I finally had enough time to read the post properly! As usual, very interesting!! Thank you Richard!
I didn't know about the decrease of the SPF in sunlight compared to the one tested in the lab, I wonder why that is the case, shouldn't the power of UV rays be the same?? It seems a bit odd, what about the other studies done with real people in the sun that say that SPF50 decreases by x% after n hours and so on, didn't they test the actual SPF? What do you think about it?
You definitely gave me some food for thought, I actually used a spreadsheet to compare the time to get erythema with different SPFs (of course not taking into account the decrease caused by the amount used and the fact that in sunlight it seems to be lower) and I found out that with SPF 5 and UVI 10, you would get skin damage after around 75 minutes, the exact same amount of time it takes to get erythema with UVI 2. Which means that if you use the dose to assess skin damage, you would just need SPF 5 even with UVI 10 if you don't stay in the sun longer than 75 minutes. Isn't it fascinating??
With SPF50, you would need to stay in the sun for 750 minutes with UVI 10 to get erythema, which is a bit overkill if you ask me.
This means that if you apply SPF50 in the morning in the right amount, theoretically you would still have around SPF22 after 8h (https://pubmed.ncbi.nlm.nih.gov/19000186/), which means that even after 8hours, you would still need 6 hours of sun with UVI 10 to have the first signs of damage.
In the worst case scenario where SPF 50 in sunlight is like SPF 20, then you would start with 20 and go down to 11 after 8hours, which still means 3hours with UVI 10 to get damage.
Let's say you apply half the amount required, you would start with around 85% less protection, then let's consider that SPF50 is actually 20, it means that you start the day with SPF 3 (erythema after 45mins) and after 8h you would have SPF 1.3, which is the same as not having sunscreen on at all. That's maybe why people get a tan (damage) even with SPF 50, because they actually have SPF 3 or less.
Let's not even talk about applying 1/4th of the required amount.
So definitely the higher SPF might be that high for that reason, and I guess also the "apply every 2 hours" because they consider that people don't put enough on.
If you put enough sunscreen on (unless you are doing an activity where it's very likely you rub it off, in that case of course you need to reapply), as far as we can see from your posts and what I've written so far, you might get away with putting it on just once, you could stay in the sun a lot of hours with high UVIs and still don't get erythema. This might explain why in this study (https://pubmed.ncbi.nlm.nih.gov/7947191/) they didn't see a lot of difference between 1 and 4 applications over an 8h period. Even more so if you stay under the beach umbrella most of the time and are careful not to rub it off.
Of course this is just theoretical, if you feel safer reapplying it 3 times, so be it! Better safe than sorry!
What do you think Richard? I would love to hear your thoughts about it!
The latest research indicates that Melanoma is usually caused by an interaction of three variable factors, namely, (1) genetic predisposition or skin type, (2) one or more occasions of serious sunburn in childhood or adolescence, and (3) inadequate immune system. The third factor might also be a combination of genetic factors and lifestyle including nutrition, stress, sleep deprivation, not getting a little daily sunshine. Personal risk of Melanoma increases when each of the contributing factors are high on the spectrum of causation. Melanoma might break out on a part of the body which was never exposed to sunshine but in that case, the other two factors would be very high risk.
You've touched on a few points I think are worth emphasizing.
I started worrying about vitamin D a long time ago because I wondered why evolution would act so quickly to make a life threatening compromise (paler skin) if the stakes weren't really high. Presumably, as our ancestors migrated north after the last ice age, they quickly lost their melanin (well, only the paler skinned folk survived) so they could synthesize enough vitamin D.
So I've always been a little concerned about the advice to completely cover up. After the Iranian revolution, women had to cover up for religious reasons and their rates of MS shot up. I worry we are not finding the right balance in our own culture for two reasons.
First of all we are increasingly recognizing our lack of winter sun means most of us don't have the vitamin D to get through the winter.
But after winter, we work in the office until Christmas and then suddenly expose our white, now untanned (and unprotected) skin at the worst possible time of year. Even if we use sunscreen, as you pointed out previously, many people don't apply it evenly or in sufficient volume. So building up a little bit of melanin during spring as a second layer of defense seems like common sense to me. Like the Swiss cheese model. Maybe we're not making good use of our body's own adaption mechanism?
Since we move around the planet so much, we're rarely genetically adapted for the environments we find ourselves in. So I like the 'precision health' concept where different skin types, different levels of tanning, different parts of our body, different times of day, and times of year all have different requirements (for vitamin D production) and different sensitivities (to UV damage).
Nice in theory but a lot more complicated than 'slip, slop, slap'.
Hi Nick. Agreed. Unfortunately though, it seems that such nuanced messaging is considered to complicated by the Health advisory people. And to make the right choices in any situation, one needs to know the UVI. When all else fails, let common sense prevail. :)
I finally had enough time to read the post properly! As usual, very interesting!! Thank you Richard!
