All Things Sunscreen: A primer from the experts at burnd

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Sunlight shapes our very biology - it nurtures the food we enjoy, sets our circadian clocks, and lifts our moods. Yet sun exposure is a double edged sword: celebrated for its health benefits, but notorious for its lifelong impact on our skin.

Navigating the world of sun protection can feel overwhelming. Labels are confusing, marketers stretch the truth, ingredients are debated, all making it hard to know what truly matters.
We hope this paper helps clarify things so you can make empowered, educated choices for yourself.

Why We Need Sun Exposure

There’s nothing quite like the feel of sunshine on your skin. Turns out that is not simply a feeling; it’s serious biology.

Sun exposure helps your body produce Vitamin D, which is essential for healthy bones, a strong immune system, and mood regulation. That wonderful sense of well-being you get from a sunny day? You can thank serotonin, the “happy hormone,” which your brain produces more of in response to sunlight.

Morning sunlight, sensed by special cells in your eyes called retinal ganglion cells (ipRGCs), signals your brain’s internal clock. This suppresses melatonin (the sleep hormone) to promote wakefulness and raises morning cortisol for alertness and raises daytime serotonin. At night, your brain turns some of that daytime serotonin into melatonin, which is one of the key hormones that helps you sleep...

Why We Need Sun Protection

When combined with fresh air and activity, mindful sun exposure is a powerful tool for both physical and mental health but along with all of the great and essential stuff sunlight does for us, there are potential downsides to overdoing it. Skin cancer is one of the most common - and most preventable - health problems in the United States accounting for more new cancer cases each year than all other cancers combined. Each year, there are over 5,000,000 new cases of skin cancer in the US. Of those, over a 100,000 are melanoma, the most serious form of sun related skin cancer and this is increasing at a rate of about 6% a year.i Worldwide, approximately 180 people die each day from melanoma, with about 25 of those in the US. Melanoma and other skin cancers, occur more often in men than in woman.

This isn’t just an “old-age” problem. Your relationship with the sun in your early years sets the stage for your entire life. Estimates for the percentage of lifetime sun exposure one gets prior to 18 have varied a lot over the years but most recent data, depending on lifestyle, puts it at around 23% i. The impact of childhood sunburns on melanoma and other skin cancers is still debated: some studies suggest they are disproportionately important ii, while others argue that the total number of sunburns over a lifetime is what matters most. All of them, however, agree that childhood sunburns increase the risk of skin cancer, including melanoma ii.

This risk continues in adulthood, especially for those who spend a lot of time outdoors iii. Consider a group with a lot of exposure: golfers. Peer-reviewed studies from Australia found that 27% of golfers had received a skin cancer diagnosis, compared to 7% of the general population. That’s more than 1 out of 4! Even after adjusting for other factors, golfers had a 2.4 times greater risk.
Aside from the skin cancer risk, sun exposure is the main cause visible aging and is responsible for an estimated whopping 80% of the visible aging iv. Further, in an another Australian study v, the daily use of a sunscreen with an SPF of 15 or greater was shown to decrease visible aging by 24%.

All of this scary stuff doesn’t mean you need to live in your basement, that’s impractical, unhealthy and not very enjoyable, at least for most of us. It just means be smart. Get outside. It is wonderful for your mental and physical health. Just don’t be careless:

Wear a hat and protective clothing when possible.
Avoid peak sun hours (typically 10 a.m. to 4 p.m.) when you can. UVB is strongest at noon. UVA is also but is generally more even throughout the day (See Ray Basics below).
And, of course, use a well-formulated, broad-spectrum sunscreen correctly and consistently. The consistent part can’t be stressed enough. Skin damage happens gradually - it’s cumulative - so protection has to follow the same pattern. In medicine, this is called compliance: following the prescribed regimen. That’s why products that feel good and are enjoyable to use matter so much, because we humans rarely stick with things we don’t like, no matter how good they are for us.

