Sunscreen: How It Works

As skin cancer rates have increased in recent decades, sun protection has become a growing concern—especially for outdoor enthusiasts.

Our companion sun protection articles cover the basics:

This article looks at the science behind sunscreen.

Sunscreen: A Brief History

In the 1940s, a World War II airman and pharmacist, Benjamin Green, used “red vet pet” (red veterinary petrolatum) in the 1940s to protect himself and fellow soldiers from ultraviolet rays. He later added cocoa butter and coconut oil to his original greasy concoction, and it evolved into Coppertone suntan cream.

Early sunscreens focused on protecting skin from sunburn-causing UVB rays; the less-immediate aging effects of UVA rays were unknown or an afterthought.

In 1978 the Food and Drug Administration (FDA) began regulating sunscreen products as an over-the-counter (OTC) drug, but not until 1988 did it approve a product that contained a UVA-focused inhibitor, avobenzone. The agency did not permit sunscreen-makers to tout avobenzone in products until 1997.

Today, however, a sunscreen’s ability to provide “broad spectrum” protection against UVA and UVB rays is considered an essential function.

Why “Broad Spectrum” Matters

Solar energy includes ultraviolet (UV) radiation. UV rays, part of the electromagnetic spectrum, travel in wavelengths: UVA in longer, more flowing lines, UVB in tighter patterns. Here’s a closer look at both:

UVA: Can prematurely age skin—wrinkles, spots, leathery texture. It can also contribute to melanoma, a skin cancer. It is the hard-to-detect “stealth bomber” segment of the UV spectrum. UVA:

  • deeply penetrates skin, though its effects may not become visible for years.
  • affects skin at any time of day, and in winter, too.
  • passes through clouds and glass.
  • travels in of elongated wavelengths (400-320 nanometers; 1 nanometer = 1 billionth of a meter).
  • accounts for about 95% of UV radiation that reaches earth (though earth’s ozone layer absorbs most solar energy).

UVB: Can burn skin. UVB rays are the “flamethrowers” of solar energy. UVB:

  • penetrates the skin’s outer layer only.
  • affects skin primarily between 10am and 2-4pm, most intensely during summer.
  • causes near-immediate effects (reddening, sunburn, blisters).
  • can partially penetrate clouds but not glass; this explains why your skin does not burn while you ride inside a windows-up vehicle—though glass-penetrating UVA rays can still affect you.
  • travels in shorter waves (320-290nm).
  • can contribute to skin cancer.
  • accounts for about 5% of the sun’s energy that reaches earth.

Note: The sun also generates UVC rays (290-200nm)—harmful to humans (they sit next to X-rays on the electromagnetic spectrum), but they are absorbed by earth’s atmosphere.

UVA rays can be split into 2 subcategories:

  • Longer UVA1 rays (400-340nm).
  • Shorter UVA2 rays (340-320nm).

It is believed that UVA2 rays produce most of the aging impact of UVA rays. It’s possible UVA1 rays have some cancer-causing potential.

In order to claim broad spectrum coverage, a sunscreen formula must in laboratory testing demonstrate the ability to provide protection up to a “critical wavelength” of 370nm. Two ingredients on the FDA’s list of 16 approved active ingredients are prime filters of UVA1: avobenzone (a chemical) and zinc oxide (a mineral).

New labeling guidelines approved in 2011 by the FDA allow sunscreens rated SPF 15 or higher to label themselves as offering broad spectrum protection they meet the critical wavelength criteria in lab tests. Seeing “broad spectrum” on a product label is a good thing.


SPF = Sun Protection Factor or, more precisely, Sunburn Protection Factor.

SPF is a rating. Based on lab tests and human subjects, it indicates how well a sunscreen shields unprotected skin from damage caused by photons (quantum particles) of sunburn-causing UVB rays.

An SPF rating offers no clues to how well a sunscreen guards skin from age-hastening UVA rays, and no comparable rating for UVA exists.

SPF ratings range between 2 and 50+. SPF 15 is the minimum rating dermatologists recommend, but SPF 30 or higher is preferred for active outdoor people.

SPF protection chart

“Even if you are just going for a walk or having a picnic, you should put on SPF 30,” says Brian Adams, MD, MPH and Interim Chairman of the Department of Dermatology at the University of Cincinnati School of Medicine. “Not only does it have a moderate increase in protection over SPF 15, it is well known that people do not apply enough sunscreen, so putting on SPF 30 can help overcome some of the inadequacy effect.”

If a random dose of sunlight emits, for example, 100 photons, an SPF 15 sunscreen would interrupt 93 of them. An SPF sunscreen, 97 of them. This shows SPF 30 does not offer twice the protection of SPF 15.

