The late 19th century and the early 20th century must be the most exciting time in the history of human civilization. People were liberated from the country, and crowded in the cities for work. Many discoveries saved lives, while many creations spoiled them. Human’s understanding of the world reached an unprecedented new level, but it came with the ability to destroy it. No one could say it better than Charles Dickens, “It was the best of times, it was the worst of times; it was the age of wisdom, it was the age of foolishness; it was the epoch of belief, it was the epoch of incredulity; it was the season of light, it was the season of darkness; it was the spring of hope, it was the winter of despair.”  

The industrialization and rapid urbanization brought many novel opportunities and unprecedented problems. The crowded population in cities accelerated the spread of infectious disease like tuberculosis. Since the 1880s, several scientists, including German physician scientist Robert Koch, discovered that UV in the sunlight can kill bacteria. In 1893, Danish physician Niels Ryberg Finsen advocated sun baths for all forms of tuberculosis. By 1903, Finsen had treated several hundred tuberculosis patients with ultraviolet radiation from

the arc lamps he had invented, an endeavor for which he was awarded the Nobel Prize. The arc from the lamp emitted light with a similar spectrum to sunlight. The rock crystal lenses they used were specially designed to emit UV, filtering out other wavelengths. They were shown to be very effective. 

Meanwhile, in the early 1900s, a rise in rickets, a bone disease, was noted in children living in cities, and it was observed that exposing them to sunlight or artificial UV light helped to cure it. This led to the hypothesis that sunlight was producing something that prevented or cured rickets. In 1924, Harry Steenbock and his colleague Alfred Black found that irradiating food with ultraviolet (UV) light gave it the ability to cure rickets. They correctly reasoned that the UV irradiation increased the content of vitamin D, which is essential for preventing and treating rickets. This discovery paved the way for the fortification of common foods, especially milk, with vitamin D. The widespread adoption of this practice was credited with virtually eradicating rickets as a public health concern.Sanatoriums offering heliotherapy- the treatment of illness with sunlight- sprang up across Europe and America to become a mainstay of treatment for diseases like tuberculosis and rickets, and remained so until the mid-20th century.

When the new middle class wanted to enjoy the vacation in the summer, they left the crowded city for the beach. Even though people got the idea that sunbathing benefited urban life, and UV was good for health, they avoided getting tan, as pale complexion dominated comptemporary aesthetic standards in general perception. Tanned skin was regarded as a symbol associated  with the working class who spent long hours in outdoor jobs. In one summer of the 1920s, Coco Chanel spent a vacation at the seaside resorts in the French Riviera by the Mediterranean, and accidentally got skin tanned under sun. When she returned, people were fascinated by her tanning, making it a new symbol of luxury and leisure for the upper class. She helped inspire the trend of sunbathing. Soon "sunlight therapy" was prescribed for almost every ailment from fatigue to tuberculosis. The envy of darker skin tone is robustly lasting in Europe and America even today. 

(In Asia, oppositely, the envy of pale skin tone is robustly lasting to nowadays. That’s another story I will talk about later.)  

The danger of sunlight, especially the UV in its composition, did not go fully unnoticed. For sure, it is common sense that over-exposure with sunlight results in sunburn of skin, sometimes with blisters. The skin can recover by avoiding more sunbath. Other than that, it doesn’t seem to harm the skin in any manner. The first clue appeared in 1894, when the German dermatologist Paul G. Unna linked skin change such as hyperkeratosis and cancer in sailors with chronic exposure to UV of sunlight in his paper 'Carcinom der Seemanshaut' (Cancer of sailor’s skin). Just a few years later, William Dubreuilh undertook an epidemiological study, finding that skin cancer was more common amongst rural workers who were routinely exposed to the sun. What they identified was the most common form of skin cancer, basal cell carcinoma, which is developed from the keratinocytes, the most abundant cells in the epidermis. If left untreated, basal cell carcinoma can invade and destroy a wide range of the neighboring tissues. If that happened on the face, it could cause facial deformation including losing nose or eyes.  

Further evidence emerged in 1928, when George Findlay investigated the carcinogenic effects of UV by exposing mice to mercury arc radiation. He found that UV could either accelerate the occurrence of papilloma, a benign tumor grown from the upper layer of skin cells, induced by coal tar, or induce the papilloma formation by itself though a much longer time (after 8 months) at a much lower incidence (papilloma is benign). This is the first experimental evidence of the carcinogenic effect of UV light.     

Basal cell carcinoma can be easily diagnosed and can be cured by surgical resection, especially when diagnosed at an earlier stage. In fact, the five-year survival rate of the patients with localized tumors is almost 100%. Meanwhile, a more rare form of skin cancer (~1/50 of basal cell carcinoma cases) had not received much attention. Its relation with sun exposure or UV light was also not as clear as basal cell carcinoma. However, in a few decades, people will learn that it is the most deadly skin cancer. It’s melanoma.