How does sunscreen work, and what does SPF mean?

Sunscreen is one of the few skin products with a number on the front that comes from a test on real people. Most people know a higher SPF is stronger. Fewer know what the number measures, why SPF 50 is only a small step up from SPF 30, or what "broad spectrum" adds. This guide explains how sunscreen protects skin, what the label numbers mean, and how the FDA tests them. It is part of Skinli's guide on how to choose a sunscreen for your face.

What sunscreen protects you from

Sunlight contains light you can see and light you cannot. The invisible part that harms skin is ultraviolet light, often shortened to UV. Two kinds reach the ground, and the American Academy of Dermatology (AAD) describes them this way:

• UVB rays are "the primary cause of sunburn." The AAD calls them the burning rays. Window glass blocks them.
• UVA rays "can prematurely age your skin, causing wrinkles and age spots, and can pass through window glass." The AAD calls them the aging rays.

The AAD says too much of either kind can lead to skin cancer. The FDA says sun damage comes from this invisible radiation, and that it can cause sunburn, early skin aging, eye damage and skin cancer.

So a sunscreen has two jobs: stop the burning rays and stop the aging rays. Two different label claims tell you how well it does each job.

How sunscreen stops the light

Sunscreen forms a thin layer on the surface of your skin. The active ingredients in that layer take in ultraviolet light before it reaches the living cells underneath. The FDA describes sunscreen as "a chemical barrier that absorbs or reflects UV radiation before it can damage the skin."

For many years, people said mineral sunscreens (zinc oxide and titanium dioxide) worked by bouncing light away, while chemical sunscreens absorbed it. A 2016 laboratory study tested that idea. It found the two mineral ingredients reflect only 4 to 5 percent of ultraviolet light, and do most of their work by absorbing it. The AAD now says that all sunscreen types "form a protective layer on your skin that absorbs the sun's rays," and that mineral types also reflect some.

This is why the thickness of the layer matters so much. A thin, patchy layer has less active ingredient in the path of the light. The NHS says, "If sunscreen is applied too thinly, the amount of protection it gives is reduced."

What the SPF number means

SPF stands for sun protection factor. The FDA says the SPF value "indicates the level of sunburn protection provided by the sunscreen product." Because the test measures sunburn, the FDA adds that SPF values "primarily indicate a sunscreen's UVB protection," the burning rays.

The AAD gives the share of burning rays each common number filters out:

SPF on the label	Share of burning rays filtered out (AAD)
SPF 15	93 percent
SPF 30	97 percent
Higher than 30	A small amount more; never 100 percent

The AAD recommends SPF 30 or higher. The FDA's own minimum for lowering skin cancer risk is SPF 15 with broad spectrum protection. The NHS asks for at least SPF 30. Skinli's plan uses SPF 30 or more, in line with the AAD and the NHS.

Why SPF 50 is only a small step up

Going from SPF 15 to SPF 30 cuts the burning rays that get through from 7 percent to 3 percent. Going higher cuts that remaining 3 percent further, but there is less left to cut. The FDA says "higher SPF values (up to 50) provide greater sunburn protection." In 2021, the FDA proposed that labels should top out at "SPF 60+."

The AAD gives one good reason to choose a higher number anyway. It says many people apply only 20 to 50 percent of the amount of sunscreen needed to get the SPF on the label, and a higher SPF "helps to compensate for this under-application."

SPF measures an amount of sun

A common belief is that SPF 30 lets you stay in the sun 30 times longer than you could with bare skin. The FDA calls this belief incorrect. It says SPF "is not directly related to time of solar exposure but to amount of solar exposure." The sun is stronger at midday than in the early morning, and stronger closer to the equator. The same hour outside can deliver different amounts of ultraviolet light.

The AAD adds that "high-number SPFs last the same amount of time as low-number SPFs." SPF 50 needs reapplying as often as SPF 30. Our guide on how often to reapply sunscreen covers the timing.

What "broad spectrum" means

The SPF number says little about the aging rays. That is the job of the words "Broad Spectrum." The FDA says broad spectrum sunscreens "help protect against both forms of UV radiation - UVA and UVB."

Under federal labeling rules (21 CFR 201.327), a sunscreen may print "Broad Spectrum SPF" with its number only if it passes a lab test. In that test, the sunscreen is spread at 0.75 milligrams per square centimeter on a roughened clear plastic plate. The plate is first exposed to a measured dose of light from a lamp that copies sunlight, because some sunscreen ingredients break down in light. Then a machine measures how much ultraviolet light passes through at each wavelength from 290 to 400 nanometers, and works out how far into the longer, aging rays the protection reaches. A product must reach a set point, 370 nanometers, to pass. The rules ask for at least three plates and at least 15 measurements for each product.

This label claim changes what the product is allowed to promise. Every sunscreen may say it "helps prevent sunburn." Only a broad-spectrum product with SPF 15 or higher may also say that, used with other sun protection, it "decreases the risk of skin cancer and early skin aging caused by the sun." A product that fails either test must carry a warning that it has been shown only to help prevent sunburn.

In the UK, the NHS tells readers to look for a star rating for UVA protection, with at least 4 stars, and says sunscreens that protect against both kinds of rays are sometimes called broad spectrum.

How the FDA tests SPF

The SPF on a U.S. label comes from a test on people, set out in 21 CFR 201.327:

1. Volunteers with skin that burns easily or moderately take part. The rules name skin types I, II and III on a six-type scale. Our guide to the Fitzpatrick skin scale explains the scale.
2. The sunscreen is spread on small marked areas of the back at 2 milligrams per square centimeter.
3. The tester waits at least 15 minutes, then shines measured doses of light from a lamp that copies sunlight.
4. The tester compares how much light causes redness on protected skin against unprotected skin.
5. The panel must produce at least 10 valid results before a number is set.

Water resistance is tested the same way, after the volunteers spend 40 or 80 minutes in a pool or hot tub.

The key number for everyday life is the 2 milligrams per square centimeter. That is a thick layer. For the face, the AAD suggests at least one teaspoon, enough to cover the length of your index and middle fingers. Skinli's plan uses the same two-finger measure for the face and neck.

What sunscreen does and does not do

The evidence on sunscreen is strongest for sunburn. The National Cancer Institute says sunscreen "has been shown to prevent sunburns and actinic keratoses," rough patches that can turn into a type of skin cancer. An Australian trial published in 2013 found that people who used sunscreen every day had 24 percent less skin aging over four and a half years than people who used it when they chose to.

No sunscreen blocks everything. The FDA says "no sunscreen completely blocks UV radiation," so shade, clothing and sunglasses are part of sun protection too. For the full safety picture, including what the NCI says about skin cancer, see our guide on whether sunscreen is bad for you.

If you are treating acne, the label rules above still apply, with one addition: choose a product labeled non-comedogenic. Our guide to the complete acne skincare routine explains why sunscreen matters while you use an acne treatment, and Skin school has a short lesson on each step.