FWIW, I was told that when the Sun is above 45 degrees over the horizon you will accrue DNA damage, and it stacks up over your entire lifetime. There is no "reset" or "heal", it just adds up.
For that reason I'm doing my darnest best to stay in the shade between 9am and 3pm. Or covering clothing.
Sort of.
Every time your skin gets 'red' due to sun exposure... that's due to DNA damage.
However, cells have several mechanisms to repair DNA - otherwise we would be in serious trouble after a single radiation burn (which is what UV light does).
They might fully repair an event successfully. Or they might not - in which case the damage may be severe and the cell will either die due to its effects or detect and trigger apoptosis. If the error isn't serious, it might not be detected and be passed on to future cell generations. Those are the ones you need to worry about.
Specifically for the skin, given that skin cells divide quite frequently, they might be caught mid-division, which is a more vulnerable state.
As you get more exposure and more damage, the chances of defects not being properly repaired increase. So you are right that, if you keep letting your skin bake, chances are you will accumulate damage that can't be repaired(over a lifetime, that's a certainty).
Cancer is not the only issue. Have you seen how the skin of people that spent a lifetime working under the sun without adequate protection look like?
A lot of what I've read lately suggests we're discovering a lot of benefits of vitamin D that were previously unknown, and some evidence that the recommended vitamin D levels should be higher than they are.
For a generation or so we've told people the sun is dangerous because of skin cancer, and obviously skin cancer is really bad. But I wonder if we have a case of need to weight risks that are high cost, low probability (skin cancer) compared with low cost, high probability (low vitamins D complications). What is the overall effect of these two things?
Short excerpt: People don’t realize this because several different diseases are lumped together under the term “skin cancer.” The most common by far are basal-cell carcinomas and squamous-cell carcinomas, which are almost never fatal. In fact, says Weller, “When I diagnose a basal-cell skin cancer in a patient, the first thing I say is congratulations, because you’re walking out of my office with a longer life expectancy than when you walked in.” That’s probably because people who get carcinomas, which are strongly linked to sun exposure, tend to be healthy types that are outside getting plenty of exercise and sunlight.
My grandpa died due to complications from a basal-cell skin cancer. He was almost 90 years old. The cancer itself was a few decades old. He served in the Navy during WWII, and likely got it from years of tropical sun exposure with no sunscreen.*
So, yeah, as far as cancers go, that's one you'd rather get if given a choice.
* (Well, and the additional years of fishing and other outdoor activities. Obviously the cause can't be pinpointed like that, but it must have contributed)
Note that if we didn't have UV repair mechanisms, we'd blister in a few minutes.
Rhee, H. J. van der, E. de Vries, and J. W. Coebergh. “Regular Sun Exposure Benefits Health.” Medical Hypotheses 97 (December 1, 2016): 34–37. https://doi.org/10.1016/j.mehy.2016.10.011.
"Since it was discovered that UV radiation was the main environmental cause of skin cancer, primary prevention programs have been started. These programs advise to avoid exposure to sunlight. However, the question arises whether sun-shunning behaviour might have an effect on general health. During the last decades new favourable associations between sunlight and disease have been discovered. There is growing observational and experimental evidence that regular exposure to sunlight contributes to the prevention of colon-, breast-, prostate cancer, non-Hodgkin lymphoma, multiple sclerosis, hypertension and diabetes. Initially, these beneficial effects were ascribed to vitamin D. Recently it became evident that immunomodulation, the formation of nitric oxide, melatonin, serotonin, and the effect of (sun)light on circadian clocks, are involved as well. In Europe (above 50 degrees north latitude), the risk of skin cancer (particularly melanoma) is mainly caused by an intermittent pattern of exposure, while regular exposure confers a relatively low risk. The available data on the negative and positive effects of sun exposure are discussed. Considering these data we hypothesize that regular sun exposure benefits health."
Hoel, David G., Marianne Berwick, Frank R. de Gruijl, and Michael F. Holick. “The Risks and Benefits of Sun Exposure 2016.” Dermato-Endocrinology 8, no. 1 (October 19, 2016). https://doi.org/10.1080/19381980.2016.1248325.
"This review considers the studies that have shown a wide range health benefits from sun/UV exposure. These benefits include among others various types of cancer, cardiovascular disease, Alzheimer disease/dementia, myopia and macular degeneration, diabetes and multiple sclerosis. The message of sun avoidance must be changed to acceptance of non-burning sun exposure sufficient to achieve serum 25(OH)D concentration of 30 ng/mL or higher in the sunny season and the general benefits of UV exposure beyond those of vitamin D."
This change in thinking has been a long-time coming. There have been results showing studies since the 90s showing lower melanoma mortality from those having more sun exposure, as described in this review:
Egan, Kathleen M., Jeffrey A. Sosman, and William J. Blot. “Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D?” JNCI: Journal of the National Cancer Institute 97, no. 3 (February 2, 2005): 161–63. https://doi.org/10.1093/jnci/dji047.
Do you know if the time of day makes a different? I avoid the Sun between 9am-3pm as that seems relatively easy to get sunburned during that time, but I wonder if the alleged benefits are tied to the same time window?
Webb, A. R., L. Kline, and M. F. Holick. “Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D3: Exposure to Winter Sunlight in Boston and Edmonton Will Not Promote Vitamin D3 Synthesis in Human Skin.” The Journal of Clinical Endocrinology and Metabolism 67, no. 2 (August 1988): 373–78. https://doi.org/10.1210/jcem-67-2-373.
Also, here's a web calculator that can help you calculate UV exposure required to get a desired amount of Vitamin D (and to avoid a sunburn) based on location, time of year and day: https://fastrt.nilu.no/VitD_quartMEDandMED_v2.html