Ray Basics

The sunlight that reaches us spans a spectrum of energy, including, among others, ultraviolet radiation (UVR), visible light and infrared radiation (IR). The IR is what warms you but we are not going to get into the details in this paper. For UVR, scientists divide it into 3 different regions:
UVC: Don’t worry about this one, it’s filtered out by the ozone layer.
UVB: It’s the primary cause of sunburn and is strongly linked to most skin cancers. It varies greatly by season and time of day, peaking around noon.
UVA: This is the silent one because it’s not a major cause of sunburn, you don’t “feel” it. It penetrates deeper into the skin and, along with UVB, contributes to premature aging (wrinkles, sun spots) and is also increasingly linked to skin cancers. UVA is divided into UVA II and UVA I, shorter and longer wavelength respectively

Summer, Winter and Clouds

Why is the sun “less strong” in the winter? The Earth orbits the Sun on a tilted axis. This tilt means that as the Earth travels around the Sun, the Northern and Southern Hemispheres receive different amounts of direct sunlight at various times of the year.

When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight, resulting in summer, while the Southern Hemisphere is tilted away and receives more indirect sunlight, causing winter. The reverse happens when the Southern Hemisphere is tilted towards the Sun. For this same reason, the closer one gets to the equator the less the seasons change.

If the sun is weaker during the winter, why are sunburns so common in winter sports? Snow and ice are the reason. They reflect a huge amount of visible light, which is why everything looks so bright and white, and they also reflect a large amount of UVR. That double reflection means the UVR hits you from above and then again from below after it bounces off the snow. In fact, hard core mountain climbers have been known to severely sunburn the roof of their mouth from this reflected UV!

Clouds also play a big role, especially for UVB radiation. About 95% of the UV energy that reaches us on the ground is UVA. Our ozone layer is great at blocking UVC and most of the UVB, but not so much UVA. But because, photon for photon, UVB carries more energy than UVA, even the relatively small amount that reaches the surface can cause significant damage. Broadly speaking clouds filter out more UVB, visible and IR than they do UVA. But, as many of us know from experience, its still possible to get badly sunburned on a cloudy day...

As above, during the summer, when the sun is more directly overhead with a greater contact angle, the UVR has a more direct path to the earth’s surface. The less direct wintertime path gives the atmosphere a greater opportunity to absorb and scatter some of the energy. Seasonal fluctuations in the ozone layer also affect UVR intensity. Because UVB is more affected by this change in path-length, it varies more than UVA with seasonal changes. In addition to the sun, there are some manmade sources of UV exposure. They include welding arcs, germicidal lamps, laboratory equipment and the still popular, but dangerous, tanning lamps.

SPF: What Does “SPF” Really Mean?

SPF stands for Sun Protection Factor, and it is only a measure of protection against burning rays. Note that UVB rays are largely responsible for sunburn but UVA II rays also play a role, albeit a smaller one.

You’ve seen the numbers: SPF 15, 30, 50 and even 100. The difference can seem big but it’s not as big as it might appear, especially in the higher numbers (see graph to the right). An SPF 30 blocks about 97% of burning rays, while an SPF 50 blocks about 98%. This is why experts say an SPF 30, or even 15 which blocks 93% of UVB, is sufficient if properly applied. That “if” is a big deal, and more on that below. Note that no sunscreen is a true “sunblock”; nothing blocks 100%, and that’s why the FDA doesn’t let brands use the term “sunblock” and why wearing a sunscreen doesn’t eliminate all the good stuff sun does for us.

So, where does the SPF number come from? In a lab, testers put a standard amount of sunscreen on a small patch of a person’s skin and shine a controlled amount of UV radiation on it. The lamps used in the test are designed to mimic the UV radiation spectrum that we are exposed to in the real world. On unprotected skin, they watch to see how long it takes before the area just starts to turn pink; say that happens at 10 minutes. That first hint of redness is called the Minimal Erythemal Dose (MED). If that same person, with sunscreen on, now takes 300 minutes to reach the same level of redness, that product is rated SPF 30 (300 divided by 10). The test is then repeated on a group of volunteers, not just one person, to make sure the number holds up. Getting back to the big “if” mentioned above. That lab test uses 2 milligrams of sunscreen per square centimeter of skin. That’s about 2 mg per a 0.6 x 0.6 inch square. For the average adult body, that’s about the same as one full ounce (a shot glass) for a single, full body application.

