You can easily get the feeling that sunscreen is a whole science in itself — and fun fact, it is! To make it a little easier to navigate, we've put together this guide covering everything you need to know about UVA, UVB and SPF.

UVA, UVB and UVC are all types of ultraviolet radiation travelling toward Earth from the sun. They differ mainly in wavelength — and that difference determines how deeply they penetrate your skin and what kind of damage they can do.

UVC is almost entirely absorbed by the ozone layer before it reaches the surface, so you don't need to worry about that one!

UVB rays (280–320 nm) are mainly absorbed by the outermost layer of your skin — the epidermis. They're the main culprit behind sunburn, and they're also what triggers your skin to produce melanin, the pigment responsible for tanning. The ozone layer blocks some UVB, but not all of it — which is why you need sunscreen!

UVA rays (320–400 nm) have a longer wavelength and penetrate much deeper, reaching the dermis — the inner layer of skin where your collagen and elastin live. UVA generates free radicals that damage DNA and increase the risk of skin cancer over time. What makes UVA particularly sneaky is that it causes no immediate reaction — no redness, no burn — so it's easy to have no idea how much you've been exposed to. On top of that, UVA rays penetrate both glass and cloud cover, meaning you can be exposed on overcast days or even sitting by a window indoors. Not ideal!

Both UVA and UVB contribute to skin cancer and premature ageing, which is why broad spectrum protection against both really matters. You can read more about this in our article on why daily sun protection is important.

SPF stands for Sun Protection Factor — and it tells you how well a sunscreen protects your skin from UVB rays. One thing that surprises a lot of people is that the SPF scale is not linear. The jumps in protection get smaller the higher you go: SPF 15 blocks around 93% of UVB rays, SPF 30 blocks 97%, and SPF 50 blocks 98%. That means the difference between SPF 30 and SPF 50 is just one percentage point. Crazy, right?

No matter how high the SPF, all sunscreens need to be reapplied at least every two hours — a higher number doesn't buy you more time in the sun. Reapply after swimming or sweating too, and you're good to go!

Wondering whether SPF 30 or SPF 50 is right for you? Head over to our guide on the difference between SPF 30 and SPF 50 for everything you need to know.

As we just learned, SPF only tells you about UVB protection. So how do you know if your sunscreen also shields you from UVA? The answer is broad spectrum protection.

If your sunscreen carries a label reading "broad spectrum" or displays a circled "UVA" logo, it means the product has been tested and shown to block an appropriate amount of UVA rays — as defined by the relevant regulatory authorities. It's a simple label check, but an important one — because a high SPF number alone is no guarantee of meaningful UVA protection.

All Suntribe sunscreens offer broad spectrum protection — but definitely not all sunscreens do, so it's always worth checking before you buy!

If your sunscreen is labelled water resistant, it means it maintains at least 50% of its labeled SPF after a set period of water exposure — whether that's swimming, surfing or sweating heavily. There are two levels:

• Water resistant — the sunscreen still has at least half its labeled SPF after 40 minutes of water exposure.
• Very water resistant — the sunscreen still has at least half its labeled SPF after 80 minutes of water exposure.

One thing worth knowing: no sunscreen is completely waterproof. The FDA actually banned the term "waterproof" from sunscreen labels in 2011 precisely because it's misleading — all sunscreens will eventually wash off. And because the test is done without towel drying, any rubbing or wiping removes the protective layer faster than the label suggests. So when in doubt, reapply!

As we've seen, SPF is really all about UVB protection. But how exactly is that number on your sunscreen bottle determined?

Across the world, scientists measure UVB protection using the same core principle: how much UVB energy does it take to cause sunburn on skin with sunscreen, compared to skin without it?

The same science, different rules

This approach is consistent globally. Whether a sunscreen is sold in Europe, the US, or Asia, SPF is still based on how much extra UVB energy the skin can tolerate before burning — as long as the product is applied correctly.

