As pointed out upthread, most supplements are hilariously low-dose. I take 20k IU, 3 times per week. (most supplements are like 1000IU per pill, but 3000IU pills can be found)
Moderate exposure is healthy, and may decrease the risk of many other cancers and heart disease.
https://www.medicalnewstoday.com/articles/260247#3
https://www.telegraph.co.uk/news/2018/03/07/dose-sunshine-ex...
https://www.marksdailyapple.com/why-some-sun-exposure-will-p...
Another way to look at it is that we evolved with the sun over millions of years, and that wouldn't have happened if it were too bad for us...
On your evolution comment, most Australians don't have dark skin, but the ones who were here before Europeans do. I think that is a good indicator that the body is adjusting to something harmful. On the other hand, many dark skinned people in Britain aren't in a climate they evolved for either. We all have to be mindful that people move around the world faster than evolution now and may need to adjust our habits.
This paper [1] has the recommended exposure time for a few cities in Australia. If you live in the same latitude, as long you live in the same latitude, this can be used as a guideline.
https://staging.mja.com.au/system/files/issues/194_07_040411...
I couldn't believe it the first time I went to Singapore, in 35C heat all day in the sun at a theme park, and didn't even get a minor blush on my pale skin due to the humidity and latitude. Definitely eye opening coming from NSW, Australia.
From the table there, for example, in Townsville in summer, with 11% of your body exposed, it takes just 6 minutes to synthesis 1000 IU of vitamin D.
A fair-skinned female friend who studied at James Cook University used to complain she got sunburnt walking between classes... So DWG, if you read this, apologies for telling you off for exaggerating!
Vitamin D is produced on UVB spectrum [1 Fig. 1] that decreases with the solar azimuth degree [2].
[1] https://www.direct-ms.org/wp-content/uploads/2018/01/Vit-D-s...
[2] https://www.researchgate.net/publication/285056396_Vitamin_D...
Over exposure to sunshine is a bad idea. Depending on latitude, time of year, skin colour and a few other very well understood factors determines what is a safe exposure to the sun. We all know this already. Getting outside generally involves fresh, moving air. There is also the risk of gentle, through moderate to vigorous exercise.
Wear a hat and sunscreen if required but a good thing you can do to fight this nasty (all other things being equal) is take moderate exercise outside daily. Don't overdo it and keep away from other people.
I won't recommend a dose of Vit D because I am not a doctor but this: https://en.wikipedia.org/wiki/Vitamin_D#Recommended_levels is probably a good start.
It depends on a lot of things. Time of year, time of day, skin colour, where you live, if you're under an Ozone hole, UV index, personal sensitivity/medications, etc.
This is a good summary[0] for the situation in Australia, possibly the worst in the world when it comes to the sun being deadly. Their strongest statement about avoiding sun:
"During summer in Australia, all states experience long periods during the day when the UV Index is 3 or above (see Table 1). During these periods, a combination of sun protection measures (broad brimmed hat, covering clothing, sunscreen, sunglasses and shade) is recommended when outdoors for more than a few minutes. In summer, most Australian adults will maintain adequate vitamin D levels from sun exposure during typical day to day outdoor activities."
So a few minutes in the sun without protection is enough to raise your cancer risk during the worse times on the worst days and you shouldn't be seeking any sun intentionally for vitamin D during these times. Growing up it was easy to get sunburned in 10-15 minutes on some days, and the advice was that if you burn you definitely raised your cancer risk.
[0]: https://wiki.cancer.org.au/policy/Position_statement_-_Risks...
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.
[1] https://www.direct-ms.org/wp-content/uploads/2018/01/Vit-D-s... [2] https://www.researchgate.net/publication/285056396_Vitamin_D... [3] https://staging.mja.com.au/system/files/issues/194_07_040411...
Assuming 'linear no-threshold'? I find that pretty suspect.