Interest in the health-promoting effects of sunlight seems to be cyclical. In the late 19th century, when it became clear that lack of sunlight caused rickets, solar therapy became all the rage. Doctors advised patients to sunbathe, and children living at higher latitudes spent three hours per week soaking up rays in carbon or mercury arc lamps. People came to believe that the sun cured a variety of disorders, including gout, ulcers and arthritis. Patients suffering from tuberculosis spent months living in sanatoriums where sunlight was among the chief prescriptions. Around this time, tanned skin, previously associated with a life of labor and poverty, became a mark of wealth. Tanned people could afford vacations, while the poor labored in the dark recesses of factories.
Beginning in the 1930s, public health authorities began to sound the alarm about skin cancer. Over the next few decades, sunlight went from panacea to toxin, as doctors urged patients to retreat into the shade.
UV light is currently experiencing something of a medical renaissance. Studies come out weekly linking sunlight deprivation to everything from multiple sclerosis to asthma. Human skin color varies roughly according to the latitude of our ancestors' origin, and anthropologists attribute this range to our need to absorb vitamin D.
Sunlight skeptics remain, however, and it has been difficult to definitively link vitamin D to such long-term problems as diabetes and heart disease.
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Explainer thanks Michael Holick of Boston University School of Medicine and Nina Jablonksi of Penn State.