I very appreciate the article, it was very good start for me, but someway I actually wanted to know more... :)
Here's a short summary of what I found:
Whenever you need protection (sunscreen, sunglasses) depend on so-called UV Index. UV Index varies during the hours of day (it's much higher during midday) and time of year. You can find actual value at local meteorological office. If the index is greater than 2 during the time of day/year you are outside, you need protection. Except you are near water or snow, which may increase intensity greatly.
E.g. in my country (Central Europe) you are safe from around begin of October to the end of February. In other countries, you may need protection around the year.
You may need more protection depending on your skin type (search for Fitzpatrick test) - lighter skin needs more protection
Now if you need protection, it's a BIG difference, whenever you have sunblock or not. E.g. one hour with SPF30 equals about 2 minute without protection (against UV-B rays... which primarily cause sunburn or tan)).
Its more complicated with UVA rays, which primarily cause skin aging (but no or just a little tan). Both UVA and UVB cause skin aging, but UVA much more than UVB. UVA is more difficult to filter/block.
You need to make sure, the sunblock you are using is marked "Broad Spectrum SPF [value]" (USA) or has UVA seal in circle (Europe). If the seal is present, then the sunblock has protection against UVA in proportion to declared SPF against UVB. There are various other systems measuring UVA/UVB protection. The actual "proportion" UVA/UVB is not really much clear to me...
E.g. if the "UVA in circle" (Europe labeling) is present - then you should have at least 1/3 of declared UVB SPF also against UVA. Very roughly, SPF30 then lets through 1/30 UVB and 1/10 UVA rays.
There is not much real difference between SPF30 and SPF50 against UVB (should be a difference of ~1.3% of rays which get through), but it could be more against UVA, as only a proportion of UVA is blocked.
Further: SPF factor value is only valid if you apply enough of the lotion... most people use too little. Also, most people don't use sunblock on all uncovered skin.
If found many references, that you need to reapply the sunblock each 2 hours OR after swimming, sweating ... E.g. if you use day cream in the morning which has SPF30 ... I have no idea (didn't succeed to find any reference) how big your protection is, if you go outside from office during noon, when you need most of the protection... It seems it is much lower than original SPF30 or even maybe zero (?).
Regarding vitamin D... according to Skin Cancer Foundation and American Cancer Society... The danger of not using sunblock is clearly higher than not getting vitamin D from sun exposure (as you may safely get vitamin D from different sources than sun).
Cancer: lifetime risk of getting melanoma is about 2.5% (1 in 40) for white people.... which doesn't look alarming, but your individual risk could be higher (e.g. if you have lighter skin type or did already a lot of sun "bathing" previously).
Most important info/quite a surprise/against "common wisdom": there is nothing like a healthy or safe way to tan. Every time you tan, you damage your skin. As this damage builds, you speed up the aging of your skin and increase your risk for all types of skin cancer. ....except using a self tanning lotion could be a safe way to add color to your skin.
Doesn't matter whenever you used sunblock or not: if your skin changes color after being exposed to sun - it's evidence of genetic damage. Even if no sunburn happened. Even if there is no color change, damage still could happen as UVA cause only little skin coloring effect.
I very appreciate the article, it was very good start for me, but someway I actually wanted to know more... :) Here's a short summary of what I found:
Some of the sources I used - https://www.who.int/news-room/questions-and-answers/item/radiation-the-ultraviolet-(uv)-index - https://www.sunsmart.com.au/uv-radiation/what-is-uv - https://www.skincancer.org/skin-cancer-prevention/sun-protection/vitamin-d/ - https://www.cancer.org/latest-news/are-you-getting-enough-vitamin-d.html - https://www.aad.org/media/stats-sunscreen