technical committee, Commission internationale de l'éclairage, that sets UVR standards will be able to discuss the findings of this paper to re-evaluate the wavelength dependency of vitamin D. Further research from our group will determine the risk/benefit calculations."
WIND: I’d bet based on the vague statements that sunlight is much more beneficial that thought, and that the risk assessment will shift sharply to more sunlight. Any other finding seems totally nonsensical. Still, all studies should be taken with a healthy dose of skepticism.