I didn't know about the decrease of the SPF in sunlight compared to the one tested in the lab, I wonder why that is the case, shouldn't the power of UV rays be the same?? It seems a bit odd, what about the other studies done with real people in the sun that say that SPF50 decreases by x% after n hours and so on, didn't they test the actual SPF? What do you think about it?
You definitely gave me some food for thought, I actually used a spreadsheet to compare the time to get erythema with different SPFs (of course not taking into account the decrease caused by the amount used and the fact that in sunlight it seems to be lower) and I found out that with SPF 5 and UVI 10, you would get skin damage after around 75 minutes, the exact same amount of time it takes to get erythema with UVI 2. Which means that if you use the dose to assess skin damage, you would just need SPF 5 even with UVI 10 if you don't stay in the sun longer than 75 minutes. Isn't it fascinating??
With SPF50, you would need to stay in the sun for 750 minutes with UVI 10 to get erythema, which is a bit overkill if you ask me.
This means that if you apply SPF50 in the morning in the right amount, theoretically you would still have around SPF22 after 8h (https://pubmed.ncbi.nlm.nih.gov/19000186/), which means that even after 8hours, you would still need 6 hours of sun with UVI 10 to have the first signs of damage.
In the worst case scenario where SPF 50 in sunlight is like SPF 20, then you would start with 20 and go down to 11 after 8hours, which still means 3hours with UVI 10 to get damage.
Things change drastically if you don't apply the right amount as shown in the table in your post (https://uv.substack.com/p/sunscreen-use)
Let's say you apply half the amount required, you would start with around 85% less protection, then let's consider that SPF50 is actually 20, it means that you start the day with SPF 3 (erythema after 45mins) and after 8h you would have SPF 1.3, which is the same as not having sunscreen on at all. That's maybe why people get a tan (damage) even with SPF 50, because they actually have SPF 3 or less.
Let's not even talk about applying 1/4th of the required amount.
So definitely the higher SPF might be that high for that reason, and I guess also the "apply every 2 hours" because they consider that people don't put enough on.
If you put enough sunscreen on (unless you are doing an activity where it's very likely you rub it off, in that case of course you need to reapply), as far as we can see from your posts and what I've written so far, you might get away with putting it on just once, you could stay in the sun a lot of hours with high UVIs and still don't get erythema. This might explain why in this study (https://pubmed.ncbi.nlm.nih.gov/7947191/) they didn't see a lot of difference between 1 and 4 applications over an 8h period. Even more so if you stay under the beach umbrella most of the time and are careful not to rub it off.
Of course this is just theoretical, if you feel safer reapplying it 3 times, so be it! Better safe than sorry!
What do you think Richard? I would love to hear your thoughts about it!
Thanks Fabio. I’ll address those concerns in a followup post. But I have couple is others in the queue, so it might be in a month or so…
Of course Richard! You are already so kind to help us with your insights!! Looking forward to other posts!
The latest research indicates that Melanoma is usually caused by an interaction of three variable factors, namely, (1) genetic predisposition or skin type, (2) one or more occasions of serious sunburn in childhood or adolescence, and (3) inadequate immune system. The third factor might also be a combination of genetic factors and lifestyle including nutrition, stress, sleep deprivation, not getting a little daily sunshine. Personal risk of Melanoma increases when each of the contributing factors are high on the spectrum of causation. Melanoma might break out on a part of the body which was never exposed to sunshine but in that case, the other two factors would be very high risk.
Agreed.
You've touched on a few points I think are worth emphasizing.
I started worrying about vitamin D a long time ago because I wondered why evolution would act so quickly to make a life threatening compromise (paler skin) if the stakes weren't really high. Presumably, as our ancestors migrated north after the last ice age, they quickly lost their melanin (well, only the paler skinned folk survived) so they could synthesize enough vitamin D.
So I've always been a little concerned about the advice to completely cover up. After the Iranian revolution, women had to cover up for religious reasons and their rates of MS shot up. I worry we are not finding the right balance in our own culture for two reasons.
First of all we are increasingly recognizing our lack of winter sun means most of us don't have the vitamin D to get through the winter.
But after winter, we work in the office until Christmas and then suddenly expose our white, now untanned (and unprotected) skin at the worst possible time of year. Even if we use sunscreen, as you pointed out previously, many people don't apply it evenly or in sufficient volume. So building up a little bit of melanin during spring as a second layer of defense seems like common sense to me. Like the Swiss cheese model. Maybe we're not making good use of our body's own adaption mechanism?
Since we move around the planet so much, we're rarely genetically adapted for the environments we find ourselves in. So I like the 'precision health' concept where different skin types, different levels of tanning, different parts of our body, different times of day, and times of year all have different requirements (for vitamin D production) and different sensitivities (to UV damage).
Nice in theory but a lot more complicated than 'slip, slop, slap'.
Hi Nick. Agreed. Unfortunately though, it seems that such nuanced messaging is considered to complicated by the Health advisory people. And to make the right choices in any situation, one needs to know the UVI. When all else fails, let common sense prevail. :)