“Ladies and gentlemen of the class of ’97: Wear sunscreen.” So begins a hypothetical graduation speech written by Chicago Tribune columnist Mary Schmich that rippled through pop culture that year — long before “going viral” was even a thing. “If I could offer you only one tip for the future, sunscreen would be it,” Schmich continued. “The long-term benefits of sunscreen have been proved by scientists, whereas the rest of my advice has no basis more reliable than my own meandering experience.”

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Australian film director Baz Luhrmann would later reach the number 10 spot on the Billboard charts with a spoken-word version that adapted Schmich’s words over a soulful beat. And while the “speech” is still sometimes misattributed as a real commencement address delivered by Kurt Vonnegut, the writer went on record with The New York Times to deny authorship. “What I said to Mary Schmich on the telephone was that what she wrote was funny and wise and charming,” Vonnegut told the newspaper, “so I would have been proud had the words been mine.”

More than two decades later, it remains a delightful essay, even if its most enduring conceit — that sunscreen is a certain and safe defense against the cancer-causing effects of the sun — is less rooted in science than most of us have come to believe. Indeed, the campaign to convince us to wear sunscreen began years before Schmich penned her wry essay, so that by now it has been burned into our collective consciousness. The Skin Cancer Foundation says on its website that “anyone over the age of six months should use a sunscreen daily” — even indoors. The American Academy of Dermatology also recommends it for everyone: “Sunscreen use can help prevent skin cancer by protecting you from the sun’s harmful ultraviolet rays. Anyone can get skin cancer, regardless of age, gender, or race.”

But here’s the rub: While it’s plausible that sunscreen prevents skin cancer (more on that later), robust evidence to back that up is hard to come by. Such is the conclusion of a review study published in the Journal of the American Academy of Dermatology in February. “Sunscreen is a multibillion-dollar industry, and its efficacy in the prevention of skin cancer is often taken as fact,” the authors note. “Despite this, there are only four prospective studies that examine sunscreen’s role in preventing skin cancer, and none of these studies examine the efficacy of sunscreen in preventing skin cancer in otherwise healthy individuals.”

You might be thinking “So what? What’s the harm in using sunscreen every morning — particularly if there’s a chance it might prevent skin cancer, right?” Well, maybe. But even that is complicated. For starters, not everyone has the same skin cancer risk, with darker skinned people facing a far lower risk overall than lighter skinned people. On top of that, a recent study published in the Journal of the American Medical Association (JAMA) raised questions about the safety of various sunscreen ingredients. In a pilot study of 24 adults, scientists at the U.S. Food and Drug Administration (FDA) showed that four active ingredients commonly used in sunscreens — avobenzone, ecamsule, octocrylene, and oxybenzone — were absorbed into the bloodstream at high enough levels to trigger the need for additional safety studies.

While, the health implications of minute amounts of sunscreen chemicals in your blood is unknown, other research mostly in test tubes and lab animals, has raised a few concerns. In particular, oxybenzone has been shown to affect sex hormones and cause allergic reactions.

In an editorial accompanying the JAMA study, former FDA commissioner Robert Califf and the editor in chief of JAMA Dermatology, Kanade Shinkai, suggested that our cultural certainty about sunscreen is unearned. “Sunscreen users reasonably presume that companies that manufacture and sell sunscreens have conducted basic studies to support the safety and effectiveness of their products and that the medical profession would demand high-quality evidence,” they wrote, adding that despite decades of widespread use: “[S]unscreens have not been subjected to standard drug safety testing.”

To be clear: The argument here is not against sunscreen. Sunlight can damage the skin, and sometimes lead to cancer, so it’s prudent to protect yourself. The problem is that good data on precisely how effective sunscreen is, in what formulations, and for which people, is significantly more sparse than most consumers have been led to believe — though scientists and public health officials have known this for a long time. They simply chose to gloss over the uncertainties in deference to a highly profitable industry and an easy-to-digest health message. “The problem is, as humans we don’t like uncertainty and we can’t handle assessing risk very well,” said Adewole Adamson, an assistant professor at Dell Medical School at the University of Texas at Austin. While the public should be made aware of where the science stands, Adamson told me, if messaging conveys uncertainty, “it is possible that misinformation can creep in more easily, especially in the era of social media where misinformation travels fast.”

