Using sunscreen properly is most of the job

The gap between the SPF on a bottle and the protection people actually get is almost entirely a matter of how much and how often. SPF is measured in a laboratory using 2 milligrams of product per square centimeter of skin. Studies of real-world use consistently find people apply between a quarter and a half of that — and the protection you get falls faster than the amount you skip.

That is the single most useful fact in sun care, and it costs nothing to act on. An SPF 30 applied properly outperforms an SPF 50 applied thinly.

The numbers worth memorizing

Two consequences follow. First, a standard bottle should not last a family a whole summer — if yours does, you are under-applying. Second, buying the large tube is a protection decision, not just an economy. How much sunscreen to apply has the full breakdown by body area.

SPF 30 or SPF 50?

SPF 30 filters about 97% of UVB; SPF 50 filters about 98%. The gap looks trivial written that way, and larger written as transmitted light: 3% versus 2% getting through is a third less. Neither framing settles it on its own. What settles it in practice is that higher SPF gives you a buffer against under-application, which everyone does. Choose SPF 30 as a floor, SPF 50 when you will be out for hours, and do not treat either as a reason to stay out longer. SPF 30 versus SPF 50 has the numbers.

Format: spray, stick or lotion

Lotion is the format that most reliably delivers the tested dose. Sprays are the least reliable — the quantity landing on skin is hard to judge, wind carries product away, and the FDA advises against inhaling them, which means never spraying near a face. If you use a spray, spray until skin is visibly wet and then rub it in; the rubbing is not optional. Sticks are excellent for faces, ears and small areas and impractical for a torso. Stick versus lotion versus spray compares all three on the things that matter.

What water resistant means, precisely

US labels may claim 40 minutes or 80 minutes of water resistance and nothing else — those are the only two tested durations permitted, and the words waterproof and sweatproof were banned outright in 2012. The clock measures immersion time, and toweling off ends it regardless of how long you were in. Water-resistant sunscreen explained covers what that means for a pool afternoon.

Expiry, storage, and when to throw a bottle out

FDA rules require sunscreen to maintain its stated strength for three years, so an undated bottle is presumed good for three years from purchase. Past that, the filters degrade and the emulsion often separates. Heat accelerates all of it, which is why the bottle that lived in a car all summer is the one to bin. Signs to act on: separation, a color change, a grainy texture, or a smell that has turned. Does sunscreen expire covers storage and what actually degrades.

When a burn is more than a burn

Most sunburn is managed at home with cool compresses, moisturizer, fluids and time. What people call "sun poisoning" is severe sunburn with systemic symptoms — blistering over a large area, fever, chills, nausea, dizziness or confusion — and it warrants medical attention rather than a home remedy. Sun poisoning versus sunburn sets out the line. This site publishes general information; for a burn you are worried about, call a clinician.