A common perception of SPF is that it provides a measure of time. That’s not entirely true. Say your unprotected skin shows signs of sunburn after 1 hour. The use of an SPF 15 sunscreen is not a guarantee your skin is safe from sunburn for 15 hours. It may be a rough approximation, but many variables can alter that calculation:

  • Skin type (fair-skinned people can burn quickly)
  • Time of year (a dose of sunlight in summer is more intense than winter sunlight)
  • Time of day (sunlight is more intense at 1pm than 9am, for example)
  • Altitude (the higher you climb, the more intense the sun)
  • Latitude (tropical and polar regions offer more intense sun exposure)
  • Activity level (heavy perspiring can diminish a sunscreen’s performance)

“We want to be clear that this [SPF rating] is the dose, not an amount of time,” says FDA spokeswoman Andrea Fischer.

SPF 30 and higher is considered a good choice for active outdoor types because it sits relatively lightly on the skin, not inhibiting the breathability of human pores.

Water Resistant

“Water resistant” means the sunscreen is formulated to perform well, despite water or sweat, for 40 or 80 minutes (as determined via a standardized lab test). The FDA’s new labeling guidelines bans the use of “waterproof” or “sweatproof” on product labels. Active (sweaty) outdoor people should seek out sunscreens labeled water resistant.

Active Ingredients

As of this writing the FDA has approved 16 ingredients generally recognized as safe and effective (GRASE) for use in sunscreen sold in the U.S. Here’s an overview:

Eight additional sunscreen ingredients have been submitted for inclusion in the FDA’s OTC monograph system through TEAs (time and extent applications). Unlike the GRASE sunscreens, the TEA sunscreen ingredients cannot be marketed as approved active ingredients until FDA issues a final rule. 

The most recognizable name in this group is ecamsule, sometimes marketed as Mexoryl SX. A few approved sunscreens in the U.S. contain ecamsule and are categorized as “new drug applications.” Ecamsule ranks “extensive” (level 4) in UVA protection, per the EPA, “limited” (level 2) for UVB.

The FDA has taken heat from critics for not acting quickly to approve high-performing UVA-blocking filters that are in use in European sunscreens. One example, says Susan M. Swetter, MD, Professor of Dermatology and Director of the Pigmented Lesion and Melanoma Program at Stanford University Medical Center, is Tinosorb. “It will be a great broad spectrum agent once it reaches the U.S. markets,” she says.

Ingredient Safety

Debates over the safety and effectiveness of active ingredients seemingly are never-ending. “No ingredient is without hazard or perfectly effective,” the Environmental Working Group, a nonprofit advocacy group well known for its annual sunscreen surveys, states on its website. The EWG tends to favor mineral sunscreens for their low skin penetration and UVA effectiveness.

The EWG worries that some active ingredients in chemical sunscreens can be too easily absorbed by our skin. The FDA says that if this occurs, it occurs in levels that pose no human risk.

“Very little is absorbed into the body,” Dr. John Wolf, Professor and Chairman of the Department of Dermatology at the Baylor College of Medicine in Houston, says of sunscreen ingredients. “I don’t know if anybody knows precisely how much is absorbed. Anything that is absorbed through the skin would be picked up by small blood vessels and circulated through the body. Most chemicals are detoxified and excreted either by the kidneys [as urine] or the liver [through the bowels].”

With minerals, the EWG worries that some brands shrink zinc oxide and titanium dioxide down to nanosized particles—tiny enough to be absorbed by our skin. In response, several mineral sunscreen-makers promote their formulas as “Non-Nano” on product labels.

Other issues or ingredients that have stirred debate:

Aminobenzoic acid (PABA): A potential cause of allergic and photosensitivity reactions; it also creates stains. Products that do not use it often state "PABA-free" on their labels. PABA is rarely found in high-quality sunscreens today.

Oxybenzone: Said to have the potential to trigger allergic reactions and, when exposed to UV light, release free radicals that could possibly damage cells. Some animal studies suggest the chemical may mimic the hormone estrogen in the body. Some question those findings. Steven Q. Wang, MD, a dermatologist at Memorial Sloan-Kettering Cancer Center in New York, published his own study showing that to reproduce those results, a human would need to be covered in oxybenzone from head to toe daily for 34 years.

Parabens: Preservatives (e.g., methylparaben) found in some skin-care products including some sunscreens. Some health questions surround them, so some brands promote their sunscreens as paraben-free.

Avobenzone (aka Parsol 1789 and other names): Unless this UVA-blocking agent is made “photostable” by other ingredients, in sunlight it can degrade into unknown chemicals (not good for sunscreen) and diminish the effectiveness of other UV-absorbing chemicals. A common stabilizer for avobenzone is octocrylene.

Sunscreen-maker Sawyer, a popular brand offered at REI for years, remains skeptical of avobenzone’s stability in natural (non-laboratory) light and excludes it from its chemical formulas even though doing so means it cannot state “broad spectrum” protection on many of its product labels.

Kurt Avery, Sawyer’s president, says his brand’s formulas use ingredient mixes that offer protection against UVA2 rays, which his company’s independent tests show to be the harmful portion of the UVA spectrum.

“If a formula using avobenzone also contains octocrylene or benzophenone (an alternate name for oxybenzone), then these 2 chemicals can slow down, but not eliminate, the dissipation of the avobenzone,” he tells REI. “They don’t slow it down enough for us and is not a feature we’d like to add.”

