How does sunscreen work? SPF, broad spectrum and sunblock explained

How sunscreen protects your skin from UV

Sunscreen is a layer on your skin that absorbs or reflects UV radiation before it reaches living tissue. The FDA describes it as a chemical barrier that absorbs or reflects UV before it can damage the skin.

The goal is to lower your risk of sunburn, skin cancer, and early skin aging. The FDA frames that as one piece of a larger plan, used alongside other sun-protection measures, not as a standalone shield.

The protecting is done by the active ingredients. Everything else, like the water or oil, is the vehicle that makes the formula spreadable. The FDA also notes that protection happens on the skin's surface, which is why how you apply it matters so much (more on that below).

Every sunscreen decision after this, from SPF number to mineral vs. chemical, is a variation on that one job. For product-level picks, see our sunscreen coverage.

What SPF measures, and whether SPF 50 beats SPF 30

SPF is a ratio, not a clock. Each sunscreen is tested for how much UV exposure it takes to cause sunburn with the product on, compared with bare skin. That comparison becomes the number.

The old idea that SPF 15 lets you stay out 15 times longer is wrong. The FDA says SPF relates to the amount of solar exposure, not to time. UV intensity changes with the hour (strongest around midday) and with latitude (stronger nearer the equator), so the same minutes in the sun are not equal.

SPF is also mainly a UVB indicator. UVB is the sunburn part of the story, which is why the number alone doesn't tell you the whole picture. That's where broad spectrum comes in.

So is SPF 50 much better than SPF 30? The FDA says higher SPF values, up to 50, give greater sunburn protection. The American Academy of Dermatology recommends SPF 30 or higher. The sources we used don't give percentage figures for the 30-to-50 jump, so we won't invent one.

What you can control, and what the FDA puts numbers on, is how much you apply and how often you reapply. A generous SPF 30 that you reapply is likely to matter more in practice than an SPF 50 you apply thinly once; the sources we used don't quantify this.

Sunscreen vs. sunblock: the word that means nothing

We'd argue there is little practical difference, because 'sunblock' is a marketing word that the sources we used don't define. No bottle blocks all UV, and SPF itself is a ratio that describes how much longer it takes you to burn, not an on/off switch.

The same logic applies to 'waterproof.' The FDA says there is no such thing as waterproof sunscreen and that all sunscreens eventually wash off. A 'water resistant' claim has to be tested, and the label must state either 40 or 80 minutes.

The FDA page we relied on doesn't discuss the word 'sunblock' directly, so treat this as a plain reading of how SPF works rather than a citation about the term. If a label promises total blocking, trust the fine print instead.

What broad spectrum means: UVA vs. UVB

Broad spectrum means the product helps protect against both UVA and UVB. The FDA says not all sunscreens are broad spectrum, so you have to check the label. The AAD uses the same definition.

This matters more than it sounds. Because SPF mainly tracks UVB, a high number doesn't guarantee UVA coverage. The FDA requires a warning on products that aren't broad spectrum, or that are below SPF 15: they have been shown only to help prevent sunburn, not skin cancer or early skin aging.

Here is the label decoder, using the FDA and AAD guidance:

The AAD's three-part check is simple: broad spectrum, SPF 30 or higher, and water resistant.

How mineral and chemical sunscreens work differently

The honest answer from the sources we have: both types are legitimate, and the choice is mostly about what you will actually wear. The FDA's list of active ingredients includes titanium dioxide and zinc oxide (the ones in mineral products) alongside avobenzone, octocrylene, oxybenzone, and bemotrizinol, which the FDA says it added in June 2026.

The FDA's general description is that sunscreen absorbs or reflects UV. The sources don't spell out which ingredient does which, and the popular shorthand that minerals 'reflect' while chemicals 'absorb' is more tidy than the science may be. We'd rather leave that line unsourced than repeat it.

One safety note from the FDA: protection takes place on the skin's surface, but at least some active ingredients are absorbed into the body. The agency says it has asked industry for safety data. That is an open question, not a verdict, and it isn't a reason to skip sunscreen.

The FDA also says a September 10, 2026 order removed PABA and trolamine salicylate, and that it knows of no marketed products containing them.

Why you apply sunscreen 15 minutes before going out

Put it on about 15 minutes before sun exposure. The FDA says that gives the sunscreen time to provide its maximum benefit.

Amount matters as much as timing. The FDA says an average adult or child needs at least 1 ounce, about a shot glass, to cover the whole body. Reapply at least every 2 hours, and sooner after swimming or sweating.

Spots people often miss:

• Ears, nose, and lips
• Back of the neck and hairline
• Exposed scalp
• Hands and tops of the feet

If you use a spray, the FDA says some are flammable and some have been recalled for benzene contamination. Don't spray directly onto your face.

Tanning, cloudy days, and daily use: what we can say

Sunscreen is not a tan-prevention device, and no label promises that. Because SPF is a ratio of how much exposure it takes to burn, a sunscreen reduces UV exposure rather than eliminating it. The sources we used don't address tanning directly, so we're not going to make claims about it beyond that.

On clouds and windows, the notes we worked from are silent too. What the FDA does say is to use broad spectrum SPF 15 or higher regularly and as directed, and that UV intensity varies with time of day and latitude. A gray sky doesn't change the instruction to use it as directed.

Who invented sunscreen, and how is it made?

We can't give you a name you can trust. The origin story is told several ways, and we don't have a source that settles it, so we're not going to pick a winner.

The making part is simpler at the level the FDA describes. A formula is built from active ingredients, which do the UV protection, plus inactive ingredients such as water or oil that make it a lotion, gel, or spray. Detailed manufacturing is beyond the sources we used.