How Is a Sunscreen's SPF Calculated?

Rawpixel/iStock via Getty Images
Rawpixel/iStock via Getty Images

I’m a pale person. A very pale person. Which means that during these hot summer months, I carry sunscreen with me at all times, and apply it liberally. But I’ve never really understood what those SPF numbers meant, so I asked some sun care to break it down for me—and to tell me how to best apply the stuff so that I can make it through the summer without looking like a lobster.

Soaking up the sun ... safely

SPF stands for Sun Protection Factor, and it indicates a sunscreen’s ability to block UVB rays. The concept was pioneered at the Coppertone Solar Research Center in 1972; in 1978, the FDA published an SPF method based on Coppertone’s system, according to Dr. David Leffell, chief of Dermatologic Surgery and Cutaneous Oncology at Yale.

The numbers themselves stand for the approximate measure of time a person who has applied the sunscreen can stay out in the sun without getting burned. Say you get burned after 20 minutes in the sun without sunscreen; if properly applied (and reapplied), SPF 30 will allow you to stay in the sun 30 times longer without burning than if you were wearing no protection at all. So, theoretically, you should have approximately 600 minutes, or 10 hours, in the sun. But it’s not an exact science because the amount of UV light that reaches us depends on a number of factors, including cloud cover, the time of day, and the reflection of UV rays off the ground, so it’s generally recommended that you reapply sunscreen every two hours (or even sooner).

What gives a sunscreen a higher SPF comes down to the product’s formulation. “It’s possible that an SPF 50 might contain slightly more of one or more sunscreen active ingredients to achieve that higher SPF,” Dr. Patricia Agin, president of Agin Suncare Consulting, says. “But it’s also possible that the SPF 50 might contain an additional active ingredient to help boost the SPF performance to SPF 50.”

No matter what SPF your sunscreen is, you’ll still get a burn if it’s not properly applied. So let’s go over how to do that.

How to apply sunscreen

First, make sure you have a water-resistant, broad spectrum sunscreen—which means that it protects against both UVB and UVA radiation—with an SPF of at least 30. “Typically, you don’t have to buy sunscreen that has an SPF higher than that unless you have very sun sensitive skin,” Leffell says. “That’s a very small percentage of the population.” (Redheads, people with light eyes, and those who turn pink after just a few minutes in the sun—you’ll want to load up on SPF above 30.)

Twenty minutes before you go out to the beach or the pool, begin to apply your sunscreen in an even coat. “Don’t apply it like icing on a cake,” Leffell says. “I see these patients and they’ve got the tops of their ears covered with thick, unevenly applied sunscreen, and that’s not a good sign.” Sunscreen sprays will easily give you that even coat you need.

Whether you’re using lotion or a spray, when it comes time to apply, Leffell recommends starting with your scalp and face, even if you plan on wearing a hat. “Make sure you’ve covered the ears and nose and under the eyes,” Leffell says. “Then, I would move down to the shoulders, and make sure that someone can apply the sunscreen on your back beyond the reach of your hands.”

Other areas that are important that you may forget to cover, but shouldn’t, are the tops of your feet, the backs of your hands, and your chest. “We see it all the time now—the v of the chest in women has become a socially and aesthetically huge issue when they are 50 and beyond. Because even though they can treat their faces with all sorts of cosmetics and procedures, the chest is much harder, and they are stuck with the face of a 40-year-old and the chest of a 60-year-old. You want to avoid that using sunscreen.”

Another important thing to keep in mind: Water-resistant doesn’t mean waterproof. “I always tell patients to reapply every couple of hours while you’re active outdoors," Leffell says, "and always reapply when you come out of the water or if you’ve been sweating a lot, regardless of whether the label says water resistant."

Determining whether or not you’ve succeeded in properly applying your sunscreen is easy: “You know you’re applying your sunscreen properly if, after the first time you’ve used it, you haven’t gotten a burn,” Leffell says.