What does vary between regions is how sunscreens are regulated and labelled. In the EU, sunscreens are classified as cosmetic products under Regulation (EC) No. 1223/2009, and a product must reach at least SPF 6 to be labelled as a sunscreen at all. Results above SPF 50 must be labelled simply as "SPF 50+", rather than showing an exact figure. In the US, sunscreens are not classified as cosmetics but as over-the-counter drugs, and must comply with FDA requirements to be considered generally recognized as safe and effective before being marketed.

Why UVB and sunburn?

UVB rays (wavelengths of roughly 290–320 nm) are the main culprit behind sunburn. Because sunburn is a visible, measurable skin reaction, scientists use it as a reliable reference point for gauging how well a sunscreen shields you from UVB radiation.

How the test works

The current international standard for SPF testing is ISO 24444:2019, which relies on testing conducted with human volunteers. Here's what happens step by step:

1. Apply the sunscreen. A standardized amount of 2 mg per cm² of skin is spread onto small marked areas on volunteers' backs. This is worth keeping in mind: using less sunscreen than this in real life means less protection than the SPF on the label suggests.
2. Expose to UV light. The test uses a xenon arc lamp solar simulator of defined and known output, exposing the skin in gradually increasing doses.
3. Check for redness. Within 16 to 24 hours after UV irradiation, a trained examiner visually assesses the skin for the first sign of redness. This threshold is called the minimal erythema dose, or MED — measured both on protected and unprotected skin.
4. Calculate the SPF. The SPF value is simply the ratio between the two:

SPF = MED (with sunscreen) ÷ MED (without sunscreen)

So an SPF 30 means it took 30 times more UV energy to cause redness on protected skin than on bare skin — a reassuringly straightforward number once you know what's behind it.

To make things even more confusing, the UVA test is related to the UVB / SPF test. So when a sunscreen is tested to confirm broad spectrum protection — meaning it provides appropriate protection against UVA — that protection is measured relative to its SPF value. To pass, a sunscreen must block at least 1/3 of the labelled SPF in UVA.

UVA standards differ around the world

Unlike SPF, where the same basic sunburn test is used globally, there is no single universal standard for measuring or labelling UVA protection. Different regions use different methods, which is why UVA claims can look quite different depending on where a sunscreen is sold. In the EU, UVA protection is assessed using the UVA-PF ratio and critical wavelength method under ISO 24442/24443, requiring the UVA-PF to be at least 1/3 of the labelled SPF and a critical wavelength of at least 370 nm. In the US, the FDA uses a binary "broad spectrum" label based on the same 370 nm critical wavelength threshold — but does not require or display a graded UVA number. In Japan and South Korea, the PA system (PA+ through PA++++) is used instead, grading UVA protection based on the Persistent Pigment Darkening (PPD) test — a method that measures how long protected skin takes to visibly tan when exposed to UVA. The UK adds yet another layer with its own star rating system, which scores the balance between UVA and UVB protection rather than measuring the absolute amount of UVA blocked. The practical takeaway: a sunscreen labelled "broad spectrum" in the US, "UVA" in the EU, or "PA++++" in Japan may all be offering strong protection — but the labels aren't directly comparable, and none of them tells the full story on their own.

Suntribe is based in Europe which is why our sunscreens are tested according to EU standards. This means that the UVA protection of our mineral sunscreens is measured using two key parameters: UVA-PF and critical wavelength, tested according to ISO 24442 (in vivo) or ISO 24443 (in vitro). The minimum requirements — and the typical results for Suntribe sunscreens — are outlined below (note that values can vary slightly between formulas, which is completely normal):

• UVA-PF: The minimum required to pass is 10. Suntribe sunscreens consistently achieve values between 22 and 27.
• Critical Wavelength: The threshold is 370 nm — the point at which a sunscreen qualifies as broad spectrum. Our sunscreens measure between 377 and 380 nm.

This means our sunscreens don't just meet the standard; they go far beyond it. That's thanks to our use of Non-Nano Zinc Oxide — a 100% mineral UV filter that is the only single active ingredient recognised to provide complete broad-spectrum protection across both UVA and UVB.

