5 ms·

Care to explain? Isn't UVB more harmful than UVA?

pazimzadeh 2mo ago
No, UVA is stronger and more harmful. UVA penetrates deeper in the skin, since it's stronger. UVB stimulates Vitamin D production, while UVA doesn't. So almost anything that is "UV-blocking" (sunscreen, UV-treated glass) gets rid of all of the beneficial UVB before UVA starts getting reduced. So you are actually enriching your exposure to the most harmful UV type, and not making Vitamin D. This might explain the paradoxical finding that people who use sunscreen more can have higher rates of skin cancer https://pmc.ncbi.nlm.nih.gov/articles/PMC10840669 https://pmc.ncbi.nlm.nih.gov/articles/PMC10840669
- miladyincontrol 2mo ago
  Confounding by indication. People who spend more time outdoors are more likely to try to wear sunscreen, as are people who already got a lot of pre-existing risk to excessive sun exposure. Key word on try as its greatly assuming there isnt a wide audience who do use sunscreen, then just apply it in poor amount, or never re-apply but enjoy staying out far longer than most do in the sun feeling they're oh so very protected. Spray sunscreens for example are just about useless, as are those solid stick applicators. They only get a fraction the amount you need for the rated protection. The biggest issue with sunscreen relates to companies selling sub par "convenience" slop, often right beside their other products some of which might actually be half decent.
  - pazimzadeh 2mo ago
    Sure that’s a theory, but hasn’t been proven. If I recall, the study showed that heavy sunscreen use amongst sunscreen users was associated with higher cancer rates. Secondly, if sun exposure is really that bad then how do you explain why most skin cancers are on the trunk and thighs rather than more exposed areas like hands and arms? The simplest answer is it’s not sun exposure that’s bad but sudden high exposure after non-exposure (repair mechanisms can’t keep up with damage). Not sure why I’m being downvoted. I guess there’s a lot of sheep among us. If anything I said is incorrect, please show me the evidence.
    - piguin 2mo ago
      The arms represent very little surface area: https://wikem.org/wiki/Estimating_body_surface_area https://wikem.org/wiki/Estimating_body_surface_area A critical factor for doing a screening is how many bad burns you've had on the chest and back which would probably be similar to many times the burns on the arms in terms of surface area x added probability.
      - pazimzadeh 2mo ago
        I like the idea of checking for that, but not sure the numbers add up. Anterior head + legs + arms = 31.5% Anterior + Posterior torsos = 36% > which would probably be similar to how many times the burns on the arms in terms of surface area x added probability I disagree, my whole point is that frequent low/moderate exposure (such as arms) reduces the chance of burning during high exposure I looked up the number, and after normalizing by surface area it looks like the skew towards torso is only for basal cell carcinoma (BCC) not (SCC). https://jamanetwork.com/journals/jamadermatology/fullarticle/2586167 https://jamanetwork.com/journals/jamadermatology/fullarticle... For melanoma the trunk was the dominant site in men, while the legs were the dominant site in women https://pmc.ncbi.nlm.nih.gov/articles/PMC2939095/ https://pmc.ncbi.nlm.nih.gov/articles/PMC2939095/ Unfortunately, that I know of there are not many studies that compare 'frequent low/moderate exposure followed by high exposure' vs 'very low exposure (high sunscreen use) followed by high exposure' Here is what I found: 1. https://pubmed.ncbi.nlm.nih.gov/9335442/ https://pubmed.ncbi.nlm.nih.gov/9335442/ These results show the specificity of the positive association between melanoma risk and intermittent sun exposure, in contrast to a reduced risk with high levels of occupational exposure. 2. https://www.nature.com/articles/s41598-026-48477-4 https://www.nature.com/articles/s41598-026-48477-4 Both the low-intermittent (HR = 1.26; 95%CI = 1.17–1.38) and high exposure groups (HR = 1.13; 95%CI = 1.01–1.26) had higher rates of melanoma than indoor workers. The low-intermittent exposure group had elevated melanoma rates on the arms and trunk (body), while the high exposure group had an elevated rate on the head and trunk. Digging into the data, the moderate exposure group had no detectable increase at any body site and a weak suggestion of lower risk (not statistically significant): Head: 0.99; Arms: 0.95; Trunk: 1.00; Legs: 0.90 Plotted: https://imgur.com/a/HBp8SnH https://imgur.com/a/HBp8SnH Why would the low-exposure group have a higher rate on the arms and trunk? Could it be because when they do go to the pool/beach, their arms are not protected? Whereas in the high exposure group, their arms are protected due to everyday exposure, but their trunk is only exposed when they go to the pool? Then again, you would think the low-exposure group would also have high trunk incidence, so there might be some confounding factors like ethnicity/behavior/dress. Anyway, hopefully I convinced you that it's not as simple as more sun = always bad.

