SPF is often framed as a “multiple of how long it takes to get burned”, but this is just a reframe of what it actually is, which is “the denominator of UV reduction”.
SPF 70 means that 1/70th of the UV gets through. It just happens to be that the “time to get burned” is just however many minutes it takes to reach a certain threshold. So, a 70X reduction in UV just means it takes 70X longer to reach any given threshold.
After chatting a bit with ChatGPT, no SPF 70 does not mean what you say it means. Theofficial definition is about erythema thresholds (whether the skin becomes red). It’s a sunburn related metric, which means it focuses on UVB. Outside of sunburn, UVA is probably more important for skin health.
From a not yet implemented FDA proposal from 2021:
Among other things, this proposal eliminates the potential confusion permitted by the current labeling regime, in which a higher numbered product (for example, one labeled SPF 30) may provide inferior protection against UVA radiation than a lower numbered product (for example, one labeled broad spectrum SPF 15).
FDA sunscreen policy seems stupid all the way down. Between giving bad advise about how to use it, banning valuable ingredients and bad labeling guidelines that ignore a prime effect of the product the FDA manages to get all the points wrong.
To get back to the more general question, here are some positive effect of UV radiation such as vitamin D3 synthesis. There are also some negative effects such as damage to elastin and collagen. Net harm arguments for sun exposure strong enough to cause sunburn seem strong, but arguments for lower amounts seem less strong.
As Bucky mentioned, health advise by dermatologist seems to be more focused on advising stronger sunscreen because it’s easier to just use stronger sunscreen than getting people to apply the 2mg/cm^2. As far as broad policy, the 2mg/cm^2 is arbitrary.
If you are the FDA and see that people seem to more often apply 0.5-1.5mg/cm^2, the optimal course of action would be to just add add language that says SPF X at 0.5mg/cm^2 and SPF Y at 2mg/cm^2. You could also have a “highly concentrated” marketing label for sunscreen that are optimized to do a good job at 0.5mg/cm^2.
Yes, erythemal dose is set as the threshold, and yes, it applies specifically to UVB, so when I said “the UV dose”, the more precise thing to say would have been the “UVB” dose. However, the point stands.
What you’re not appreciating is that the “erythemal dose” is irrelevant to the numerical rating of the sunscreen. You seem to be suggesting that if the threshold were different, then the SPF rating would be different as well. See below
If we were to change the SPF threshold from “enough UVB to begin to burn you” to “enough UVB to burn you to a crisp until your skin turns black and falls off your body”, the SPF rating of every sunscreen on planet earth would be the same.
By the same token, if we were to change the SPF threshold to “enough UVB to give you a little bit of vitamin D but not burn you”, then the SPF rating would remain the same.
This is the reason we don’t need to adjust SPF ratings based on how dark your skin is. A person with dark skin requires a threshold of UVB that is far greater than a white person, and yet SPF50 remains SPF50 for both parties.
The main point is that there are few contexts where someone spends enough time in the sun to get sunburn with SPF 26 worth of practical protection. SPF 26 is good enough protection against sunburn. You need to violate general advise about how to deal with sun or have a very unusual situation to get that problem.
If you want to protect against skin aging, SPF is not a measure that tells you how well sun screen protects against it.
What you’re not appreciating is that the “erythemal dose” is irrelevant to the numerical rating of the sunscreen. You seem to be suggesting that if the threshold were different, then the SPF rating would be different as well. See below
You seem to assume that a complex biological system behaves the way your simplified model behaves without making any arguments for that assumption being true.
The people who wrote the actual regulations seem to think that the system is complex enough to get people to do actual effectiveness testing based on the erythemal dose.
By the same token, if we were to change the SPF threshold to “enough UVB to give you a little bit of vitamin D but not burn you”, then the SPF rating would remain the same.
That would only be true if the spectrum that results in vitamin D production is exactly the same as the spectrum that results in sunburn. The are quite similar but exactly the same is probably not a true claim.
If “nuanced complexities of biology” were non-negligible factors, then we would need to have different SPF ratings for the time of year, time of day, ethnicity, hydration status, vitamin C intake, and so on. All of these have nonzero implications for the dose/response curves. All can be totally ignored in practice.