However, most people apply less, maybe as little as 3/4 or even ¼ that amount vi. And because of the physics of how sunscreen films work on the skin, applying half as much of an SPF 40 for example does not give you an SPF 20. It gives you something far less, likely an SPF of 12 or 15.

The takeaway: Apply sunscreen generously. When in doubt, apply more. Don’t game the process by saying to yourself I’ll use an SPF 50 and just put a little on. Its unpredictable and doesn’t work well.

A Note on Water Resistance

A product’s SPF is also tested for its durability in water. This is what “Water Resistant” means. It does not mean “waterproof.” Just like sunblock, no sunscreen is truly waterproof...

In the U.S., the FDA test is rigorous. A group of 10 people has the sunscreen applied and then they sit in a circulating water bath (like a hot tub, but not hot) for two 20-minute sessions. After 40 minutes of water immersion, they are air-dried (no towels), and then the SPF test is performed. If the product still provides its labeled SPF (e.g., SPF 30), it can be called “Water Resistant (40 minutes).” For an “80-minute” claim, they must pass the test after four 20-minute water sessions.

The EU system, governed by Cosmetics Europe guidelines, is conceptually similar, using human volunteers and immersion cycles but in the EU, the product only has to maintain at least 50% of the stated SPF to be considered water resistant. This is one test that is more stringent in the US than in the EU. The key point however is that, in both regions, the claim means the product is still effective to a meaningful degree after water emersion. Note that the tests don’t look at abrasive activities that might take the sunscreen off like surfing or toweling off so it’s always best to err on the side of caution.

Understanding “Broad-Spectrum” Protection

Getting sunburned is bad, for sure, and you should avoid it at all costs but it’s also a warning signal that you have had way too much UV exposure. And because you get out of the sun when you get too much sunburned, you also limit your UVA exposure.

But if you turn off that sunburn “alarm” with a non-broad-spectrum sunscreen, you could theoretically stay out longer and get unnaturally high amounts of UVA. Since SPF effectively measures only UVB and some UVA II, you need another test to measure total UVA to make sure your sunscreen blocks both. That’s where “Broad-Spectrum” comes in.

This claim means the sunscreen has been tested and proven to protect against both UVA and UVB rays proportionately.

In the U.S., a sunscreen can only claim “Broad-Spectrum” on the label if it passes a specific FDA test by demonstrating a “critical wavelength” of 370 nanometers. Without going into the math, critical wavelength is a way of measuring how much UVA protection the product provides relative to its SPF. A value of 370 or greater passes. You never see the number on the label; you just see the “Broad-Spectrum” claim if it passes. Other regions, like the EU, have different systems, but the goal is the same: to ensure you are not just protected from a burn, but also from the deeper, silent damage from UVA rays. Today, the vast majority of products from reputable brands are Broad Spectrum and one should not use a product if its not.

How to Read the Sunscreen Label

In the United States, sunscreens are regulated as over-the-counter (OTC) drugs like, for example, aspirin, cortisone cream or allergy medications. This impacts a lot things like how they are developed, what ingredients they must use, tests that are legally required and how they are manufactured. It also means their labels must follow a standardized format, which helps you find the key information. The FDA dictates what information must be listed and, for much of it, the exact wording that is allowed. This is all designed so that we consumers can easily locate important information and compare products and it’s the reason why OTC drug packaging all looks very similar.

“Mineral” vs. “Chemical” Ingredients

First, a note from our inner scientist: everything is a chemical. Water is a chemical. Zinc oxide is a chemical. The term “chemical-free” is pure marketing, not reality. The real terms are mineral (or inorganic) for zinc oxide and titanium dioxide and organic for all the others and that’s what we will use from this point forward. Both are safe and effective, but they have different pros and cons.

