Education, Wellness / Self-Care

SPF 101: everything you need to understand Sunscreen labels

10 questions answered.

words by: Adam Hurly
May 3, 2022

It seems obvious enough: You are heading to the beach, and you want some sun protection, but perhaps just enough to prevent a burn. Maybe if you’re seeking a tan, you opt for SPF15 instead of 30; it’s half the strength, but still strong enough, no? Is that how it works?


No, not exactly. SPF is something we all know that we need, but very few of us actually understand how it is measured, and how to properly use it to our advantage. Ditto for all the other things on a sunscreen label, from PA+ to broad-spectrum to water resistance. (Hey , at least we all know what water-resistant means, but how is it even measured?)


We’ll get into all of these questions in this SPF overview. Read on before you next lather up.


What does SPF measure, and what do the SPF numbers actually mean? 

SPF, which stands for Sun Protection Factor, measures the amount of time a product shields your skin from the carcinogenic and burn-causing ultraviolet B (UVB) rays. (There are also ultraviolet A rays. We’ll get into the two types later in this article.) So, if you apply an SPF15 product, it will protect your skin 15 times longer from burn than if you hadn’t applied it at all.


So, is SPF30 sunscreen twice as powerful as SPF15? 

In terms of how long SPF 30 lasts on your skin, yes, it is twice as strong as SPF15. However, the SPF strength does not directly scale up and down in terms of how many rays it’s filtering. SPF15 blocks approximately 93% of these UVB rays, while SPF30 blocks 97%. SPF50 blocks 98%, and SPF100 blocks 99%.


What amount of SPF should you use?

While SPF15 is a terrific shield, it makes sense to up the ante to SPF30, at a minimum. (This is more or less agreed upon across the FDA and the American Academy of Dermatology, aka AAD.) Fairer skin types should absolutely use SPF50 as a baseline.


What is broad-spectrum sunscreen?

Broad-spectrum sunscreen provides protection against both types of UV rays—UVA and UVB. There is a third type of UV ray (UVC, but of course), though its rays cannot penetrate Earth’s atmosphere. Whew.


What is the difference between UVA and UVB rays?

UVA rays have longer wavelengths and can penetrate deeper into the skin. They trigger melanin production and cause tanning, but are also responsible for “photo aging”—those skin conditions caused by excessive sun exposure. Think fine lines, rough texture, loss of firmness, hyperpigmentation, etc.


UVB rays have shorter wavelengths. They do not penetrate as deeply as UVA rays, but they can fry the outermost layers of skin. They cause sunburn, and are also a major culprit behind skin cancer.


If SPF measures UVB defense, then is there something measuring UVA protection?

While it isn’t as common to declare a product’s UVA defensiveness (yet, at least), you can look out for it. Watch for “PA+” all the way up to “PA++++”. The number of + symbols determines how strong its protection against UVA rays is, and PA stands for “Protection Grade of UVA”. It’s still a flawed system of measurement, since it only measures how strong the initial shield is, rather than how long it lasts.


What does water resistance mean? How is it determined?

If an SPF product promises water/sweat resistance, then that means it can endure 40 or 80 minutes of exposure or submersion to liquids without compromising efficacy. These products endure significant testing by the FDA (as do all SPF products, for each of the claims made on the label).


If a product lasts more than 40 minutes, it earns the initial distinction (the minimum to be called “water resistant”. If it exceeds 80 minutes, then it can call itself such, but there is no benchmark above 80 minutes, and no product may advertise itself for water resistance beyond that point.


What is mineral sunscreen?

There are two categories of sunscreen ingredients: chemical and mineral (also called physical). Chemical shields soak into the skin and neutralize UV rays upon encountering them. Mineral defenses rest atop the surface of the skin (the cheaper ones leaving a physical, white chalky cast), and they deflect UV rays.


Mineral/physical shields are hands down superior to chemical ones in terms of assured safety to humans and the ecosystem (namely the coral reefs, which can absorb particles of certain chemical sunscreens). Which one you use is a matter of preference, since the chemical options on shelves have been cleared by the FDA.


Just note that purely mineral options are rarely recalled the way we hear about chemical options. And if you do get a mineral option, make sure to get one that uses zinc oxide as its primary shield. Titanium dioxide is a sufficient option too, but its broad-spectrum abilities are decidedly inferior to zinc oxide.


How much sunscreen should you apply?

The FDA and AAD suggest applying one shot-glass worth of sunscreen for the entire average-sized adult body. They also recommend reapplying every 2 hours in the sun (unless you’re in the water or sweating and need to reapply after the allotted 40 or 80 minutes, depending on the product’s water resistance, if it has any at all; if not, then reapply immediately after emerging from the water). Be liberal with it, too. You’re not getting nearly the protection you need with a sparse layer.


Do you need to wear SPF year round?

Yes. UVA rays can penetrate clouds, and even glass. So their photo-aging abilities work around the calendar, and even come for you when you drive your car or sit near a window.


More and more, scientists are learning that the blue lights from our screens are also aging the skin, particularly by compromising firmness and causing fine lines and wrinkles. Wearing SPF while using your devices (in other words, all day, every day?) shows significant promise in slowing signs of aging. At the very least, you can use an SPF-packed moisturizer during the day. If you’re applying a moisturizer in the morning, then it may as well have SPF30+. (Be sure to reapply it throughout the day, too!) So yes, that means you should wear sunscreen indoors too.


Before you head to the beach this summer, check to see if your sunscreen expired and do a check on all your grooming products while you’re at it.