And yet, hiding the uncertainty carries risks, too. Buried questions don’t go away, after all, they mushroom. “If we don’t do the research needed to address uncertainties,” Adamson added, “ultimately physicians, public health officials, and industry are at risk of losing public trust.”

There’s no doubt that ultraviolet (UV) radiation from sunlight takes a toll on our skin. Over time, exposing naked flesh to the sun causes wrinkling, sagging, and dark patches called lentigos, more commonly known as age spots. UV exposure can also increase the risk of skin cancer by damaging the DNA in skin cells, causing them to develop abnormally and multiply out of control.

Skin cancer is the most commonly diagnosed cancer in the U.S. according to the American Cancer Society. An estimated 3.3 million Americans will be diagnosed with either squamous cell carcinoma (SCC) or basal cell carcinoma (BCC) each year — cancers that can be disfiguring, but are seldom life threatening. Fewer than 100,000 people will be diagnosed with melanoma, which can be deadly if it spreads, but is highly curable if detected while still confined to the skin, as most cases are in this country.

Melanin, the pigment that gives skin its color, absorbs and scatters UV light. So, in general, the darker your skin, the lower your risk of skin cancer, although data for non-melanoma skin cancers are very limited for people of color says Susan Taylor, an associate professor of dermatology at the University of Pennsylvania and founder of the Skin of Color Society, a professional organization dedicated to skin health issues in people of color. Melanoma is extremely rare in African Americans, and typically develops in areas not exposed to the sun, such as the palms and soles of the feet.

Given the sun’s probable role in most skin cancers, though, it’s surprising that there isn’t stronger evidence that sunscreen prevents all forms of the disease. In the Journal of the American Academy of Dermatology review, the authors found only four randomized controlled trials conducted in the last four decades. The analysis found strong evidence that sunscreen prevented squamous cell, but not basal cell carcinomas, most likely because those cancers develop too slowly for studies to detect a trend, said lead author Reid Waldman, a dermatology resident at the University of Connecticut.

Only one study looked at melanoma, Waldman and his co-author found. Although that study is widely cited as proof that sunscreen halves the risk of melanoma, that’s misleading, Waldman told me. People given sunscreen and told to use it daily during the four years of the original trial had a 1.5 percent risk of developing melanoma 10 years later, compared to a 3 percent risk in those not given sunscreen or instructions — a difference that barely reached statistical significance.

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Melanoma, meanwhile, is a complex disease: Genetics, patterns of exposure (regular sun exposure may be less harmful than intermittent, high-intensity sunlight, for example), and other still-unknown factors contribute to the risk. This may partly explain why melanoma rates in the U.S. have tripled since the 1970s, even as the use of sunscreen has increased. A lot of the blame, says Waldman, may be due to the use of tanning beds, which studies show sharply increases the risk of melanoma and other skin cancers. Another theory is that sunscreen may actually increase people’s skin-cancer risk if it allows them to spend more time in the sun, especially if they are not protected against the broad spectrum of UV radiation — including what’s known as UVA radiation and its counterpart, UVB.

UVB rays cause sunburn; UVA rays, the primary form of radiation used in indoor tanning, penetrate the skin more deeply and damage DNA without blistering. Unlike in Europe, U.S. regulations allow sunscreen makers to sell products that filter out UVB to prevent sunburn, but provide less protection against DNA-damaging UVA.

“If you just use a sunscreen that just protects against sunburn,” said David Andrews, a senior scientist at the nonprofit Environmental Working Group, which recently published a comprehensive report on sunscreens, “you are effectively getting the same sun exposure as you would from a tanning bed.”

Meanwhile, researchers have known for at least two decades that the chemicals in many sunscreens get into the blood and potentially cause harm. “The American public should be furious that the active ingredients in these products have not been adequately tested for safety,” said Andrews.

In February, the FDA proposed new regulations that would require manufacturers to test 12 active ingredients to see if they are absorbed into the blood at levels above a specified threshold. If they are — as the recent FDA study proved is likely — then companies will be required to do further tests in lab animals to see if the chemicals increase the risk of cancer or cause reproductive or developmental problems. The FDA has also proposed classifying the two chemicals used in mineral sunscreens, zinc oxide and titanium dioxide, as “generally recognized as safe and effective,” exempting them from further tests. Those ingredients act as a physical barrier to UV radiation and do not appear to penetrate the skin.

