In his recent LessWrong post, Some takes on UV & cancer, Steve Byrnes comes out against the "Public Health Orthodoxy" on UV. Among other topics I won't be addressing, Brynes claims that non-sunburn sun exposure does not increase risk of skin cancer, and suggests that people should aim to "wean off" sunscreen and develop a permanent tan.[1]

Brynes is wrong and his advice is dangerous.

Non-sunburn UV exposure causes cancer

Our mechanistic understanding of UV-induced carcinogenesis is consistent with non-sunburn exposure causing cancer

We have a pretty solid understanding of how and why sun exposure causes cancer. UVB exposure causes direct DNA damage, whereas UVA causes damage primarily through oxidative stress.[2] Both of these pathways involve the formation of abnormal structures in DNA called photoproducts, such as cyclobutane pyrimidine dimers (CPDs). CPDs are a primary cause of melanoma.[3]

CPDs do not require sunburn to form. For example, look at the second graph of Figure 4 of Miyamura et al. 2010, a small (n=7) study on the tanning process.[4]

The black boxes show CPD presence after repeated sub-sunburn threshold UV exposure, designed to induce tanning. The red boxes show CPD presence after another exposure to 2 MED UV.[5] The [...]

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Outline:

(00:34) Non-sunburn UV exposure causes cancer

(00:39) Our mechanistic understanding of UV-induced carcinogenesis is consistent with non-sunburn exposure causing cancer

(02:31) Empirical observations of indoor tanning provide evidence that non-sunburn exposure causes cancer

(04:04) Rebuttals to specific arguments from Brynes

(04:22) Sunscreen/cancer correlation studies do not control for sun exposure, but randomized trials support sunscreen use

(06:23) Skin color variation demonstrates strong evolutionary pressure to mitigate UV damage

(07:30) Carcinogens rarely exhibit linear dose-response relationships, and occupational exposure data is hopelessly muddled

(10:31) Conclusion

(10:34) Subject-level takeaways

(11:10) Meta-level takeaways

(11:20) Prioritize mechanistic understanding

(11:47) Beware the streetlight effect

(12:32) The orthodoxy is usually right

The original text contained 15 footnotes which were omitted from this narration.

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First published:
April 14th, 2026

Source:
https://www.lesswrong.com/posts/EFiCGc3YwadZBzvZ3/contra-byrnes-on-uv-and-cancer

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Narrated by TYPE III AUDIO.

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Images from the article:

Box plot comparing DNA damage across control, UVA&UVB, UVA, and UVB treatment groups.

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“Contra Byrnes on UV & Cancer” by HedonicEscalator

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