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The ABCs of sunscreen

How to read a sunscreen label to ensure you get optimal sun protection.

May 20, 2022 | UCI Health
Young mother applies sunscreen to her daughter's nose.

“I recommend mineral-based sunscreens to my patients,” says UCI Health dermatologist Dr. Kristen Kelly. “They are protective and less likely to irritate the skin or cause allergic reactions.”


Sunscreen is a year-round fact of life for Southern California residents, but choosing a product to protect against skin cancer-causing ultraviolet rays can be confusing and complicated if you don’t know what to look for.

How much SPF is enough? Which is better, a mineral-based or chemical-based sunscreen? Is it waterproof?

“The two most powerful defenses against skin cancer are protective clothing and sunscreen,” says UCI Health dermatologist Dr. Kristen Kelly. “But many people don’t understand how to read a sunscreen label or know what’s necessary for optimal protection.”

Here’s a primer.

What SPF really means

Each bottle, tube or spray can of sunscreen is required to list its sun protection factor, or SPF number, to show how much it shields the skin against the sun’s ultraviolet B (UVB) rays, which damage the superficial layers of skin. The SPF number can range from 2 to 100.

The SPF number is a measure of how much longer it takes untanned skin to start to redden with the sunscreen applied vs. how long without it.

Contrary to popular belief, an SPF of 30 is not twice as effective as a sunscreen with an SPF of 15,” says Kelly, professor and chair of the UCI School of Medicine’s Department of Dermatology.

To determine a product’s SPF, manufacturers are supposed to use a simple formula: Divide the number of seconds it takes an untanned patch of skin to slightly redden when covered in sunscreen by the number of seconds it takes unscreened skin to slightly redden.

For example, if it takes 300 seconds of sun exposure for skin to turn slightly red with a particular sunscreen, and 10 seconds without, 300 divided by 10 equals 30. That sunscreen’s SPF factor is 30.

In general, a sunscreen is supposed to offer protection at the following SPF levels:

  • SPF 15 protects against 93% of UVB rays
  • SPF 30 protects against 97% of UVB rays
  • SPF 50 protects against 98% of UVB rays
  • SPF 100 protects against 99% of UVB rays

What SPF doesn't tell you

But Kelly cautions that the SPF number is not a measure of how much time you can stay in the sun. That's because it's difficult to determine with precision how long it takes any one person to get a sunburn without wearing sunscreen.

“A person’s skin type, where they live, the time of year and even the time of day are all factors to consider,” she says.

Someone with a light complexion sitting in the Southern California sun at 2 p.m. on a summer day burns a lot faster than they would outdoors in Idaho during the winter. A person with a darker complexion would also be affected differently in the same scenarios.

Broad-spectrum protection

A key problem with the SPF rating, Kelly says, is that it only measures a product’s effectiveness against UVB rays, not ultraviolet A rays, which can penetrate the skin more deeply. There is currently no FDA-approved rating system to measure UVA protection levels.

But ample data shows that both UVA and UVB can cause premature skin aging and skin cancer.

So while the American Academy of Dermatology recommends using a sunscreen with SPF 30 or higher, Kelly advises selecting products labeled “broad spectrum”, which means they shield the skin from both UVA and UVB rays.

“Make sure to read the entire sunscreen label, front and back,” she says. “Check to see that the product you’re considering has a ‘skin cancer/skin-aging alert’ in the drug facts section of the label.”

Beware of sunscreens that aren’t labeled broad spectrum or are broad spectrum but have only an SPF of 2 to 14. These products are required to carry a warning label that they have been shown to reduce sunburn — but not cancer or early aging.

“In this case, put it back on the shelf and search for better sun protection,” she says.

Stronger rules proposed

Last fall, the U.S. Food and Drug Administration (FDA) issued a proposed order to help consumers identify key sunscreen safety information to ensure that the product you buy has sufficient UVA protection and doesn’t overstate its SPF factor.