Agin has a caveat, though: "It’s not a good idea to think of sunscreens only as a way to extend your time in the sun," she says. "One must also understand that even before becoming sunburned, your skin is receiving UV exposure that causes other damage to the skin. At the end of the 600 minutes, you will have accrued enough UV to cause a sunburn—one Minimal Erythema Dose or MED—but there is pre-MED damage done to skin cells’ DNA and to the skin’s supporting structure of collagen and elastin that is not visible and happens even before you sunburn. These types of damage can occur without sunburning. So you can’t measure all the damage done to your skin by only being concerned about sunburn."

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An earlier version of this post ran in 2014.

Why Are Shower Doors in Hotel Rooms Getting Smaller?

sl-f/iStock via Getty Images
sl-f/iStock via Getty Images

Shower doors are shrinking in posh hotels, and minimalism is to blame, Condé Nast Traveler reports.

In lieu of hanging shower curtains or providing full shower doors, many newer hotels are opting for glass panels that cover only half the length of the shower. That’s frustrating for many travelers, who complain the growing trend is inconvenient and leaves bathroom floors sopping wet and slippery after shower use.

According to Condé Nast Traveler, the half-door trend began in European hotels in the 1980s. “A lot of it comes down to people trying to design hotel rooms with limited space,” boutique hotel designer Tom Parker told the magazine. “It’s about the swing of the shower door, because it has to open outward for safety reasons, like [if] someone falls in the shower. You have to figure out where the door swing’s going to go, make sure it’s not [hitting] the main door. It’s just about clearances.” A smaller door also has the added benefit of making the space appear larger than it really is, according to the magazine.

The trend is also connected to the birth of minimalist “lifestyle hotels,” which cater to a younger, hipper clientele that gravitates toward sleek lines and modern design. Plus, half-size glass doors are easier to clean than shower curtains, which tend to trap bacteria and need to regularly be replaced, which can add up to significant additional costs for a hotel.

Theoretically, even half-door showers are designed to minimize water spillage. Designers try to level the floors in bathrooms so water doesn’t pool in random areas, and they place shower heads and knobs in areas that are more protected by glass paneling. And where design doesn’t work, hotels try to pick up the slack.

“Hotels tend to mitigate the risks by offering non-slip interior shower mats, cloth bath mats for stepping out of the shower, grab bars, [and] open showers or no-sill showers which avoid having to step up and over the ledge,” designer Douglas DeBoer, founder and CEO of Rebel Design Group, told Condé Nast Traveler.

But the half-door trend still has yet to gain much love from hotel guests. “The older generation much, much prefers having a shower door,” Parker told Condé Nast Traveler. “I’m like a 70-year-old man at heart anyway. I like [a shower door] if it’s in keeping with the style of the rest of the room.”

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Does Pushing the Button at a Crosswalk Actually Do Anything?

Pressing this crosswalk button may or may not do something.
Pressing this crosswalk button may or may not do something.
David Tran/iStock via Getty Images

Since crosswalk signals rarely seem to give you the green light (or more accurately, the white, human-shaped light) right after you press the button, you may find yourself wondering if those buttons actually work. The potentially exasperating answer is this: It depends.

First and foremost, it’s important to understand that crosswalk buttons aren’t designed to have an immediate effect; they’re just supposed to tell the system that a person is waiting to cross. As CityLab explained, some systems won’t ever give pedestrians the crossing signal unless someone has pressed the button, while others are programmed to shorten the wait time for walkers when the button has been pressed. No matter what, the system still has to cycle through its other phases to give cars enough time to pass through the intersection, so you’ll probably still have to stand there for a moment.

During busy traffic times or under other extenuating circumstances, however, cities can switch the system to what’s known as “recall mode,” when pedestrian crossings are part of the cycle already and pressing the button quite literally changes nothing. Unfortunately, there’s no way to tell if a particular button is in recall mode, short of calling your city officials and asking an expert to come inspect it.

But if you feel like a button isn’t doing anything, there’s a pretty good chance it’s been permanently deactivated. As congestion has increased and the systems to manage it have become more advanced over the years, cities have moved away from using crosswalk buttons at all. In 2018, for example, CNN reported that only around 100 of New York City’s 1000 buttons were still functioning. Since actually removing the buttons from crosswalks would be a costly endeavor, cities have opted to leave them intact, just waiting to be pummeled by impatient pedestrians who don’t know any better.

What about 'close door' buttons on elevators, you ask? That depends, too.

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