hckrnws
The UV index is not the warm sensation of sunlight on bare skin
by evakhoury
by evakhoury
Not a skin-scientist, but this seems misleading: Sunburn is not caused by some bad DNA floating around, nor damaged cells making making mistakes or pumping out wrong proteins to inflame their neighbors.
Instead, your cells have techniques to measure a variety of risk factors and signals (which does include DNA-molecule weirdness [0]) and when things get too risky the well-behaved ones will self-destruct, to avoid the chance of turning into a cancer.
So the sunburn you see and feel is instead due to cells signalling "something damaged me and I can't be trusted, I'm shutting down, recycle me" to your immune-system, plus the attendant bustle of hazmat-cleanup and increased security-sweeps for cancer-leftovers and other potential causes of the damage.
Or the other way radiation sickness is the deepest worst sunburn you have ever had.
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.
Or for something a bit more immediate and casual, the Kurzgesagt playlist of immune-system videos. [0]
[0] https://www.youtube.com/show/VLPLFs4vir_WsTyS2cy4vj4obl5igqC...
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.
Details: https://pdb101.rcsb.org/motm/91
Usually, it involves water, water cools you down, so you don't feel the heat, but it doesn't block UV, so you don't get the warning.
There is a certain look to the sky (slightly whiter-bluer) looking against the clouds, and I am not sure if it is because I am directly seeing UV, whether there is some fluorescence, or whether it’s placebo effect.
I moved from Vermont to Texas and had to purchase an umbrella with UV protection to walk around during the day. The highest the UV index hits in the Vermont's summer is 6. In Texas, it's 11+ !!! 15 minutes and you're toast
https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...
I've found that some sources report lower UV index on cloudy days, whereas some seem to indicate the same index as a sunny day.
It's not clear if these sources all have a different model for UV, some being more sophisticated or conservative, and whether any of them are backed up by a UV meter.
https://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index...
It allows you to gauge whether you should maybe wait under that tree over there while waiting for a friend.
In Australia for example, this is a matter of life and death! There are skin cancel clinics around every corner (no exaggeration).
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This is very misleading. By some coincidence I was just looking into this earlier today.
The lower the UV index, the smaller the window is between the amount of time needed to get enough Vitamin D and the time to get burnt (erythema). So the best thing to do is get sunlight mid-day for a shorter amount of time.
UV Radiation: Balancing Risks and Benefits https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...
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.
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).
https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da3...
> 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.
I feel that's more of a restatement than a contradiction.
The cell shutdown is (eventually) violent and messy, signalling involves cytokines, and the "hazmat and security" metaphor is an inflammation response.
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It usually goes together with bright direct sunlight in the summer, so I think most people don't learn to notice or distinguish it.
But I find that it's on particular overcast days in the summer, when there is no direct sunlight, that you can feel it intensely without the accompanying sunlight/heat. It reminds me to put on sunblock. But it's not all overcast days -- it's strongest when there's a light cloudy "haze". In contrast, when it's thick clouds approaching thunderstorm, you obviously don't feel it at all.
So it's that kind of thing that's hard to judge before you go outside, but as soon as you do, you get a clear "oh, I definitely need to go back in and put in sunblock" or "oh, this is fine, no sunblock needed today".
The best way I can explain it is as kind of an "oppressive" feeling, almost like a kind of pressure on your skin. Like, you can describe heat that way too -- oppressive or like a kind of pressure -- but in this case, you're feeling those things when there is no heat.
And when you are at the beach on a hot sunny day, I find it kind of magical how the sunblock removes the feeling entirely. It's still sunny and bright and hot, but you don't feel the oppressive/pressure feeling together with it.
Actinic?
(Edited, fixed)
https://care360.basf.com/emea/en/industries/personal-care/co...
This is in Australia where the hole in the ozone layer often results in 10x ‘normal’ UV levels, you can get noticeably burned in a few minutes.
Some parts of South America have a UV index twice that of Sydney’s maximum. All of the actually extreme UV index locales are extreme altitude and near equatorial.
Indeed looking at Oceania; Sydney, Perth, and Auckland all hit 14/15 regularly in the Summer months. Auckland in particular has ~40% more UV than comparable northern latitudes.
Given the genetics and immigration is heavily skewed in favour of the British and Irish diaspora, it's about 3x the UV Index of what the UK and Ireland see. Dunno where 10x came from.
By 10x you mean 10% more, and by ozone layer you mean the fact that the southern hemisphere's summer coincides with Earth's perihelion while the northern hemisphere's summer coincides with aphelion.
https://www.abc.net.au/news/science/2025-02-04/sun-summer-uv...
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
The right thickness marine layer can make the sky seem dim but it's like one massive UV diffuser from horizon to horizon.
She has her entire family wearing SPF20 24/365, constantly reminding me 'it's the overcast days that actually do the long-term damage'.
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.
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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?
on the other hand, uva is the one that is still substantial in winter and away from solar noon, goes through glass easily, and can cause sunburns and skin cancer.
A sun lower in the sky means your shadow is longer. Maybe I'm confused?
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.
You're allowed to just say "cool, here's another important thing to think about it". You don't have to pick a fight.
I am not trying to be negative, but this is very important if you have pale skin. Being outdoors in morning and late afternoon is dramatically less damaging than mid day.
If your goal is "hang out in the sun but don't get burned", then morning/evening is better. If your goal is "get exactly as much vitamin D as I need while minimizing UV damage, perhaps using a timer", then it sounds like technically mid-day is better (assuming I'm reading the parent right).
Mineral - Reflects
Chemical - Absorbs
https://cen.acs.org/business/consumer-products/What-in-sunsc...
Black means absorbed, of course.
The highest UV index I can find for Sydney is 14, which is typical of many places in the tropics.
I've spent time in the 20+ UV of the northern Atacama and it is a different beast. The indirect UV reflected by the soil is strong enough to burn you even if you are not exposed to direct sun. It requires precautions that I never needed in Australia or elsewhere in the tropics.
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.