Why Does the Sun Damage Skin?
The sun emits energy in the form of electromagnetic radiation. This radiation can be divided into different ranges, each with different effects. Some of it we can perceive directly - we feel it as warmth on our skin or see it as visible light. Other parts, such as ultraviolet (UV) radiation, are invisible to us. This is what makes it dangerous: UV radiation can harm us without us ever noticing it.
UVA and UVB: What Is the Difference?
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UVA: This part of UV radiation penetrates deep into the skin and can promote the development of skin cancer.
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UVB: This part penetrates less deeply. It can cause sunburn and damage the skin, but it is also responsible for the production of Vitamin D, which our body needs.
We depend on UV radiation - and yet we should avoid doses that are too high.
When Is UV Radiation at Its Strongest?
The intensity of UV radiation depends on the time of day, location, and season. As a general rule:
- It is lower in the morning, increases through the late morning, reaches its peak at midday, and then decreases again.
- The closer to the equator, the more intense it is.
- It is particularly strong on the water, on snow, and at high altitudes.
On sunny days, it is therefore important to use sun protection from the morning through well into the afternoon, and to seek shade regularly. This becomes even more important the closer you are to the equator. Those who want to know exactly can use the WHO-recommended SunSmart Global UV App to check the UV index at their location.
What Does the Sun Protection Factor (SPF) Mean?
The most reliable protection is staying in the shade. In the sun, we can use products that neutralise part of the incoming UV radiation - extending the time we can safely spend outdoors. The Sun Protection Factor (SPF) indicates how effective that protection is.
Common values range from SPF 15 to SPF 50+:
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SPF 15 – medium protection, approximately 93% of incoming UVB radiation is filtered
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SPF 20 – medium protection, approximately 95% of incoming UVB radiation is filtered
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SPF 30 – high protection, approximately 97% of incoming UVB radiation is filtered
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SPF 50 – high protection, approximately 98% of incoming UVB radiation is filtered
What About UVA Protection?
In addition to the SPF, sun protection products always carry a UVA label - recognizable by the letters "UVA" inside a circle. This confirms that the product achieves at least one third of its UVB filter effectiveness for UVA radiation as well. A product with SPF 30 and a UVA label therefore not only protects against UVB rays, but also provides a defined minimum level of protection against the deeper-penetrating UVA rays.
How Long Can I Stay in the Sun With Sunscreen?
Depending on your skin type, the amount of time you can spend in the sun without protection varies.
As a rule of thumb: the possible time you can safely spend in the sun multiplies by the SPF value.
If you can safely spend 10 minutes in the sun without protection (as a typical Northern European), you theoretically have:
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SPF 15: 10 min × 15 = 150 min/2.5 h of UVB protection
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SPF 30: 10 min × 30 = 300 min/5 h of UVB protection
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SPF 50: 10 min × 50 = 500 min/8.3 h of UVB protection
However, the German Federal Office for Radiation Protection (BfS) recommends using no more than 60% of this theoretical protection time. Beyond that point, skin damage is already likely. In practice, this means:
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SPF 15: max. 90 min/1.5 h
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SPF 30: max. 180 min/3 h
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SPF 50: max. 300 min/5 h
Safety Insight: For water sports, the BfS recommends at least SPF 50+. Apply 20-30 minutes before going outside, in sufficient quantity - and reapply every two hours, at the latest after every time you leave the water.
SPF Calculator: Find Your Personal Protection Factor
UV radiation is particularly intense on the water. Reflection increases exposure by up to 40%. Enter your situation and receive a science-based recommendation: RESTUBE SPF Calculator.
Sunscreen: Mineral or Chemical UV Filter?
Sunscreens are applied to the skin and protect against incoming UV radiation through the use of chemical or mineral UV filters. Since these products are so widely used, large amounts end up on and in our skin and in the water when we swim. It is therefore worth taking a closer look at the available options.
Sunscreen with Chemical UV Filters
Chemical UV filters absorb into the skin without leaving a residue. The incoming UV radiation is then largely absorbed and converted into heat energy. For this reason, it is essential to allow adequate absorption time - apply at least 20–30 minutes before sun exposure. Chemical filters typically achieve high or very high (50+) SPF values and are popular because they do not leave a white cast. However, that high level of protection comes at a cost. In recent years, several widely used chemical UV filters have been found to be harmful to health or the environment - most notably oxybenzone, octinoxate, and octocrylene.