The water resistance test is conducted in a lab under standardised conditions. Test subjects have sunscreen applied to their arm, which is then immersed in water for either 40 or 80 minutes, and the SPF is measured before and after the immersion. It's not a single long soak — the test follows alternating cycles of 20 minutes in water and 15 minutes of air drying, mimicking real-world behaviour. Crucially, subjects do not towel off between cycles, since any rubbing can physically remove the protective film — which is also why reapplication after towel-drying is always recommended in real life.

To pass, a sunscreen must retain at least 50% of its original SPF value after the time in water. The test protocol is governed by ISO 16217 and ISO 18861 in the EU, and by 21 CFR 201.327 under the FDA's 2011 Sunscreen Final Rule in the US.

It's important to know that the water resistance test was designed primarily for liquid sunscreens. Thicker mineral sunscreens — like Suntribe's Face & Sport series — sit on top of the skin as a physical layer, and their water resistance depends on the film-forming properties of the overall formulation, not just the UV filter. This means they can often withstand significantly more than 80 minutes of water contact without losing meaningful SPF. With these formulations, a visible layer of protection on the skin is the most practical sign that you're still covered — and the clearest signal of when it's time to reapply.

With lighter liquid sunscreens, such as the Natural Mineral Sunscreen SPF 30, we recommend reapplying regularly after coming out of the water or when sweating heavily. This is the safest approach — and consistent reapplication every two hours, or after swimming or sweating, is what dermatologists universally recommend regardless of water resistance rating.

1. Higher SPF means longer protection.

This is one of the most widespread sunscreen myths. A higher SPF doesn't mean you can spend more time in the sun before reapplying — it simply means a higher percentage of UVB rays are filtered out. And the gains get smaller as the numbers climb: SPF 30 blocks around 97% of UVB rays, while SPF 50 blocks around 98% — a difference of just 1%. You can read more about this in our guide on the difference between SPF 30 and SPF 50. More importantly, a high SPF can create a false sense of security, leading people to skip reapplication or stay out far longer than is safe. The rule is simple: all sunscreens need to be reapplied at least every two hours, regardless of SPF. Be sure to choose a broad spectrum sunscreen and reapply it regularly. For our non-visible SPF products, that means every two hours without exception. The exception is our sport sunscreen line — with these, you are protected for several hours as long as you can see a visible layer on the skin.

2. Sunscreen blocks all UV rays.

No sunscreen blocks UV radiation completely. SPF 30 filters around 97% of UVB rays, and even SPF 100 only reaches 99% — meaning some UV always gets through. Sunscreen is an essential tool, but sun protection goes beyond it. Wearing hats, seeking shade, and covering up with clothing are all important layers of defence that no sunscreen alone can replace. You can read more about why daily sun protection matters on our blog.

3. Waterproof sunscreen doesn't need reapplication.

No sunscreen is truly waterproof or sweatproof — the FDA banned these terms from sunscreen labels in 2011 precisely because they're misleading. What water-resistant sunscreens do offer is a tested minimum: a product labelled "water resistant (40 minutes)" has shown it retains at least half its labelled SPF after 40 minutes of immersion; "water resistant (80 minutes)" after 80 minutes. But that's a lab result — real-world activities like towel drying physically remove the protective layer, which is why reapplying any liquid sunscreen after swimming, sweating, or towel drying is always the safest approach.

You can read more about debunking common sunscreen myths in our blog post on this topic.

Sun protection doesn't have to be complicated — and now you've got the science to back up every choice you make. You know the difference between UVA and UVB, what SPF actually measures, and why broad spectrum protection matters. Most importantly, you know what to look for on a label.

Ready to put it into practice? Suntribe's range of mineral sunscreens covers every skin type and activity — whether you're spending a lazy day at the beach, catching waves, or pushing hard in the mountains. All our products offer broad spectrum protection, are made with Non-Nano Zinc Oxide, and are free from harmful ingredients.

Find your match, and reach out if you have any questions — we're always happy to help.