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The sun’s ultraviolet rays damage the cells of the skin in ways that promote wrinkles, blemishes, and other signs of aging. UV damage also raises a person’s risk for skin cancer, which is the most common form of cancer in the United States. These dangers are well established, and so virtually all public health messaging advises people to apply sunscreen, don protective clothing, and take other measures to shield skin from the sun’s rays. But some doctors who have studied the interaction between sunlight and human health say that “avoid the sun” recommendations are too strident, and that the benefits of moderate sun exposure without sunscreen may counterbalance — or even outweigh — the risks. “Making people phobic about being outdoors in the sun is just so counter to our evolutionary basis — it just doesn’t make sense,” says James O’Keefe, MD, a cardiologist at Saint Luke’s Mid America Heart Institute in Kansas City who has studied the interactions between sunlight and human health. He says human beings evolved to live outdoors — “We’re not moles,” he says — and that our absence of hair or fur suggests that our skin is meant to encounter some direct sun exposure. “I think there are a lot of potential mechanisms by which sunlight could benefit health.” Asked for specifics, O’Keefe says that sun-exposed skin releases large amounts of nitric oxide into the bloodstream. “Nitric oxide keeps vessels soft and supple, and it gives them a Teflon-like surface so that platelets don’t stick,” he says. “The vessels naturally produce a lot of nitric oxide when you’re healthy, and especially when you’re young.” He points out that deaths due to cardiovascular disease — the most common cause of death in the United States — tend to peak in winter both in the United States and in Europe, and that the absence of sun and its attendant nitric oxide boost may be a contributing factor. Weller, the University of Edinburgh dermatologist, has studied the relationship between sunlight and nitric oxide, as well as the effects of both on human health. He says that sun-triggered elevations in nitric oxide could help protect people from Covid-19, and his belief is based in part on a 15-year-old Swedish study that examined another deadly coronavirus: SARS. The first SARS outbreak occurred in 2002 in the Guangdong province of China. Like its close cousin Covid-19, SARS is a respiratory illness. The Swedish group showed that, in lab models, nitric oxide prevents the SARS virus from reproducing. “Covid-19 gets into the body by binding to the same receptor as the SARS virus,” Weller says. “And this [Swedish] group found that nitric oxide stops SARS from doing damage because it stops it from binding to this receptor.” If this lab work turns out to be accurate and applicable to Covid-19 — both big ifs — this could be one way in which sunlight defends the body against Covid-19. Another possibility, Weller and others say, has to do with “the sunshine vitamin.” Vitamin D and Covid-19 Vitamin D is an essential nutrient, which means the human body needs it but can’t make it. While some foods contain vitamin D, people have traditionally gotten most of their vitamin D from the sun: When exposed to ultraviolet light, a chemical reaction takes place in the skin that results in the production of vitamin D. For a just-published study in the journal Aging Clinical and Experimental Research, researchers examined the average vitamin D levels among residents of different European countries. They found a correlation between low vitamin D levels and higher rates of Covid-19 infections and — even more so — Covid-19 deaths. “Previous studies have shown that vitamin D protected against acute respiratory tract infection overall, and older adults — the group most deficient in vitamin D — are also the ones most seriously affected by Covid-19,” says Petre Cristian Ilie, PhD, co-author of the study and a research director at the Queen Elizabeth Hospital in the U.K. “Our finding was that getting vitamin D levels into the normal range might help.” Ilie says there are several mechanisms by which vitamin D could counteract Covid-19. First, vitamin D enhances the expression of an enzyme called angiotensin-converting enzyme 2, or ACE2. “Previous studies identified associations between higher levels of ACE2 and better coronavirus disease health outcomes,” Ilie says, adding that, in the lungs, ACE2 has demonstrated the ability to protect against acute lung injury.