Oxybenzone and octinoxate have been shown to be damaging to fish and coral reefs, so damaging that some countries, including Hawaii, have banned sunscreens containing these filters entirely. This has led to a growing number of manufacturers labelling their products as "ocean & reef safe". However, this is not a standardised certification - it simply indicates that no demonstrably harmful UV filter has been used. Octocrylene is a widely used UV filter that converts over time into benzophenone, which was classified as potentially carcinogenic in 2013. For this reason, it is strongly advised not to use sunscreen past its expiry date. Some studies have even detected benzophenone in products before their expiry date.
This does not mean that chemical filters are generally bad. But it does make sense to research the ingredients carefully before choosing a product with chemical filters. A possible alternative is provided by mineral filters.
Sunscreen with Mineral UV Filters
Mineral UV filters are small particles that form a visible protective layer on the skin, reflecting and absorbing incoming sunlight. As a general rule:
The smaller the particles used, the higher the UV protection.
For this reason, increasingly smaller filter particles have been produced over time. Very small particles - known as nanoparticles - also eliminate the white cast on the skin. However, this comes with its own risks: nanoparticles can potentially penetrate cells and accumulate there, posing potential risks to both health and the environment. In the EU, products containing nanoparticles must be labelled accordingly. With larger, non-nano particles, this risk does not apply.
Zinc Oxide and Titanium Dioxide: What Is the Difference?
The two most common mineral UV filters differ in their filtering properties and risk assessments:
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Zinc oxide – broad-spectrum UV filter, protects equally against UVA and UVB radiation, considered safe
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Titanium dioxide – very strong UVB filter, often combined with zinc oxide, though its safety assessment remains controversial
The two filters are frequently combined: zinc oxide provides reliable UVA protection, while titanium dioxide boosts UVB protection and raises the overall SPF. Unfortunately, this combination does not come without drawbacks. While zinc oxide is considered safe, the assessment of titanium dioxide remains controversial. A potential health risk has been raised. However, an EU ban on the use of titanium dioxide in food products has recently had to be withdrawn.
What Is UV Protective Clothing?
In principle, any fabric that covers the skin or provides shade offers some level of UV protection. However, regular clothing and hats are not tested for UV protection and therefore cannot carry an SPF rating. As a general rule: the more tightly woven a fabric, the higher its UV protection.
Those who want precise protection can turn to specialist UV protective clothing. The fabrics used are tested for UV transmittance and may therefore be sold with an SPF rating.
UV protective clothing offers several advantages:
- Protection is active immediately, no application time needed
- Cannot be compromised by incorrect application
- Not reduced by water or sweat
- No risk of harmful ingredients affecting health or the environment
This makes UV clothing particularly attractive for athletes and children. The one downside: wearing clothing in the sun adds warmth and changes the feeling of being outside. For many people, feeling the sun on their skin is part of summer, which is why sunscreen remains the first choice for most beach and swim days.
Sun Protection for Water Sports: The Key Takeaways
Prolonged sun exposure is a health risk. The UV index helps assess that risk. Using sun protection extends the time you can safely spend in the sun. To avoid additional health risks or environmental harm, it is worth researching ingredients carefully or considering UV protective clothing as an alternative.
According to the German Federal Office for Radiation Protection (BfS):
- UV radiation on and in the water is up to 40% more intense than on land due to reflection: use at least SPF 50+
- Apply sunscreen (including water-resistant formulas) 20-30 minutes before going outside or entering the water, so it can absorb fully and is not immediately washed off
- Reapply at least every two hours and immediately after leaving the water or sweating: this ensures full protection is maintained at all times
- Reapplying sunscreen does not extend the maximum safe time in the sun - it only ensures the stated protection level is actually achieved
- A 200ml bottle provides only around 5 full-body applications for an adult
- For children and people with skin type I or II: avoid direct sun exposure as much as possible, especially for babies and toddlers. Otherwise: use at least SPF 50+
- When in doubt: a zinc oxide sunscreen with SPF 50+ is a safe and environmentally friendly choice
