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How is SPF Calculated?

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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,” says 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.”

But 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

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."


An earlier version of this post ran in 2014.

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Big Questions
Why Is Soda Measured in Liters?
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Never a nation to fall in line, America is one of the few countries to resist the metric system. We stubbornly measure distance in miles and weight in pounds. So what’s with those two-liter bottles of soda?

First, a clarification: Soda is far from the only substance we measure in metric units. Heck, it’s not even the only beverage. Wine, liquor, and bottled water are sold by the milliliter. The healthcare field is all about metric units, too, from cholesterol levels to prescription, over-the-counter, and supplement dosages. We run 5-kilometer races, ride on 215-millimeter tires, and use 8-millimeter cameras, or at least we used to.

In most other things, we determinedly cling to our imperial measurements. Attempts to convince Americans to join the rest of the metric-measuring world have been met with great resistance.

Ken Butcher of the National Institute of Science and Technology has been working with the government’s tiny Metric Program for years. Speaking to Mental Floss back in 2013, Butcher explained that we’re so entrenched in our way of doing things that switching measurement systems now would be both chaotic and expensive.

"If we were going to start a new country all with the metric system, it would be easy," he said. "But when you have to go in and change almost everything that touches people’s everyday life and their physical and mental experience, their education, and then you take that away from them—it can be scary."

Here and there, though, when it’s convenient, we have been willing to budge. The soda bottle is a good example. Until 1970, all soft drinks in the U.S. were sold in fluid ounces and gallons, mostly in glass bottles. Then the plastic polyethylene terephthalate (PET) bottle came along, and soft drink makers decided it was time for a product redesign.

The redesign process coincided with two key factors: a short-lived wave of government interest in going metric, and the burgeoning environmental movement.

The folks at PepsiCo decided to meld all three into its exciting new vessel: a lightweight, cheap, recyclable, metric bottle, with built-in fins so it could stand up on supermarket shelves. Two liters: the soda size of the future.

The two-liter bottle took off. The rest of the soft drink world had no choice but to get on board. And voila: liters of cola for all.

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

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Big Questions
Where Is the Hottest Place on Earth?
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The summer of 2017 will go down as an endurance test of sorts for the people of Phoenix, Arizona. The National Weather Service issued an extreme heat warning, and planes were grounded as a result of temperatures exceeding 120 degrees. (Heat affects air density, which in turn affects a plane’s lift.)

Despite those dire measures, Phoenix is not the hottest place on Earth. And it’s not even close.

That dubious honor was bestowed on the Lut Desert in Iran in 2005, when land temperatures were recorded at a staggering 159.3 degrees Fahrenheit. The remote area was off the grid—literally—for many years until satellites began to measure temperatures in areas that were either not well trafficked on foot or not measured with the proper instruments. Lut also measured record temperatures in 2004, 2006, 2007, and 2009.

Before satellites registered Lut as a contender, one of the hottest areas on Earth was thought to be El Azizia, Libya, where a 1922 measurement of 136 degrees stood as a record for decades. (Winds blowing from the nearby Sahara Desert contributed to the oppressive heat.)

While the World Meteorological Organization (WMO) acknowledged this reading as the hottest on record for years, they later declared that instrumentation problems and other concerns led to new doubts about the accuracy.

Naturally, declaring the hottest place on Earth might be about more than just a single isolated reading. If it’s consistency we’re after, then the appropriately-named Death Valley in California, where temperatures are consistently 90 degrees or above for roughly half the year and at least 100 degrees for 140 days annually, has to be a contender. A blistering temperature of 134 degrees was recorded there in 1913.

Both Death Valley and Libya were measured using air temperature readings, while Lut was taken from a land reading, making all three pretty valid contenders. These are not urban areas, and paving the hottest place on Earth with sidewalks would be a very, very bad idea. Temperatures as low as 95 degrees can cause blacktop and pavement to reach skin-scorching temperatures of 141 degrees.

There are always additional factors to consider beyond a temperature number, however. In 2015, Bandar Mahshahr in Iran recorded temperatures of 115 degrees but a heat index—what it feels like outside when accounting for significant humidity—of an astounding 163 degrees. That thought might be one of the few things able to cool Phoenix residents off.

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

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