Most people apply sunscreen the way they were taught as kids and never think about it again. That works most of the time. But the questions people actually search (does it expire, does it block vitamin D, is it safe) all trace back to one thing: how sunscreen works at the level of the active ingredient, not the marketing on the bottle.
Sunscreen works through one of two types of filter: mineral (physical) filters that sit on the skin's surface, and chemical filters that absorb into the top layers of skin. Both absorb UV and release it as heat.
SPF measures UVB protection only. "Broad spectrum" is the label claim that covers UVA too, and the two are not interchangeable.
For anyone outdoors for hours, the water-resistance rating and reapplication matter as much as the filter itself.
The short version: sunscreen is two different mechanisms wearing the same label. Knowing which one you are using explains almost everything else.
Physical sunscreen vs. chemical sunscreen
Sunscreens fall into two categories. Physical sunscreen, also called mineral sunscreen, uses zinc oxide, titanium dioxide, or both. These filters sit on the surface of the skin. Chemical sunscreen uses organic filters such as avobenzone, homosalate, octisalate, and octocrylene, which are absorbed into the upper layers of skin.
The old line that "mineral reflects, chemical absorbs" is an oversimplification. Research shows mineral filters absorb most of the UV they encounter and reflect only a small share. The real difference is where the filter sits and how the skin interacts with it. We go deeper on that trade-off in mineral vs. chemical sunscreen for athletes.
What UVA and UVB mean, and why broad spectrum matters more than SPF
UV radiation reaching the skin splits into two bands that matter. UVB is the shorter wavelength, the main cause of sunburn and a major contributor to skin cancer risk. UVA penetrates deeper, causes less visible burning, and is associated with premature aging - the fine lines, loss of elasticity, and pigment changes that show up years after the exposure that caused them.
SPF measures UVB protection only. A sunscreen can carry a high SPF and still offer thin UVA coverage, which is why "broad spectrum" is the claim worth checking first. It means the product has been tested against both bands.
SPF 50 vs. SPF 30: how much difference is there? Less than the numbers suggest. When applied at the tested thickness, SPF 30 filters about 97% of UVB and SPF 50 about 98%. In real life, the bigger gap comes from how much sunscreen goes on and how often it is reapplied. Sunscreen 101 covers the SPF number in full.
Why zinc oxide holds up the way it does
Zinc oxide protects across UVB, UVA2, and UVA1 with a relatively flat absorption curve, which is why it is considered one of the most complete broad spectrum filters available. It is also photostable, meaning it does not break down under sustained sun the way some chemical filters can over a long day. For anyone outside for hours - a swim practice, a long ride, a full day on the water - that stability matters. How zinc compares to the other mineral filter is covered in zinc oxide vs. titanium dioxide.
Does sunscreen expire?
Yes. The FDA requires sunscreen to stay effective at its labeled SPF for three years unless an earlier expiration date is printed on the package. Heat, direct sun, and humidity speed up breakdown, which is why a bottle left in a car or gym bag all summer degrades faster than one kept in a cabinet. If a sunscreen has separated, changed color, or lost its original texture, replace it regardless of the date.
How long does sunscreen last on skin? This is the question that matters day to day. Reapply at least every two hours during sun exposure, and immediately after swimming, heavy sweating, or towel-drying. A water-resistant sunscreen is tested and labeled for either 40 or 80 minutes of water or sweat exposure; after that, protection drops. All Dermasport sunscreens are water-resistant (80 minutes). What the other label terms mean is in what sunscreen labels actually mean.
Is sunscreen safe? Does it cause cancer?
This question gets searched constantly, usually because of concerns about specific chemical filters. The evidence runs the other way: regular sunscreen use is associated with a lower risk of skin cancer, including melanoma, because it reduces cumulative UV exposure. In 2019, the FDA asked manufacturers for more data on several chemical filters, including how much is absorbed into the body. That request is not a finding of harm. It is also part of why zinc oxide and titanium dioxide, the only two filters the FDA has proposed as generally recognized as safe and effective, are often recommended as the more conservative choice.
Is sunscreen bad for you? No. It is one of the most studied skin products available, and the documented risks of unprotected UV exposure - sunburn, premature aging, and skin cancer - far outweigh concerns about a properly formulated sunscreen. More on common misconceptions in top 5 sunscreen myths.
Does sunscreen block vitamin D?
It reduces vitamin D production but does not eliminate it. Even well-applied sunscreen lets some UVB through, and most people get incidental sun exposure throughout the day. If you are concerned about vitamin D levels, dietary sources and supplements are a more reliable path than cutting back on sun protection. Talk to your doctor about testing if you are unsure.
Where formulation comes back into it
None of this explains what most people actually experience with sunscreen - stinging eyes, a white cast, a greasy finish that does not survive a workout. Those are formulation problems, not filter problems. The active ingredient decides what gets blocked. Everything else in the bottle decides whether the sunscreen survives sweat, water, and reapplication long enough to matter.
That is why sunscreen for active use has to be judged on more than SPF. Some formulas include film-forming ingredients that create a water-repellent layer on the skin, which helps the sunscreen stay in place instead of running into the eyes with sweat. Texture, finish, and how easily a product layers over itself also affect whether people reapply it as often as they should.
How often should you actually reapply? At least every two hours during continuous exposure, and immediately after swimming, heavy sweating, or towel-drying, whatever the SPF on the label.
The takeaway
Sunscreen is not one mechanism, and it does not do one job. It is a UV filter, chosen for its wavelength coverage and stability, inside a formula that has to survive the conditions it will be used in. Check for broad spectrum first, then the water-resistance rating, then whether you will actually reapply it.
References
- U.S. Food and Drug Administration. Sunscreen: How to Help Protect Your Skin from the Sun
- American Academy of Dermatology. How to Apply Sunscreen
- Abdel Azim S, et al. Sunscreens Part 1: Mechanisms and Efficacy. Journal of the American Academy of Dermatology, 2025.
- Cole C, et al. Metal Oxide Sunscreens Protect Skin by Absorption, Not by Reflection or Scattering. Photodermatology, Photoimmunology & Photomedicine, 2016.
- Passeron T, et al. Sunscreen Photoprotection and Vitamin D Status. British Journal of Dermatology, 2019.
Read next: Waterproof vs. Water Resistant Sunscreen: What's the Difference? · Reef-Safe Sunscreen: Is Zinc Oxide the Best Choice? · Blue Light Sunscreen: Do You Really Need It?
