UV index weighs UVB more heavily than UVA. UVB falls much faster than UVA and visible sunlight as the sun gets lower because photons travel through increasingly more ozone and atmosphere. So the higher in the sky the sun is, minus cloud clover, the higher the UV index is.

I don't think so. If you controlled for temperature and season I bet you wouldn't be able to tell.

You can use simple heuristics to get a very good idea.

1: How high in the sky is the sun? (Many other factors like time of year, time of day, and latitude feed into this) 2: What is the chance for you at a given time of the sun being blocked by clouds, buildings, etc?

Maybe how different things look with sunglasses on vs off?

Sunlight is associated with lower risk of almost every cancer except skin cancer, and skin cancer is the easiest to detect early.

I think the original statement was misleading wrt sunburn for the reason I stated. I think we have different definitions of fight.

Ohh, I see. I'm not sure it was "very misleading", but I agree somebody trying to decide when to get sunlight for the purposes of vitamin D could easily misapply the lesson from this article.

You only need 5 minutes at midday in the summer. At 7am, you get essentially no vitamin D.

What you are saying is exactly the wrong takeaway. Please read the article:

https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...

Beneficial UV is UVB, which is low when the sun is low because it can’t get through the ozone and atmosphere as well when the sun is at an angle. Instead, you are getting mostly UVA, which has an easier time passing through the skin (and atmosphere) and doesn’t stimulate vitamin D production.

After reading your reply here and the one I was responding to, I think I understand (You spelled out) the disconnect; the post I replied to was about Vitamin-D-Maxxing, which isn't something I had in my cross check. My primary concern is "Which times should we do outdoors things to prevent sun damage [photoaging + cancer]" (And/or "Do I need to apply sunscreen before going out now / how long can I stay out)

Yes, mid-day for a shorter amount of time is better for optimal Vitamin D. The article I linked above goes into great detail about it, more than the two quotes I pulled out

You can get sunburn from light reflecting off water as well, in hot places. Maybe not on its own, but certainly in conjunction with direct sunlight.

One of my worst sunburns was in San Diego on an extremely gray day. I should have known better.

The right thickness marine layer can make the sky seem dim but it's like one massive UV diffuser from horizon to horizon.

They are also the worst because you get mostly UVA since UVB has a harder time penetrating through cloud cover

It depends a lot on the position of the sun in the sky too. It can be warm around sunset and sunrise, but because the sun is so low to the horizon, it is passing through more atmosphere.

I spent a lot of time out in the sun by the Dead Sea and did not burn at all, since the very low altitude seems to have provided me with extra protection, whereas nearby on the shores of the Red Sea, at sea level, I did burn with similar levels of exposure.

I wish people would stop saying UV without clarifying what wavelength they mean. Damage from UVA and UVB is very different.

UVB light can promote DNA repair pathways:

Enhancement of UVB-induced DNA damage repair after a chronic low-dose UVB pre-stimulation

https://www.sciencedirect.com/science/article/abs/pii/S15687...

UVA can slow down repair of damage from UVB:

Long-wavelength UVA enhances UVB-induced cell death in cultured keratinocytes: DSB formation and suppressed survival pathway

https://pubmed.ncbi.nlm.nih.gov/33978941/

This whole article shouldn't even exist, and should just point to wikipedia or mayo clinic.

DNA is the instruction set for protein synthesis. If your DNA is damaged beyond repair that's an issue, and sure apoptosis is the usual result and not cancer. At the same time however when cells die either from the direct impact of DNA damage or apoptosis they both stop working and spill their contents into the area... including signalling molecules that promote inflammation.

The sunburn you feel isn't a distress signal from cells warning that they can't be trusted, it's tissue death and an inflammatory reaction triggered by a cytokine response.

Toasting my DNA is quite a way to put it. It's an informative read, it helps me understand why the noonday sun does feel so intense. Even if it's not hot, the ability to find shade at noon is so poor, it feels inescapable.

Here are the predictions and observations for Australia: https://www.arpansa.gov.au/our-services/monitoring/ultraviol...

I don't know about this. I can feel where there are holes in my sunblock (which only block UV) which lets me know where to apply it, and the sensation is not one of heat of the sun, it's different. I'm not sure about the mechanism, but the presence of the sunblock stops the "UV" type sensation. This from the moment I put it on to long after the water component of the sunblock has evaporated. And, the type of sunblock doesn't matter (the drier waterproof kind or various lotion varieties). And, I do notice a difference between the different SPF, along with some "bad" sunblock that I just kept applying but couldn't make the feeling go away (it had apparently separated, and everyone that used it was burned).

I think the "real" test would be to just shine a UV light in a dark room, to see if the source can be identified while blindfolded, or something.

As someone who has gotten sunburned in California winters and even around 7-9 PM in Alaska, I can't relate to your closing idea at all!

In some parts of South America the UV index reaches 15-20 due to near-equatorial latitude and extremely high elevation. It is strong enough that you need protection from indirect UV radiation.

Good catch, it is kind of confusing as they seem to contradict themselves, and also depends on the season.

  Figure 6 shows that for full body exposures, there is a wide window between the time for sufficient UV and the time for too much UV. As the fraction of body that is exposed decreases, the window of optimum UV exposure times also decreases. If only hands and face are exposed, there is generally only a small window between receiving insufficient UV for vitamin D production, and too much UV for skin damage (erythema).

  As the UVI becomes smaller, it becomes more difficult to produce sufficient vitamin D without inducing erythema. For low sun conditions typical of midday in the mid-latitude winter (SZA ∼65°), where R∼1, the skin area exposed must be more than 12% (see Eq. 5) This means that for these low sun conditions it is improbable that sufficient vitamin D can be produced from exposing the hands and face alone, since that area is less than 12% of the full body.

If you are able to expose a lot of your body at a time then yes later in the day is workable (in the summer). In the winter, (depending on your latitude) you still need mid day sun to make any meaningful Vitamin D.

I live in New Zealand, and my wife is Spanish so we’ve spent a bunch of time in Spain too. NZ for a combination of reasons has much higher UV than Europe, with the corresponding skin cancer rates. In midsummer the UV is around 150% of what it would be in Europe at the equivalent time of year, but here, especially if it’s windy, it might feel like 20 degrees and you’d quite like to be in the sun, whereas in Spain you’d probably be at 40 degrees and there’s no way you’d want to be out of the shade. The difference is really stark, in general there I can regulate my sun exposure by feel much more than I can here.