The rule, which if finalized would go into effect in November, also requires that products with a sun protection factor (SPF) of 15 or higher provide broad-spectrum protection — that is protection from both UVA and UVB radiation.

In an email, a spokesperson for Johnson & Johnson, one of the leading sunscreen makers, acknowledged that small amounts of sunscreen ingredients are absorbed, but deflected my question about whether that finding necessitates further safety testing. “Each sunscreen ingredient has a safety profile that substantiates their safe use when used appropriately, and each has been in use for decades with FDA approval,” the spokesperson said. “No credible evidence shows that absorption of small amounts of sunscreen ingredients, like what was found in [the FDA] study, has harmful effects on human health.”

At the end of the day, Schmich’s advice to wear some sort of sunscreen is still sound for most people, according to all the experts I spoke with. What we do know with the greatest certainty is that sunlight can damage the skin, so it’s prudent to protect yourself. In a statement about the FDA study, the Skin Cancer Foundation echoed Johnson & Johnson’s assertion that sunscreens have been used for many years without evidence of harm: “There’s simply no justification for abandoning sun-safe behaviors.”

For people concerned about possible health risks, experts advise using mineral sunscreens until we have better safety data on the chemical versions. “Out of an abundance of caution, pregnant and nursing women may want to consider using mineral sunscreens as well,” advises the Skin Cancer Foundation.

Susan Taylor of the University of Pennsylvania, who is also vice-president elect of the American Academy of Dermatology, says the advice to wear sunscreen applies to people with darker skin too. “Although blacks are less likely to develop non-melanoma skin cancer and melanoma, when we do develop it, it is at a more advanced stage with increased morbidity and mortality,” she said. “So why not recommend that blacks use sunscreen? Would we not be doing an injustice to black people if we did not recommend it until we have more data?”

In addition, both medical societies and the experts I spoke with warn against relying on sunscreen like it’s an invincible shield, which it’s not. Seeking shade, avoiding the direct midday sun when UV intensity peaks, and wearing UV-protective clothing, broad-brimmed hats, and sunglasses are also recommended. “I think that we should be discussing even more the role of sun avoidance for children as well as protective clothing more than we do,” said Taylor.

Still, the soundbite of advice to “wear sunscreen” conceals a host of unknowns, according to the JAMA editorial. There’s the “urgent question” of the effects in infants and children, who may absorb chemicals at a higher rate. There’s also the question of how much people inhale from spray sunscreens or absorb from the many personal-care products that now contain UV filters. And Adamson pointed out that research and public health advice often ignores how the risk-to-benefit calculus may differ for people of color. “Dermatology is a largely white organization,” said Adamson, who is black.

“There’s also the optics of, here you are, tone deaf, giving these recommendations to folks with darker skin that are completely devoid of any evidence at all,” he added.

Adamson told me that he recently agreed to serve on a panel for the American Academy of Dermatology to discuss more nuanced recommendations. The AAD and other organizations could do a better job relaying uncertainty, said Taylor. “I feel strongly that we must give the public accurate information (even if it is ambiguous) for each person to decide what is best for them.”

Andrews told me that he’s encouraged by the FDA’s proposal to require commonsense safety testing — though he also suggests that scientists, regulators, and even journalists like me ought to have been raising questions about the certainty of public sunscreen messaging a long time ago. Had we done so, it’s likely that the market would have evolved by now to produce products whose safety is more certain. “The companies that make sunscreens and other cosmetic and consumer products will rapidly change their products to meet consumer demand,” Andrews said.

Schmich concludes her column: “Be careful whose advice you buy, but be patient with those who supply it. Advice is a form of nostalgia. Dispensing it is a way of fishing the past from the disposal, wiping it off, painting over the ugly parts, and recycling it for more than it’s worth.

“But,” she added, “trust me on the sunscreen.”

When it comes to the science of sunscreen, that trust still needs to be earned.

Teresa Carr is an award-winning, Texas-based journalist with a background in both science and writing, which makes her curious about how the world works. She is a former Consumer Reports editor and writer, and a 2018 Knight Science Journalism Fellow at MIT. In 2019, she began penning the Matters of Fact column for Undark.