“Essentially a few things are changing,” Kelly says.

“First, the highest SPF that can be marketed will be SPF 60 plus, although some formulations may be allowed up to SPF 80. Another important rule change is that any sunscreen rated SPF 15 or higher must include sun filters that allow the product to be considered broad-spectrum.”

Water-resistant sunscreen

Another feature to look for in sunscreen is “water resistance,” Kelly says.

Until 2012, sunscreen manufacturers were free to make exaggerated claims about their products, describing them as “waterproof” and “sweatproof.” Then the FDA reined them in.

Now, even broad-spectrum sunscreen rated SPF 15 or higher can’t be labeled “waterproof” or “sweatproof” because no sun-protection product stays on the body when exposed over time to water or perspiration. Nor can a product be called a "sunblock," because no sunscreen is capable of completely blocking the sun's rays.

The clues to look for are the words “water resistant” and “very water resistant.” Products labeled “water resistant” protect for 40 minutes when submerged and those labeled “very water resistant” protect for about 80 minutes.

However, since no sunscreen is completely waterproof, you need to reapply it each time you get out of the water.

Mineral-based vs. chemical-based sunscreens

The FDA also stated in its September 2021 rule change that only two of 16 UV filters used in sunscreens are generally recognized as safe and effective (GRASE). These are the minerals zinc oxide and titanium dioxide.

The FDA also said that chemical-based sunscreens containing the compounds aminobenzoic acid (PABA) and trolamine salicylate are deemed not safe and effective for sunscreen use.

The other 12 chemical agents in sunscreens (cinoxate, dioxybenzone, ensulizole, homosalate, meradimate, octinoxate, octisalate, octocrylene, padimate O, sulisobenzone, oxybenzone, and avobenzone) require additional study to prove they are safe and effective.

“I recommend mineral-based sunscreens to my patients,” says Kelly. “They are protective and less likely to irritate the skin or cause allergic reactions.”

What about the reefs?

Some of the chemical compounds of concern — oxybenzone, octinoxate and octocrylene — also have been found to exacerbate reef bleaching already occurring as ocean temperatures warm around the world.

In an effort to preserve their reefs and ocean life, seven tourist meccas now ban sunscreens with these chemicals. They are:

  • Hawaii
  • U.S. Virgin Islands
  • Aruba
  • Key West, Florida
  • Bonaire
  • Ecotourism reserves in Mexico
  • Palau

The good news is that many mineral-based sunscreens work just as well to protect you without harming reefs. Look for non-nano (not small enough to penetrate the skin) sunscreens with zinc oxide or titanium dioxide, which are reef-safe and typically labeled as such.

Be sunscreen savvy

No matter how effective a sunscreen is in testing, it won’t work if you don’t apply it correctly.

Kelly recommends at least one ounce — or an average shot glass — of sunscreen to adequately cover your entire face and body. Give yourself at least 20 minutes for the sunscreen to be absorbed before going outdoors.

“Typically, people put on about one-third to one-half the amount of sunscreen the lab used when determining the SPF number for a product,” she says.

Don’t forget these commonly missed spots during your sun-protection routine:

  • Armpits
  • Back of hands, knees, neck
  • Ears and lips (use a lip balm with SPF protection)
  • Top of feet
  • Under swimsuit edges and bathing suit straps

You should also follow these tried-and-true, sun-safety recommendations:

  • Wear a broad-brimmed hat
  • As much as possible, stay out of the sun between 10 a.m. and 4 p.m., peak UV exposure time
  • Wear a long-sleeved shirt, long pants and other clothes that offer sun protection
  • Protect your eyes from cataract-causing UV radiation by wearing sunglasses that block 100% of UV rays

But sunscreen has to come first.

“It needs to be part of your daily routine to reduce your risk of skin cancer,” says Kelly. “It’s important for everyone, no matter your skin color or ethnicity.”

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