J͎Λ͎Y͎@TakeThiamine
This infamous photo comes from a 2012 case report in the NEJM titled “Unilateral Dermatoheliosis.” It documents a 69-year-old delivery truck driver who spent 28 years with the left side of his face exposed to the sun through his side window.
And it doesn’t indict the sun so much as it indicts chronic, unnatural, one-sided UVA-heavy exposure.
Tempered car glass windows selectively filter ultraviolet (UV) radiation. UVB (burning rays; shorter wavelengths) are mostly blocked by standard glass while 70-80% of UVA (aging rays; longer wavelengths) largely pass through unchanged.
That means car glass skews the natural profile of sunlight: you don’t feel the burn as easily and therefore get continued, cumulative damage.
Regardless, the issue here is this guy is sitting in one place blasting his face with the sun for several consecutive hours, day in, day out.
Does this mean the sun is bad? No.
Does this mean most sunscreens are good? Definitely not.
For most of human evolutionary history, vitamin D was produced primarily through cutaneous synthesis, with UVB radiation—roughly 290–315 nm, especially around 295–300 nm—acting on 7-dehydrocholesterol in the skin to form previtamin D3, which then thermally isomerizes into vitamin D3.
Natural dietary sources (fatty fish, egg yolks, liver) are extremely limited and could never meet daily needs for most populations without regular sun exposure. Food fortification and widespread supplementation are 20th-century inventions.
Yet rates of skin cancer—especially melanoma—have increased 10-17x since the 1950s. Commercial sunscreens weren’t a thing until the ‘40s. Fortification wasn’t a thing until the ‘30s.
Clearly there’s more going on here.
We evolved under the sun, but under very different exposure patterns than people today. We used to get constant moderate exposure, now we stay indoors most of the day and go out on vacation to burn.
Burning—and the resulting DNA damage from it—is bad. And what makes someone more prone to burning? Being overexposed and overloaded in PUFAs.
This isn’t esoteric, it’s well-supported by the biochemistry of lipid peroxidation:
• PUFAs contain multiple double bonds that are highly reactive. When UV radiation hits the skin, it generates reactive oxygen species (ROS) that "attack" these double bonds.
• This interaction creates a cascading chain reaction of lipid radicals, producing toxic byproducts like malondialdehyde (MDA) and 4-HNE. These byproducts are what trigger the inflammatory response we recognize as a sunburn.
• In contrast, saturated fats have no double bonds and are much more stable under the heat and oxidative stress of the sun.
So what’s the solution?
1. Limit exposure until you build some resilience.
2. Eat better. Stored PUFA takes years to detox, so you’ll have to tread somewhat lightly if you’re just beginning to stop eating garbage food.
3. Wear thin, white, cotton long sleeves and a hat.
4. Use a mineral-based, seed oil-free sunscreen (zinc oxide).
The practical takeaway is simple: avoid burning, favor regular moderate exposure over binge exposure, wear protective clothing when needed, and use a clean mineral sunscreen when you can’t control how much sun you’re getting.