Meanwhile, the number of mineral-based sunscreens has grown. Zinc oxide is the most often-used active ingredient in the category. Many boutique sunscreen makers play up its natural origins and supplement it with plant-based inactive ingredients, from cranberry seed oil to grapefruit seed extract. Its downside: Some formulas tend to be thick and difficult to apply, and they may leave a white patina on skin.

Varying Opinions on Chemicals

Many views on sunscreen safety exist.

“The EWG claims of sunscreen concerns have been largely refuted, including the potential ‘dangers’ of nanoparticles, retinol, benzophenone (oxybenzone), free radicals, and so on,” says Stanford’s Dr. Swetter. “Currently marketed sunscreens are deemed safe and effective by the FDA in preventing skin aging and skin cancer when used appropriately.”

Adds Baylor’s Dr. Wolf, an American Academy of Dermatology member: “I have not personally seen any convincing evidence that sunscreen ingredients are dangerous or hazardous”

Yet a 2013 article published in The Washington Post quotes Robert Friedman, Clinical Professor of Dermatology at the New York University School of Medicine, who explained gray areas in research remain.

“Even though the data are soft, we do know that a certain amount of the chemical sunscreens are absorbed into the body, and we don’t know exactly what their effects are,” Friedman told the newspaper.

As The Post reported, Friedman was referring to a 2008 survey by the Centers for Disease Control and Prevention (CDC) that examined urine samples from more than 2,500 people. Oxybenzone was detected in 97% of samples. The paper noted that the source for the oxybenzone is uncertain (it is used in cosmetics), and it is unclear how it may affect health.

EWG says its efforts are focused on keeping people safe. “EWG directs consumers towards products and ingredients that are the most protective from UVA rays, and which do so using ingredients of lower health concern,” says David Andrews, Ph.D. and senior scientist at EWG. “Sunscreen alone has not been shown to prevent cancer so EWG cautions not to rely on sunscreens as the first line of defense. Instead make them part of a sun-protection strategy that includes clothing and seeking shade.”

In this space we have merely attempted to make readers aware that many opinions regarding sunscreen’s active ingredients are in wide circulation. Arguments and counter-arguments on these topics abound. Readers so inclined can independently investigate them on the web in great detail.

Parting thought: The FDA worries that speculation over health issues may persuade some people to avoid using sunscreen altogether, leaving them vulnerable to the greater dangers of excessive sun exposure (sunburn, accelerated skin aging and skin cancer). That, everyone agrees, would be a wrong move. Carry and use sunscreen when outdoors.

New Sunscreen Product Labeling

The FDA monitors sunscreen development and approves active ingredients for use in the U.S. It regards sunscreen as an over-the-counter medication and sets standards for labeling. In 2011 the agency approved new standardized labeling guidelines for sunscreen. Some terminology you will see on new sunscreen labels:

  • Broad spectrum can only appear on products that offer an SPF of 15 or higher and that protect against UVA and UVB rays.
  • Water resistant means the sunscreen performs at its peak, despite water or sweat, for 40 or 80 minutes (as determined via a standardized lab test).

Based on the FDA’s final regulations, here are terms you will no longer see:

  • Sunblock: No skin treatment blocks 100% of UV rays. The term could give users a false sense of invulnerability: “I’m wearing sunblock, so I’m totally safe and can stay outside all day long.” Not true.
  • Waterproof: Swimming or getting doused with water will diminish sunscreen’s effectiveness. As mentioned above, products can now claim either 40 or 80 minutes of water resistance. Once someone stamped “waterproof” on a sunscreen product, everyone did the same to stay competitive. But the term gave users exaggerated expectations and another reason to feel falsely invulnerable.
  • Sweatproof: Same deal as waterproof. Reapply as directed.
  • Instant protection or all-day protection: Neither is true. Sunscreen needs at least 15 minutes on the skin to be effective, and no product can claim more than 2 hours of continuous protection.

A proposed FDA regulation would ban SPF ratings higher than 50+. Until it is passed, you may still see SPF 70, 80 or 100+ on product labels, but in-the-know sunscreen shoppers will recognize those numbers as marketing bluster.

As mentioned earlier, SPF 30 blocks about 97% of UVB rays, SPF 50 about 98%. Numbers such as SPF 70 or 100 imply these sunscreens will deliver huge, exponential benefits. In reality, users might get a tiny fraction of additional benefit. Don’t be fooled.

The labeling compliance date for major brands was Dec. 17, 2012. All smaller, boutique-size sunscreen-makers must have labels that comply by Dec. 17, 2013.

FDA labeling overview:

Shop REI’s selection of sunscreen.

Article contributors: Brian Adams, MD, MPH and Interim Chairman of the Department of Dermatology at the University of Cincinnati School of Medicine; Robert Friedman, Clinical Professor of Dermatology at the New York University School of Medicine; Susan M. Swetter, MD, Professor of Dermatology and Director of the Pigmented Lesion and Melanoma Program at Stanford University Medical Center; John E. Wolf, Jr., MD, MA and Professor and Chairman of the Department of Dermatology at the Baylor College of Medicine in Houston.