Sure, there are minor nuances to stimulus response in chaotic biological systems, but the bottom line is, and has always been, area under the curve of UV exposure.
For the record, I agree that SPF 26 is more than enough to protect you from sunburn in almost all cases, and that SPF doesn’t account for UVA. The UVA rating system is of the form “PA+++”, which does a similar thing, just for different wavelengths.
SPF is often framed as a “multiple of how long it takes to get burned”, but this is just a reframe of what it actually is, which is “the denominator of UV reduction”.
SPF 70 means that 1/70th of the UV gets through. It just happens to be that the “time to get burned” is just however many minutes it takes to reach a certain threshold. So, a 70X reduction in UV just means it takes 70X longer to reach any given threshold.
After chatting a bit with ChatGPT, no SPF 70 does not mean what you say it means. The official definition is about erythema thresholds (whether the skin becomes red). It’s a sunburn related metric, which means it focuses on UVB. Outside of sunburn, UVA is probably more important for skin health.
From a not yet implemented FDA proposal from 2021:
FDA sunscreen policy seems stupid all the way down. Between giving bad advise about how to use it, banning valuable ingredients and bad labeling guidelines that ignore a prime effect of the product the FDA manages to get all the points wrong.
To get back to the more general question, here are some positive effect of UV radiation such as vitamin D3 synthesis. There are also some negative effects such as damage to elastin and collagen. Net harm arguments for sun exposure strong enough to cause sunburn seem strong, but arguments for lower amounts seem less strong.
As Bucky mentioned, health advise by dermatologist seems to be more focused on advising stronger sunscreen because it’s easier to just use stronger sunscreen than getting people to apply the 2mg/cm^2. As far as broad policy, the 2mg/cm^2 is arbitrary.
If you are the FDA and see that people seem to more often apply 0.5-1.5mg/cm^2, the optimal course of action would be to just add add language that says SPF X at 0.5mg/cm^2 and SPF Y at 2mg/cm^2. You could also have a “highly concentrated” marketing label for sunscreen that are optimized to do a good job at 0.5mg/cm^2.
Yes, erythemal dose is set as the threshold, and yes, it applies specifically to UVB, so when I said “the UV dose”, the more precise thing to say would have been the “UVB” dose. However, the point stands.
What you’re not appreciating is that the “erythemal dose” is irrelevant to the numerical rating of the sunscreen. You seem to be suggesting that if the threshold were different, then the SPF rating would be different as well. See below
If we were to change the SPF threshold from “enough UVB to begin to burn you” to “enough UVB to burn you to a crisp until your skin turns black and falls off your body”, the SPF rating of every sunscreen on planet earth would be the same.
By the same token, if we were to change the SPF threshold to “enough UVB to give you a little bit of vitamin D but not burn you”, then the SPF rating would remain the same.
This is the reason we don’t need to adjust SPF ratings based on how dark your skin is. A person with dark skin requires a threshold of UVB that is far greater than a white person, and yet SPF50 remains SPF50 for both parties.
The main point is that there are few contexts where someone spends enough time in the sun to get sunburn with SPF 26 worth of practical protection. SPF 26 is good enough protection against sunburn. You need to violate general advise about how to deal with sun or have a very unusual situation to get that problem.
If you want to protect against skin aging, SPF is not a measure that tells you how well sun screen protects against it.
You seem to assume that a complex biological system behaves the way your simplified model behaves without making any arguments for that assumption being true.
The people who wrote the actual regulations seem to think that the system is complex enough to get people to do actual effectiveness testing based on the erythemal dose.
That would only be true if the spectrum that results in vitamin D production is exactly the same as the spectrum that results in sunburn. The are quite similar but exactly the same is probably not a true claim.
If “nuanced complexities of biology” were non-negligible factors, then we would need to have different SPF ratings for the time of year, time of day, ethnicity, hydration status, vitamin C intake, and so on. All of these have nonzero implications for the dose/response curves. All can be totally ignored in practice.
Sure, there are minor nuances to stimulus response in chaotic biological systems, but the bottom line is, and has always been, area under the curve of UV exposure.
For the record, I agree that SPF 26 is more than enough to protect you from sunburn in almost all cases, and that SPF doesn’t account for UVA. The UVA rating system is of the form “PA+++”, which does a similar thing, just for different wavelengths.