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 overlap in the red boxes suggests that the protection offered by tanning is minimal: tanned skin still accumulates additional CPDs.
In fact, tanning is caused by DNA damage, specifically the activity of the p53 tumor suppressor protein in response to UVB-induced damage.[6] This is why dermatologists say "there is no safe way to get a tan": tanning requires exposing your skin to carcinogenic conditions.
Sunscreen is effective at preventing CPD formation.[7] Contrary to Brynes' claims that tans are "nature's sunscreen" and "better than sunscreen," you should still wear sunscreen if you have a tan.
Empirical observations of indoor tanning provide evidence that non-sunburn exposure causes cancer
In the footnotes of his post, Bryne