> Sunburn is triggered when your DNA is damaged from light absorption [...] This interferes with protein synthesis and wreaks havoc in your body over time
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.
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.
Good post - the UV index is related to how short the "column of air" that the sun traverses to get to your skin - noon is the shortest distance. Scattering is a density function (more air, more scattering), so this is why it looks like cos^p when compared to normal irradiance (cos). It also explains why it is hard to get a sunburn in the temperate winter, since the sun always appears at an angle, it doesn't go directly overhead.
I've heard a quick way to determine if the UV index is dangerous is if your shadow is shorter than you. Though location matters too.
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
The details of UV damage are interesting. You can think of DNA as a sequence of four letters: C, G, A, and T. If there are two neighboring T's, UV can move a bond, linking the two T's together (i.e. thymine dimerization). If you're in the sun, each skin cell gets 50-100 of these pairs created per second (which is much higher than I expected). Enzymes fix most of these errors, but sometimes the errors will cause problems during DNA replication and you can end up with mutations.
I have always heard sunny snow days are the worst, because you feel cold, but the sun is still hitting you, plus reflecting off the snow to hit you twice.
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.
> the sun may still feel pretty intense in the morning or late afternoon, but the risk of sunburn is considerably lower
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.
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).
You would do well to calculate the numbers for your own city, as I did earlier today for mine:
I really appreciate learning this fact, but I can't tell how it makes the quoted sentence misleading. The author is just talking about the surprising disparity between how warm it feels and how fast you will burn. They don't mention vitamin D.
I know they don’t mention vitamin D, but I think it’s important for people to know the context and not think that late day sunlight automatically = better
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.
The statement you quoted is very useful [One of the most useful heuristics], and your statement is misleading. You accumulate damage (Photoaging, cancer risk) dramatically faster at high sun angles (i.e. mid day, close to the summer solstice, closer-to-equatorial latitudes) etc than low ones.
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).
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
it really only takes about 5 minutes of exposure to get the vitamin D your body needs. i would happily do it between 7-9AM when the UV is low than doing it at mid day.
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 doesn’t stimulate vitamin D production.
Does anyone know how UV indexes reported by weather apps (including forecasts) work?
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.
It is a prediction of UV intensity based on specific forecast location, time of day, day of year, prevailing weather, and any anomalies affecting the ozone layer. It is calculated by NOAA.
I am curious if anyone else can tell whether it is a high UV day just by looking outside.
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.
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?
UV index weighs UVB more heavily than UVA. UVB falls much faster than 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.
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.
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.
[0] https://www.nature.com/articles/newbio241074a0
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.
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
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.
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.
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...
You would do well to calculate the numbers for your own city, as I did earlier today for mine:https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da3...
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).
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 doesn’t stimulate vitamin D production.
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...
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.
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?