Mineral Sunscreens

Ingredients: Zinc Oxide and Titanium Dioxide.
How they work: These are tiny, solid particles that are suspended (not dissolved) in the lotion. They sit on the surface of the skin and, just like an organic filter, primarily absorb UVR, converting it into tiny amounts of harmless heat. The old idea that they reflect light like a mirror is a common, but largely inaccurate, oversimplification. Mineral sunscreens are often coated in something to make them easier to formulate or help with water resistance.
The Pros/Cons: They are very safe and provide good broad spectrum protection, especially zinc oxide. Because they are particles, they have different formulation challenges and can leave a white cast at higher concentrations (higher SPFs), more so with titanium dioxide than zinc oxide. The better brands can formulate around this issue, but not always completely. Also, mineral sunscreens are less efficient than organic sunscreens meaning you need to use more of them to achieve a given SPF.

Chemical (Organic) Sunscreens

Ingredients: In the US there are 14 allowed organic sunscreen actives. 2 of these are basically never used so, in practice, there are 12 organic sunscreens available to brands as of late 2025. They include, among others, Avobenzone, Octinoxate, Octocrylene, Octisalate, and Homosalate. In mid-2026, its expected that Bemotrizinol, a sunscreen long available in other parts of the world, will be officially approved in the US.
How they work: These molecules are dissolved into the sunscreen base. Think of it like a liquid dye dissolving in oil - the dye molecules spread out and in effect becomes part of the oil, creating a uniform liquid. These sunscreen molecules do the same, becoming part of the formula’s oil phase, ready to form a clear, uniform film on the skin. When UV rays hit them, these molecules absorb the energy, get “excited,” and then release that energy as harmless heat. Note that there are few water-soluble filters, e.g., Ensulizole, and they work the same way, just in the water phase instead of the oil.
The Pros/Cons: They are generally but not always more lightweight and rub in clear, making them easier to use in “cosmetically elegant” formulations. At very high concentrations over large surface areas, a small amount of some of them can be detected in the bloodstream. One of them, benzophenone-3 (oxybenzone), has been more associated with toxicity but it has pretty much been phased out and is only found in older sunscreens at this point.

A Note on “Nano” vs. “Non-Nano”

You will often see mineral sunscreens marketed as “nano” or “non-nano.” This term refers to the size of the mineral particles themselves.

Non-Nano: Traditional zinc oxide and titanium dioxide particles are relatively large, often large enough to be seen with the naked eye. For perspective, we humans can see something around 50 microns which is about the diameter of really fine hair. Particles this big are OK at blocking UV light, but they are also large enough to scatter visible light. This is what gives the thick, white, pasty look. For titanium dioxde, this so true that its the main pigment in white paint.
Nano: To solve the white-cast problem, cosmetic scientists began using much smaller particles in the early 1990’s. Known as nanoparticles (generally around 100 nanometers which is 1/10th of a micron or 1/400th of the really fine hair mentioned above), these particles are small enough that they don’t scatter visible light well (i.e., they are not white in a thin film like that a sunscreen). This cosmetic elegance is a major benefit, as it means people are more likely to apply the sunscreen properly and in sufficient amounts. From a performance standpoint, nano-sized particles are also much more efficient at blocking UV.

The regulation of these terms, nano / non nano, differs globally. In the EU, the definition is strict: if nanoparticles are used, the ingredient must be labeled as “(nano)” on the ingredient list (e.g., “Zinc Oxide (nano)”). But, like many regulations, the details really matter. The definition of nano in the EU is that 50% or more of the particles need to be less than 100nm in some dimension (diameter, length ..). This means that a brand can use a particle mix that is 49% nano and 51% non-nano and not have to say nano on the label. In fact, if a particle-based sunscreen is effective (high SPF) and more or less transparent, it almost certainly has a large number of nanoparticles even if it ducks under the definition. This is just physics, there is a tradeoff between opacity (white cast) and particle size and you can’t have it both ways. In the U.S., there is not a formal, legal definition of “nano” for cosmetics or sunscreen. The term is primarily a marketing one, and the “non-nano” claim is not standardized, meaning the particle sizes can still be very small.

Tinted Sunscreens

Many people prefer a tinted sunscreen, especially for facial products. The “tint” largely comes from various flavors of iron oxides which are, at a high level, similar in structure to zinc oxide and titanium dioxide, the mineral sunscreens. But, instead of being white, they are various shades of yellow, red and brown and are the same pigments used in colored cosmetics. When used in a sunscreen they provide color and some UV protection. They also block some blue light which is becoming a thing. So, with tinted mineral sunscreens one can get higher SPFs than with minerals alone because the tint itself provides some protection and it also hides the white film allowing for the use of higher levels of zinc oxide and titanium dioxide.