No, UVA is stronger and more harmful. UVA penetrates deeper in the skin, since it's stronger. UVB stimulates Vitamin D production, while UVA doesn't. So almost anything that is "UV-blocking" (sunscreen, UV-treated glass) gets rid of all of the beneficial UVB before UVA starts getting reduced. So you are actually enriching your exposure to the most harmful UV type, and not making Vitamin D. This might explain the paradoxical finding that people who use sunscreen more can have higher rates of skin cancer https://pmc.ncbi.nlm.nih.gov/articles/PMC10840669 https://pmc.ncbi.nlm.nih.gov/articles/PMC10840669

miladyincontrol 2mo ago
Confounding by indication. People who spend more time outdoors are more likely to try to wear sunscreen, as are people who already got a lot of pre-existing risk to excessive sun exposure. Key word on try as its greatly assuming there isnt a wide audience who do use sunscreen, then just apply it in poor amount, or never re-apply but enjoy staying out far longer than most do in the sun feeling they're oh so very protected. Spray sunscreens for example are just about useless, as are those solid stick applicators. They only get a fraction the amount you need for the rated protection. The biggest issue with sunscreen relates to companies selling sub par "convenience" slop, often right beside their other products some of which might actually be half decent.
- pazimzadeh 2mo ago
  Sure that’s a theory, but hasn’t been proven. If I recall, the study showed that heavy sunscreen use amongst sunscreen users was associated with higher cancer rates. Secondly, if sun exposure is really that bad then how do you explain why most skin cancers are on the trunk and thighs rather than more exposed areas like hands and arms? The simplest answer is it’s not sun exposure that’s bad but sudden high exposure after non-exposure (repair mechanisms can’t keep up with damage). Not sure why I’m being downvoted. I guess there’s a lot of sheep among us. If anything I said is incorrect, please show me the evidence.
  - piguin 2mo ago
    The arms represent very little surface area: https://wikem.org/wiki/Estimating_body_surface_area https://wikem.org/wiki/Estimating_body_surface_area A critical factor for doing a screening is how many bad burns you've had on the chest and back which would probably be similar to many times the burns on the arms in terms of surface area x added probability.
    - pazimzadeh 2mo ago
      I like the idea of checking for that, but not sure the numbers add up. Anterior head + legs + arms = 31.5% Anterior + Posterior torsos = 36% > which would probably be similar to how many times the burns on the arms in terms of surface area x added probability I disagree, my whole point is that frequent low/moderate exposure (such as arms) reduces the chance of burning during high exposure I looked up the number, and after normalizing by surface area it looks like the skew towards torso is only for basal cell carcinoma (BCC) not (SCC). https://jamanetwork.com/journals/jamadermatology/fullarticle/2586167 https://jamanetwork.com/journals/jamadermatology/fullarticle... For melanoma the trunk was the dominant site in men, while the legs were the dominant site in women https://pmc.ncbi.nlm.nih.gov/articles/PMC2939095/ https://pmc.ncbi.nlm.nih.gov/articles/PMC2939095/ Unfortunately, that I know of there are not many studies that compare 'frequent low/moderate exposure followed by high exposure' vs 'very low exposure (high sunscreen use) followed by high exposure' Here is what I found: 1. https://pubmed.ncbi.nlm.nih.gov/9335442/ https://pubmed.ncbi.nlm.nih.gov/9335442/ These results show the specificity of the positive association between melanoma risk and intermittent sun exposure, in contrast to a reduced risk with high levels of occupational exposure. 2. https://www.nature.com/articles/s41598-026-48477-4 https://www.nature.com/articles/s41598-026-48477-4 Both the low-intermittent (HR = 1.26; 95%CI = 1.17–1.38) and high exposure groups (HR = 1.13; 95%CI = 1.01–1.26) had higher rates of melanoma than indoor workers. The low-intermittent exposure group had elevated melanoma rates on the arms and trunk (body), while the high exposure group had an elevated rate on the head and trunk. Digging into the data, the moderate exposure group had no detectable increase at any body site and a weak suggestion of lower risk (not statistically significant): Head: 0.99; Arms: 0.95; Trunk: 1.00; Legs: 0.90 Plotted: https://imgur.com/a/HBp8SnH https://imgur.com/a/HBp8SnH Why would the low-exposure group have a higher rate on the arms and trunk? Could it be because when they do go to the pool/beach, their arms are not protected? Whereas in the high exposure group, their arms are protected due to everyday exposure, but their trunk is only exposed when they go to the pool? Then again, you would think the low-exposure group would also have high trunk incidence, so there might be some confounding factors like ethnicity/behavior/dress. Anyway, hopefully I convinced you that it's not as simple as more sun = always bad.