Sun Exposure Science: Rowan Jacobsen Interview

Two weeks before this conversation, host Liz Moody got a phone call that scared her: a biopsy had come back as melanoma, the most dangerous form of skin cancer. It was caught at stage zero and removed cleanly from her chest (00:00). The diagnosis left her with a question most dermatology does not answer well: did a lifetime of sun exposure hurt her, or did the sun protect her in ways nobody talks about?

Science journalist Rowan Jacobsen spent years chasing that question for his new book, In Defense of Sun. He is not a dermatologist and does not sell sunscreen, skincare, or supplements, which he says freed him to follow the research across cardiology, immunology, and sleep science instead of staying inside the usual skin-only lane (00:58). What he found complicates much of what the public has been told since the sunscreen campaigns of the 1980s.

Start with the puzzle that opens the episode. Outdoor workers, people who spend their careers in direct sun, have lower melanoma mortality than people who work indoors (01:55). Women in a 20-year Swedish study called the Melanoma in Southern Sweden study who actively sought sun, through winter vacations or regular sunbathing, lived longer than women who avoided it, even though sun-seekers were not necessarily more careful about sunscreen (03:08). The lie in Jacobsen's opening game turned out to be the one that sounds safest: that people with dark skin need sunscreen every day. Melanin, the pigment that gives skin its color, does so much protective work on its own that for most daily activity it makes sunscreen unnecessary for people who have a lot of it (20:50).

"Melanin is the best sun protectant ever developed. We haven't created anything that can compete with melanin." - Rowan Jacobsen [21:00]

The real risk factor for melanoma, Jacobsen says, is not steady sun exposure but a history of blistering childhood sunburns combined with fair skin (08:19). Outdoor workers get regular, moderate doses that build protection gradually, while occasional beachgoers get hit with the sudden blast their skin is not ready for.

The Light Sunscreen Does Not Block

Part of the problem is how sunscreen was built. For most of the twentieth century, scientists assumed only ultraviolet light, the high-energy band invisible to the eye, could damage skin cells enough to cause cancer, so sunscreen was designed to block it. Newer research shows about half of the free radicals, unstable molecules that damage DNA and other cell structures, generated by sunlight come from visible light, which ordinary sunscreen lets straight through (12:45). Within ultraviolet light itself there was a second miscalculation: UVA makes up about 95 percent of the ultraviolet radiation that reaches skin, while UVB is only about 5 percent, yet sunscreens from the 1970s and 1980s blocked mainly UVB (13:49). That let people stay out far longer without burning, the body's usual warning sign, while absorbing UVA the whole time. A newly approved ingredient called bemotrizinol, already used for two decades in Asian and European sunscreens, blocks the full UVA spectrum and is starting to replace older US formulas (15:10).

That distinction matters because of a finding Jacobsen calls one of the strangest things he came across. A National Cancer Institute analysis separated UVA exposure from UVB exposure across the country and found that UVA tracked closely with melanoma, while UVB, once UVA was accounted for, was linked to lower melanoma rates (105:15). This does not mean sunburn is good. It suggests the band long treated as the dangerous one may be doing protective work, likely through the vitamin D and repair processes it triggers, while UVA causes damage more quietly.

Underneath all of this sits a concept called hormesis: the idea that a small, repeated dose of stress trains the body to repair itself better. Exercise works this way, tearing muscle fibers so the body rebuilds them stronger. Jacobsen argues sunlight works the same way on skin. Ultraviolet light damages DNA in small amounts, which activates repair systems, including a gene called P53 that flags and eliminates badly damaged cells before they can turn cancerous (26:04).

"Your body gets good at fixing that damage by regularly causing the damage in small doses." - Rowan Jacobsen [25:00]

The same logic appears in cardiovascular health. Sunlight hitting skin releases nitric oxide from nitrate compounds stored there, mostly sourced from leafy greens, and nitric oxide relaxes blood vessels and lowers blood pressure (29:33). Heart disease kills far more people than melanoma does: melanoma makes up only 1 to 2 percent of all skin cancers and does not rank among the top 40 causes of death from disease (44:08). Jacobsen argues the trade-off is lopsided, a modest rise in skin cancer risk set against a documented drop in the nation's leading killer (31:05).

Sun exposure also appears to calm an overactive immune system. Children who play outside three hours a day have about one-fifth the rate of multiple sclerosis compared with children who do not, a pattern researchers have tracked since the 1970s (33:37). Jacobsen compares it to the reversal in peanut allergy advice, where exposing children to a trigger early turned out to prevent problems later rather than cause them (34:18).

The Hidden Light Gap Indoors

Much of the conversation turns out to be less about skin than about eyes and circadian rhythm, the body's internal 24-hour clock that governs sleep, hormones, and metabolism in nearly every cell (63:08). Light is measured in lux, a unit of brightness. A typical indoor room lit by artificial light sits around 100 to 200 lux. A sunny day outdoors runs 50,000 to 100,000 lux, and even an overcast or rainy day still delivers 10,000 to 20,000 lux, far brighter than almost any room people spend time in (60:57, 61:20). A UK study that tracked people's light exposure with wearable sensors found that people with the brightest days were 34 percent less likely to die of any cause, while people with the darkest nights were 20 percent less likely to die of any cause (68:25).

"If your shadow is longer than you are, you can't make any vitamin D." - Rowan Jacobsen [88:01]

That gap shows up in mental health too. Researchers gave people with severe depression smart light bulbs that automatically shifted from blue to red-orange light in the evening and simply told them to try to get outside more, without tracking whether they did. Eighty-two percent moved from severe to mild depression within four weeks, a result that held when the control group later got the same bulbs (78:06).

The vitamin D story is where Jacobsen is bluntest. Decades of observational data showed people with higher vitamin D in their blood had lower rates of nearly every disease, which led to mass supplementation. But long, large clinical trials that actually tested vitamin D pills against placebo found no significant benefits for people who were not already deficient (83:40). Jacobsen's explanation is that vitamin D levels were always a marker of sun exposure, and sun exposure brings a bundle of other benefits, nitric oxide, circadian signaling, immune regulation, that a pill cannot replicate. Below a blood level of about 16 nanograms per milliliter, disease risk climbs, and supplementing still makes sense there (87:16). Above it, he says, the pill is probably not doing much.

None of this argues for reckless sun exposure or says dermatologists are wrong to warn against sunburn. Jacobsen repeats that fair-skinned people with a childhood history of burns, people like Moody, still carry real risk and should cover up during peak hours. His argument is narrower: total sun avoidance, the message most people absorbed, trades a small reduction in skin cancer risk for measurable increases in heart disease, autoimmune disease, and depression. The sun does not have a lobby. Nobody profits from telling people to stand outside for twenty minutes. The evidence, now, is arguing for it anyway.