A Note on “SPF Boosters”

You may see a product marketed as “100% Mineral” but with a surprisingly high SPF (40 or higher) and a nearly clear finish. This is often a red flag and you should check the inactive ingredient list. You will likely see chemicals like butyloctyl salicylate and Tridecyl salicylate vii. These are good UV absorbers that are structurally very similar to regulated sunscreen filters, but because they are not regulated as such, they can be used to “boost” the SPF without being listed as actives. This is a common industry practice and is, in burnd’s view, deceptive. While we believe hybrid products are great and offer advantages, if a consumer wants to have a 100% mineral-based product, they should be able to do so and not be tricked by regulatory nuances.

Formulation Matters

Most of what you rub onto your skin in a sunscreen isn’t the UV filters - it’s everything else in the formula. These “inactive” ingredients, listed in the Drug Facts box, include water, emollients, thickeners, film formers, emulsifiers, preservatives, extracts, fragrances, and added functional ingredients like antioxidants and vitamins. Importantly, it’s this supporting cast that determines how the product feels, smells, looks, and applies.

Formulating a product that delivers high-level UVR protection and feels great on the skin is surprisingly difficult. Creating a lightweight, non-greasy, pleasant-feeling sunscreen - especially one that’s water-resistant - requires specialized expertise, extensive testing, and significant development costs. When done well, though, the result is a product people actually want to use and reapply, which ultimately leads to better real-world protection.

Our Philosophy

At Burnd, we are performance-driven. We believe in using the best tools for the job. This is why we often create hybrid sunscreens. By combining the strengths of both mineral and organic filters, we create products that achieve high, broad-spectrum water-resistant protection while feeling incredibly light and elegant, and that are invisible on the skin.

Our Final Word

Sun protection science is constantly evolving. Our promise to you is to evolve with it, formulating with the most effective, proven ingredients to keep your skin healthy.

We hope this primer empowers you to step into the sun with confidence, armed with the knowledge to protect the health of your skin for years to come.

For more information, visit www.getburnd.com

References

i Skin Cancer Foundation. Skin Cancer Facts. Available at: https://www.skincancer.org/skin-cancer-information/skin-cancer-facts/
ii Lergenmüller S, et al. Lifetime Sunburn Trajectories and Associated Risks of Cutaneous Melanoma and Squamous Cell Carcinoma Among a Cohort of Norwegian Women. JAMA Dermatology. 2022 Dec 1;158(12):1367–1377. doi:10.1001/jamadermatol.2022.4053. PMCID: PMC9535508. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC9535508/
iii Pagoto SL, et al. Photoprotection in Outdoor Sports: A Review of the Literature and Recommendations to Reduce Risk Among Athletes. Dermatology and Therapy (Heidelberg). 2022;12:329–343. doi:10.1007/s13555-021-00671-0. https://doi.org/10.1007/s13555-021-00671-0
iv BMJ Open Sport & Exercise Medicine.
Article PDF. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC10360421/pdf/bmjsem-2023-001597.pdf
v Flament F, Bazin R, Laquieze S, Rubert V, Simonpietri E, Piot B. Effect of the Sun on Visible Clinical Signs of Aging in Caucasian Skin. Clin Cosmet Investig Dermatol. 2013;6:221–232. doi:10.2147/CCID.S44686. PMID: 24101874; PMCID: PMC3790843.
Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC3790843/
vi Taylor S, Diffey B. Simple dosage guide for suncreams will help users. BMJ. 2002 Jun 22;324(7352):1526. doi:10.1136/bmj.324.7352.1526/a. PMCID: PMC1123459. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC1123459/
vii Moradi Tuchayi S, Wang Z, Yan J, Garibyan L, Bai X, Gilchrest BA. Sunscreens: Misconceptions and Misinformation. J Invest Dermatol. 2023;143(8):1406–1411.
PMCID: PMC1123459. Full text: https://www.sciencedirect.com/science/article/pii/S0022202X23019772