A man is sprayed with sunscreen at the Coachella Valley Music and Arts Festival. REUTERS/Lucy Nicholson. (REUTERS)

Boer Deng writes for Nature.

Summer’s smell — sun, sea, sandals, shore — lives in the vague sweetness of sunscreen. Sunblock is one of the season’s totems, along with pink-jacketed paperbacks and bottles of lime-favored beer. But for all its ubiquity, the elixir is often misused. Here are five myths, dispelled to help you protect yourself better.

1. You need sunscreen only on sunny days.

Plenty of people have had the unpleasant experience of going out on a cool, overcast day — too rare during Washington summers — only to return with a nasty sunburn. Yet one of the most-cited reasons for forgoing sunscreen is cloud cover. When clouds are shrouding the sun, it’s easy to think that they’re protecting you from its rays. But unlike light or warmth, the sun’s skin-damaging ultraviolet radiation can’t be perceived directly. Although clouds block some UV radiation, 80 percent still reaches the Earth’s surface. Clouds can also have the effect of reflecting UV rays, enhancing their reach.

2. The SPF is what matters.

Advice about sunscreen invariably begins with the recommendation to use something with an SPF value of 15 or greater, giving the impression that those three familiar letters are all you need to decide which sunscreen to buy. But SPF (“sun protection factor”) is more akin to meter-markers at the local pool — a gauge for what might suit a particular swimmer. It’s an approximation of how much time you can spend in the sun without burning. An SPF 15 sunscreen, for example, will allow you to stay exposed 15 times longer without burning than if your skin was unprotected, while an SPF 30 product is said to extend this 30 times. Protectiveness against UV rays does not scale in proportion to the SPF value, however: SPF 15 lotions block 93 percent of UV rays; SPF 30, about 97 percent.

Nor is SPF efficacy exact. Testers arrive at the figure by applying two milligrams of sunscreen to a square centimeter of skin. Users regularly apply less — only a quarter or half that amount, by one estimate. Moreover, not all sunscreen formulations are the same: Seventeen sun-filtering molecules are approved by the Food and Drug Administration for blocking various lengths of UV radiation, which ranges from 200 to 400 nanometers on the electromagnetic spectrum. Other ingredients confer water resistance or chemical stability, which changes how long UV protection lasts.

Sometimes, an inflated SPF value offers misleading assurance with little meaning (surely the debut of an SPF 200+ lotion would bring “a world without poverty,” quipped a New Yorker humor column). In fact, the false safety implied by ever-higher SPF numbers inspired regulators in Europe, Japan and Australia to cap SPF values. Sunscreens sold in those markets do not exceed 50.

3. Darker-skinned people have less need for sunscreen.

It’s true that some of us are more susceptible to sunburn than others. In part, this is related to the presence of melanin, a pigment in skin that absorbs wavelengths of UV radiation. But that natural shield is not so strong that sunscreen can be jettisoned: Melanin is thought to offer an SPF value of 1.5 to 2.0 and is less effective at blocking the most damaging UV rays, known as UVA radiation, which penetrate deeper into skin.

The mistaken notion that dark skin is naturally more protected propagates in insidious ways. For example, a 2006 study found that sunblock ads are much more prevalent in magazines with predominantly white readerships, compared with those aimed at black audiences. Myths about sun protection among African Americans can have lethal consequences: Though the incidence of skin cancer among blacks is low compared with other groups, their survival outcomes are poorer — a 73 percent five-year survival rate compared with a rate of 91 percent for whites. This is partly because of differences in access to medical care and partly because of the types of skin cancers that disproportionately strike dark-skinned people, including the aggressive acral lentiginous melanoma. A lack of awareness of skin cancer risks among African Americans is thought to contribute to black patients not seeking treatment immediately for suspicious skin lesions; as a result, many African Americans present with more advanced cases of melanoma.

4. That old bottle is just fine.

Like spoiled milk, expired sunscreen isn’t much good. But consumers seem befuddled about whether an expiration date represents a meaningful warning or merely manufacturer caution. According to one study, one-third of people don’t check the dates at all. Sunscreens expire because their ingredients deteriorate over time, losing their power.

Old sunblock isn’t just ineffective; in some cases, that bottle in the back of the medicine cupboard might even be dangerous: Banana Boat was forced to recall a raft of North American products manufactured between January 2010 and September 2012 after it was discovered that these posed a “potential risk of . . . igniting on the skin” if they came in contact with a combustible source (the fire from a barbecue grill, say) before completely drying. Besides, if you’re applying sunblock properly, you probably won’t have any left over at the end of the summer, anyway.

5. Sunscreen is toxic.

“Your sunscreen might be poisoning you,” declares a headline on the Web site for “The Dr. Oz Show.” “Is sunscreen dangerous?” asks U.S. News & World Report. Driving these worries are past studies linking particular sunscreen ingredients with undesirable byproducts and possible negative health effects. Some fear that it causes cancer, though the medical consensus says such concerns are overblown. The link between photodamage and skin cancer is well established; the possible adverse effects of sunscreen chemicals far less so. And sunblock products in the United States are rather well-regulated: The FDA has considered sunscreen an over-the-counter drug since the late 1970s, making it subject to more regulatory scrutiny than cosmetic lotions, fragrances or creams that also penetrate the skin.

This has drawn criticism from some quarters: Chemical & Engineering News recently lamented that American consumers do not have access to a number of sun-blocking ingredients available in Europe. Some of those compounds have been languishing under FDA review since 2002. The pace of approval is so slow that President Obama signed the Sunscreen Innovation Act in November 2014 to get things moving.

But for those to whom “chemicals” sound ominous and who disdain the thought of sunscreen, consider: Plant spores contain compounds that naturally block UV rays. Hippos secrete a pigment mixture to protect themselves against the sun. We humans would be foolhardy not to learn from nature and do the same.

Twitter: @boerdeng

Five myths is a weekly feature challenging everything you think you know. You can check out previous myths, read more from Outlook or follow our updates on Facebook and Twitter.