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."

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us atbigquestions@mentalfloss.com.

An earlier version of this post ran in 2014.

Why Do We Eat Cranberry Sauce on Thanksgiving?

MSPhotographic/iStock via Getty Images
MSPhotographic/iStock via Getty Images

While plenty of people eat turkey, mashed potatoes, and pie year-round, it seems like cranberry sauce almost exclusively exists in the Thanksgiving universe. Although we don’t know for sure whether it was eaten at the very first Thanksgiving, the jiggly, gelatinous side dish does have deep roots in the history of America’s fruited plains.

According to Insider, cranberries are one of only three commercially grown fruits native to the United States, and the Wampanoag tribe had been using them for food, dye, and medicine long before feasting with the Pilgrims in 1621. If there were cranberries at the party, they probably didn’t taste much like the sweetened sauce we’re (circumstantially) fond of today; at that point, the settlers hadn’t yet succeeded in growing sugar cane in the New World.

But a little more than 50 years later, according to a 1672 account cited by The Washington Post, the new Americans and Native Americans had both started to enjoy cranberries much like we do at Thanksgiving dinner: “Indians and English use it much, boyling them with Sugar for a Sauce to eat with their Meat.”

In 1796, Amelia Simmons—author of American Cookery, the first-ever American cookbook—took it one step further by recommending that roast turkey be served with cranberry sauce. Considering that the Library of Congress included the book on its list of “Books That Shaped America,” it’s possible that Simmons’s suggestion reverberated through kitchens across the nation, and the tradition gained momentum from there. She does mention pickled mangoes as an alternate side dish for turkey, but the then-Indian import was likely less common than the locally-grown cranberry.

Then, in the early 1800s, Ocean Spray revolutionized the labor-intensive process of hand-picking cranberries from vines with what’s called a wet harvest. Basically, farmers flood the bogs where cranberries grow, and then they wade into the water to collect the floating berries en masse.

farmer wet-harvesting cranberries
A farmer gathering cranberries during a wet harvest.
kongxinzhu/iStock via Getty Images

This was a more efficient technique, but a mass harvest meant that more cranberries got damaged. So in 1912, Ocean Spray began crushing them into canned, jellied cranberry sauce—maximizing the yield and making it easier than ever for every home in America to slice up a cylinder of solid, sugary, berry goodness.

Explore the stories behind your other favorite (or least favorite) Thanksgiving foods here.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

Why Do We Eat Pumpkin Pie at Thanksgiving?

gjohnstonphoto/iStock via Getty Images
gjohnstonphoto/iStock via Getty Images

While it’s possible—even probable—that pumpkins were served at the 1621 harvest festival that’s now considered the predecessor to Thanksgiving, attendees definitely didn’t dine on pumpkin pie (there was no butter or wheat flour to make crust).

The earliest known recipes for pumpkin pie actually come from 17th-century Europe. Pumpkins, like potatoes and tomatoes, were first introduced to Europe in the Columbian Exchange, but Europeans were more comfortable cooking with pumpkins because they were similar to their native gourds.

By the 18th century, however, Europeans on the whole lost interest in pumpkin pie. According to HowStuffWorks, Europeans began to prefer apple, pear, and quince pies, which they perceived as more sophisticated. But at the same time pumpkin pie was losing favor in Europe, it was gaining true staple status in America.

In 1796, Amelia Simmons published American Cookery, the first cookbook written and published in the New World colonies. Simmons included two recipes for “pompkin pudding” cooked in pastry crust. Simmons’s recipes call for “stewed and strained” pumpkin, combined with a mixture of nutmeg, allspice, and ginger (yes, it seems our pumpkin spice obsession dates back to at least the 1500s).

But how did pumpkin pie become so irrevocably tied with the Thanksgiving holiday? That has everything to do with Sarah Josepha Hale, a New Hampshire-born writer and editor who is often called the “Godmother of Thanksgiving.” In her 1827 abolitionist novel Northwood, Hale described a Thanksgiving meal complete with “fried chicken floating in gravy,” broiled ham, wheat bread, cranberry sauce, and—of course—pumpkin pie. For more than 30 years, Hale advocated for Thanksgiving to become a national holiday, writing regular editorials and sending letters to five American presidents. Thanksgiving was a symbol for unity in an increasingly divided country, she argued [PDF].

Abraham Lincoln eventually declared Thanksgiving a national holiday in 1863 (to near-immediate outcry from Southerners, who viewed the holiday as an attempt to enforce Yankee values). Southern governors reluctantly complied with the presidential proclamation, but cooks in the South developed their own unique regional traditions. In the South, sweet potato pie quickly became more popular than New England’s pumpkin pie (mostly because sweet potatoes were easier to come by than pumpkins). Now, pumpkin pie reigns supreme as the most popular holiday pie across most of the United States, although the Northeast prefers apple and the South is split between apple and pecan, another Southern staple.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

SECTIONS

arrow
LIVE SMARTER