SPF Terminology

Sun Protection Factor. A measure of mainly UVB protection, tested on human skin. The number tells you how much longer it takes skin to burn compared to unprotected skin, not how many hours you're covered.

The longer rays. They're the ones behind most premature aging and uneven tone. They also contribute to some skin cancers.

The shorter rays. They cause sunburn and most skin cancers, and they're what the SPF number measures.

A daily forecast of UV strength, from 0 to 11+. It's weighted heavily toward UVB, so a low number tells you you probably won't burn. It says much less about the UVA reaching your skin.

Protection against both UVA and UVB. In the US, a product has to pass a specific test to use the term. If a label doesn't say it, the product is only covering part of the problem.

The skin's response from too much UV radiation, caused by direct DNA damage. Redness, pain, swelling. It shows up hours after the exposure that caused it.

Everything UV does that damages the skin, including sunburns, photoaging, and skin cancer.

The visible changes in skin that come from UV exposure over time rather than from age itself, including wrinkles and coarse leathery texture, brown spots, sagging, and visible small blood vessels called telangiectasias.

Unstable molecules generated by UV exposure. Can damage DNA indirectly and break down collagen and elastin. Antioxidants like vitamin E help defend against them.

The active pharmaceutical ingredients (APIs), meaning the ingredients doing the actual protecting. Everything else in the bottle is for texture, hydration, and stability. There are two families: mineral filters (zinc oxide, titanium dioxide) and organic/chemical filters (avobenzone, octinoxate, homosalate, octisalate, octocrylene, oxybenzone and others). Also known as UV filters.

Zinc oxide and titanium dioxide. They absorb, reflect, and scatter UV. White by nature, they are responsible for the white cast sometimes seen with mineral-based sunscreens.

Also known as organic filters, meaning carbon-based. They absorb UV. "Chemical" is a bit of a misnomer, since everything is chemistry. Transparent by nature.

A sunscreen using both mineral and chemical (organic) filters.

Ingredients that raise SPF without being an SPF filter. They work by helping the film spread evenly, or by absorbing some UV themselves.

Mineral. Covers both UVA and UVB. It's also white by nature, which is the engineering challenge in mineral sunscreen.

Mineral. Stronger on UVB. Its white cast is even more prominent than zinc oxide.

Chemical. The most common UVA filter in US sunscreens. Effective, but it breaks down in sunlight unless other ingredients stabilize it, which is why photostability matters. Unstable with octinoxate.

Chemical. A UVB filter, valued for its potency.

Chemical. A weak UVB filter, used mainly in combination with other filters.

Chemical. A mild UVB filter, helps stabilize avobenzone from degrading in sunlight.

Chemical. A UVB and mild short UVA filter, helps stabilize avobenzone from degrading in sunlight.

Chemical. A filter covering UVB and short UVA. The most debated filter in the category.

Chemical. A broad-spectrum filter, very photostable, used in Europe and Asia for years. Sold as Tinosorb S. The FDA approved it in June 2026, the first new sunscreen filter cleared in the US in over 20 years, so expect to see it in a lot more American sunscreens from here.

Two ways to test a sunscreen. In vitro happens on a plate in a lab. In vivo happens on human skin.

A UVA scale, from PA+ to PA++++. SPF measures mainly UVB, which causes burning. PA tells you more about the degree of UVA protection. It's standard in Asia and still uncommon on US labels.

Whether a sunscreen keeps working as it sits in sunlight. Some filters degrade more quickly than others. Photostability testing measures how much protection is left after a set UV dose, which can approximate hours in the sun.

The test behind the broad-spectrum label in the US. A product has to reach 370 nanometers or higher to make the claim, and it tells you the product absorbs within the UVB-through-UVA range.

Formulated not to clog pores. It isn't a regulated term, so it's worth asking whether a brand tested it or just printed it.

A label can say 40 or 80 minutes, which is how long the SPF holds up while swimming. After that, reapply. The FDA banned "waterproof" and "sweatproof" on sunscreen labels because no sunscreen fully repels water.

A word the FDA no longer allows on labels, because no sunscreen blocks 100% of UV.

An unregulated marketing term, which does not guarantee any verified absence of ecological harm. Some regions have banned oxybenzone and octinoxate, so if this matters to you, read the ingredient list rather than the phrase.

A "nanoparticle" is one under 100 nm in diameter. Refers to particle size of mineral filters. Regulators in the US and EU permit both for topical use. But nano sprays/powders are excluded, because they should not be inhaled.

The pale white film mineral filters can leave, most visible on deeper skin tones. How much shows up depends on the formula, including the specific filters used, how finely the zinc oxide is milled, and how well it's dispersed.

Every two hours outdoors, and again after swimming, sweating, or towel drying. Some filters do break down in light, but the bigger reason is simpler: sunscreen comes off. Water, sweat, and rubbing remove it unevenly. Reapplication also partly compensates for initial under-dosing, which causes the biggest gap between labeled and real-world SPF. Two hours is the standard on every US label.