For centuries, the ideal human complexion had nothing to do with the sun. It actively avoided it.
In the courts of Europe and in Asia equally, pale skin was not merely fashionable. It was a social declaration, a biological résumé worn on one's face. The logic was airtight and everyone understood it: dark skin meant you worked outdoors, and working outdoors meant you were poor. A white complexion, by contrast, announced that you could afford to stay inside, that someone else was out in the fields turning brown on your behalf. Aristocratic figures powdered their faces white for social events. Women were instructed to cover themselves during outdoor activities and keep well away from sunlight. Wide hats and long sleeves were not merely fashion items but social armor, a daily assertion of class membership.

Skin-whitening recipes circulated through magazines, neighbors, and family members with the urgency of medical advice. Most were no better than placebos. Some were considerably worse: mercury compounds, hydrogen peroxide, and other toxic or corrosive ingredients that damaged the very skin they were meant to protect. Maria Gunning, the Countess of Coventry and one of English society's most celebrated debutantes of the 1750s, died at twenty-eight from lead poisoning, consumed in a sense by her own beauty routine. Even this cautionary tale changed nothing. The social calculus was too powerful. Pale was wealth. Tan was labor. The equation held for centuries.

Then the Industrial Revolution arrived and scrambled the equation entirely.

Through the mid-1800s, the working class was moving off the land and into factories. Mines, mills, and textile plants were dark interiors, all of them, with workers emerging at the end of the day as pale as their former aristocratic betters. The old class signals were collapsing. If factory workers were now the pale ones, if clerks and shopkeepers and seamstresses never saw the sun, then paleness meant something rather different. It meant you worked indoors at a machine, not that you lounged in a garden with a novel and a glass of claret.

The expanding middle class, accumulating disposable income and leisure time, began putting both to use at the seaside. They discovered beaches, railway tickets to the coast, and the modern project of spending their holidays doing nothing purposeful at all. Public education made young people more independent. Pale complexion began to be viewed as a sign of inactivity and introversion rather than privilege. More people got tanned. The tan was being reborn as a symbol, not yet fully respectable, but no longer automatically a mark of the lower classes.
The fashion industry responded accordingly. Clothing silhouettes that had covered necks, wrists, and ankles as a matter of course began to open up. The corset loosened. The sleeve shortened, driven by women's suffrage, the bicycle, and two world wars. Wide hats and long gloves, once the uniform of the sun-avoidant lady of leisure, gave way to lighter, more revealing styles that left considerably more skin exposed to the elements. Nobody was tracking the dermatological consequences of the shorter sleeve. The surface area of human skin exposed to the sun was quietly, steadily increasing, season by season, decade by decade, and no one was measuring what that meant.

While society was reshaping its relationship with the sun for reasons of fashion and leisure, medicine was arriving at its own enthusiastic endorsement of sunlight, by an entirely different route.

The crowded populations of European cities were suffering from diseases that sunlight seemed uniquely positioned to address. Tuberculosis, responsible for one death in four across Europe, flourished in the dark, poorly ventilated interiors that industrial housing had made universal. Children in the smoky interiors of industrial cities, rarely exposed to direct sunlight, were developing rickets at alarming rates, a bone disease causing skeletal deformities, bowed legs, and stunted growth. Physicians began looking outward for answers, and what they found, repeatedly, was sunlight.

The scientific case for sunlight's medical power was built step by step. In 1877, the British researchers Arthur Downes and Thomas Blunt published a short paper in Nature reporting that sunlight could kill bacteria, including anthrax bacilli, and that the bactericidal effect was concentrated in the shorter, ultraviolet wavelengths of the spectrum. The paper set off a chain reaction in European science. H. Marshall Ward demonstrated in the 1890s, using prism experiments, that the germicidal effect was strongest precisely in the UV and violet-blue wavelengths. The logic followed naturally: if UV light killed bacteria in a test tube, it might kill bacteria in a human body too.

Dr. John Coakley Lettsom had already established the Royal Sea Bathing Infirmary for Scrofula in Margate, England, after noticing that fishermen who spent their days outdoors in sea air and sunlight rarely suffered from certain forms of tuberculosis. Physicians began prescribing tropical climates, then retreats to the mountains. German physician Hermann Brehmer formalized a sun-based treatment protocol for tuberculosis in 1857. By the end of the nineteenth century, sanatoria had proliferated across Europe and America, their south-facing terraces lined with patients in deck chairs turning their faces to the light.

The Danish physician Niels Ryberg Finsen was the man who translated the laboratory science into clinical practice most ambitiously. Finsen was himself chronically ill, suffering from what is now thought to be Niemann-Pick disease, which left him anemic and exhausted, and he was personally convinced that sunlight might restore what his body could not produce. In 1893, he published a paper arguing that specific wavelengths of light carried specific medical benefits, and began treating lupus vulgaris, a disfiguring form of skin tuberculosis that pocked the faces of its victims, with concentrated ultraviolet light from electric carbon arc lamps he designed himself. The rock crystal lenses he used were specially engineered to emit UV while filtering out other wavelengths. They were shown to be remarkably effective. By 1903, Finsen had treated several hundred tuberculosis patients with ultraviolet radiation, an achievement for which he was awarded the Nobel Prize in Medicine. He was forty-three years old and dying of the illness that had driven him to his research. He did not live to see the consequences of what he had set in motion.

The rickets problem yielded its own UV breakthrough. In 1924, Harry Steenbock and his colleague Alfred Black showed that irradiating food with UV light increased its vitamin D content sufficiently to cure rickets. This discovery led directly to the fortification of milk and other common foods with vitamin D, a public health intervention credited with virtually eradicating rickets as a health concern. Sanatoriums offering heliotherapy, the treatment of illness with sunlight, sprang up across Europe and America. Auguste Rollier became heliotherapy's most celebrated practitioner, treating patients at Leysin in the Swiss Alps with full-body sun exposure. Children at his Alpine clinic skied in nothing but boots and loincloths, the better to absorb the beneficial rays. John Harvey Kellogg, the American health reformer and corn-flakes inventor, visited Finsen's clinic and established light therapy as a feature of his Battle Creek Sanitarium in Michigan.

The medical profession had drawn a clear conclusion: ultraviolet radiation was therapeutic. It killed bacteria, cured rickets, healed skin tuberculosis, and in Finsen's hands won a Nobel Prize. It was, in a word, good.

This conclusion was not wrong, exactly. It was dangerously incomplete. What the physicians of Finsen's era did not known was that the same ultraviolet radiation that killed bacteria also damaged DNA. The same photons that destroyed bacteria were mutagenic: capable of disrupting the genetic machinery of healthy cells in ways that could, years or decades later, turn them malignant. The knife that healed also cut. But that discovery was not yet available.