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.

What's the Difference Between a College and a University?

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Chinnapong/iStock via Getty Images

Going off to college is a milestone in any young adult’s life. The phrase itself conjures up images of newfound independence, exposure to new perspectives, knowledge, and possibly even one or more sips of alcohol.

In America, however, few people use the phrase “going off to university,” or “headed to university,” even if they are indeed about to set off for, say, Harvard University. Why did college become the predominant term for postsecondary education? And is there any difference between the two institutions?

While university appears to be the older of the two terms, dating as far back as the 13th century, schools and students in North America have embraced college to describe most places of higher learning. There is no rigid definition of the words, but there are some general attributes for each. A college is typically a four-year school that offers undergraduate degrees like an associate or a bachelor’s. (Community colleges are often two-year schools.) They don’t typically offer master’s or doctorates, and the size of their student body is typically the smaller of the two.

Universities, on the other hand, tend to offer both undergraduate and graduate programs leading to advanced degrees for a larger group of students. They can also be comprised of several schools—referred to as colleges—under their umbrella. A university could offer both a school of arts and sciences and a school of business. The University of Michigan has a College of Engineering, for example.

While many of these traits are common, they’re not guaranteed. Some colleges can be bigger than universities, some might offer master’s degrees, and so on. To complicate matters further, an institution that fits the criteria of a university might choose to call itself a college. Both Dartmouth College and Boston College qualify as universities but use the college label owing to tradition. Schools may begin as colleges, grow into universities, but retain the original name.

People tend to think of a university as being more prestigious or harder to get into, but there are too many variables to make that determination at a glance. Some colleges might ask more of applicants than universities. Some universities might be smaller than certain colleges. Either one can be public or private.

Things get a little more convoluted abroad. In the UK, students go off to university (or uni) instead of college. The British version of college is typically a two-year program where students either focus on learning one particular skill set (much like a vocational school) or use the time to prepare for exams so that they can advance to university. Language matters, too; in Spanish, colegio usually refers to high school.

While the terms aren’t strictly interchangeable, there is enough of a difference between the two to try and make the distinction. Keep in mind that some states, like New Jersey, have rules about how institutions label themselves. There, a university has to have at least three fields of graduate study leading to advanced degrees.

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Why Do We Eat Candy on Halloween?

Jupiterimages/iStock via Getty Images
Jupiterimages/iStock via Getty Images

On October 31, hordes of children armed with Jack-o'-lantern-shaped buckets and pillow cases will take to the streets in search of sugar. Trick-or-treating for candy is synonymous with Halloween, but the tradition had to go through a centuries-long evolution to arrive at the place it is today. So how did the holiday become an opportunity for kids to get free sweets? You can blame pagans, Catholics, and candy companies.

Historians agree that a Celtic autumn festival called Samhain was the precursor to modern Halloween. Samhain was a time to celebrate the last harvest of the year and the approach of the winter season. It was also a festival for honoring the dead. One way Celtics may have appeased the spirits they believed still walked the Earth was by leaving treats on their doorsteps.

When Catholics infiltrated Ireland in the 1st century CE, they rebranded many pagan holidays to fit their religion. November 1 became the “feasts of All Saints and All Souls," and the day before it was dubbed "All-Hallows'-Eve." The new holidays looked a lot different from the original Celtic festival, but many traditions stuck around, including the practice of honoring the dead with food. The food of choice for Christians became "soul cakes," small pastries usually baked with expensive ingredients and spices like currants and saffron.

Instead of leaving them outside for passing ghosts, soul cakes were distributed to beggars who went door-to-door promising to pray for souls of the deceased in exchange for something to eat. Sometimes they wore costumes to honor the saints—something pagans originally did to avoid being harassed by evil spirits. The ritual, known as souling, is believed to have planted the seeds for modern-day trick-or-treating.

Souling didn't survive the holiday's migration from Europe to the United States. In America, the first Halloween celebrations were a way to mark the end-of-year harvest season, and the food that was served mainly consisted of homemade seasonal treats like caramel apples and mixed nuts. There were no soul cakes—or candies, for that matter—to be found.

It wasn't until the 1950s that trick-or-treating gained popularity in the U.S. Following the Great Depression and World War II, the suburbs were booming, and people were looking for excuses to have fun and get to know their neighbors. The old practice of souling was resurrected and made into an excuse for kids to dress up in costumes and roam their neighborhoods. Common trick-or-treat offerings included nuts, coins, and homemade baked goods ("treats" that most kids would turn their noses up at today).

That changed when the candy companies got their hands on the holiday. They had already convinced consumers that they needed candy on Christmas and Easter, and they were looking for an equally lucrative opportunity to market candy in the fall. The new practice of trick-or-treating was almost too good to be true. Manufacturers downsized candies into smaller, bite-sized packages and began marketing them as treats for Halloween. Adults were grateful to have a convenient alternative to baking, kids loved the sweet treats, and the candy companies made billions.

Today, it's hard to imagine Halloween without Skittles, chocolate bars, and the perennial candy corn debates. But when you're digging through a bag or bowl of Halloween candy this October, remember that you could have been having eating soul cakes instead.

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