Who knows his sun protection factor …


Sun on your skin – so beautiful, so carcinogenic. So apply cream. With what? How much of it? And what about LSF, UV-A and UV-B? A guide for sunny days

One of the biggest mistakes: not using enough cream © Barbara Ködel / plainpicture; Tom Merton / Getty Images

The computer is humming inside, the sun is shining outside. Dog days started in Europe on July 22: the warmest 30 days of the year. The good weather drives people from the home office to the beach, to the lake and – with a safe distance – to the pool edge again.

Of course, one thing should not be missing: sun protection. In Germany, almost 300,000 people develop skin cancer every year, roughly the same number as Mannheim. The best-known late consequence of solar radiation is black skin cancer, malignant melanoma, from which around 3,000 people die in Germany every year. But white skin cancer – this includes squamous cell carcinoma and basalioma – is also caused by long-term UV exposure. It is much more common than malignant melanoma, but is rarely fatal.

We answer the most important questions about UV radiation, the right sun protection (cream, oil or spray) and the question of how environmentally harmful the products can be.

What is so dangerous in the sun?

UV rays only make up a small percentage of solar radiation, but are responsible for a large part of their effects on the skin. UV stands for ultraviolet and denotes rays beyond violet, i.e. beyond the visible range.

On their way from the sun to us, UV-A and UV-B rays pass through the ozone layer of the stratosphere and become a danger to our skin. UV-B rays remain relatively superficial and trigger the typical sunburn – an inflammation of the skin that, depending on the severity, is accompanied by redness, swelling, pain and blisters. UV-A rays are long-wave, penetrate deeper into the body and through window glass. They primarily lead to premature skin aging.

Both types of radiation favor the development of skin cancer (Journal of Clinical and Aesthetic Dermatology: Wilson, 2012). Correct: Sunburn is a sure sign of excessive UV exposure to the skin. But it is also important that those who do not have a sunburn are not automatically protected. Even without redness that is visible at first glance, UV-A rays can damage the DNA of skin cells and result in malignant skin changes many years later.

How safe is sunscreen?

Sunscreen is like a bicycle helmet: anyone who uses it thinks they are safe and are prone to risky behavior.

Sun protection factor 50+ and I can safely in the sun? Unfortunately it’s not that easy. First, protection against UV-A rays is often significantly weaker than against skin-reddening UV-B rays. According to the EU directive, it only has to be a third of UV-B protection. Second, a lot of people don’t apply enough sunscreen, less than half the recommended amount on average (Archives of dermatology: Neale et al., 2002). So the maximum possible sun protection factor is not even reached.

What does the sun protection factor mean?

Sunscreens have been working against UV-B and UV-A rays in Europe since 2006. A separate label identifies the additional UV-A protection. Especially in non-European countries, products can be sold that only protect against UV-B rays. So it is important to pay attention to the small print.

The amount of UV-B rays intercepted is identified indirectly via the sun protection factor (SPF). The number describes the factor by which the duration until the likely sunburn increases after we have applied the sunscreen. How long our skin can be exposed to UV light without sun protection before sunburn occurs depends on the skin type. In the case of very light skin, this intrinsic protection time is only a few minutes – 30 times longer with sun protection factor 30. If you are sensitive and get sunburned without protection after five minutes, you can only use the SPF 30 for two and a half hours in the sun. However, your own skin type is difficult to assess without medical advice. A dermatologist should determine this, advises the Federal Office for Radiation Protection (BfS). Because it’s about more than just light or dark.

How do I use sunscreen correctly?

The S3 guideline for the prevention of skin cancer recommends two milligrams per square centimeter of skin. Carola Berking, director of the skin clinic in Erlangen and co-author of the guideline, explains what this means on the beach: “All skin areas that are not protected by clothing should be covered with white paint for a short time.”

Also important: the sunscreen must be applied in good time, preferably 20 to 30 minutes before sunbathing. If you just come out of the lake or sweat a lot, you should replace the sun protection directly – even if the seal says “waterproof” or “extra waterproof”. In Europe, the seal only describes 50 percent residual protection after 40 or 80 minutes in the water. The Federal Office for Radiation Protection recommends applying a new layer of cream even after two hours without contact with water.

Cream, lotion or gel?

Which sunscreen is suitable depends primarily on the texture of the skin, says Berking. For example, creams are particularly fatty and are good for the legs of many people, while lotions and gels are better for the face. Sprays tempt you to apply even less protection. Sunbathers should also be careful not to inhale the spray aerosols.

How does sun protection work?

Conventional sun creams are an oil-water mixture that also contains special light stabilizers: the UV filters. These filters are based on two different principles: The organic UV filters, often referred to as “chemical”, absorb the UV rays and convert them into harmless heat radiation. These include, for example, the substances oxybenzone and octocrylene. To take full effect, they have to soak in the skin for 20 to 30 minutes. The inorganic or “physical” filter – the name is not entirely correct, since the inorganic filter is also a chemical compound – consists of small particles, mostly titanium dioxide or zinc oxide, which lie on the skin like a surface made of tiny mirrors and the sunlight reflect. Their effects begin immediately.

Because a substance does not cover the entire UV spectrum, several filters are usually combined. Mixing different UV filters in one cream has another advantage, says Carola Berking. So none of the individual ingredients have to be dosed particularly high.

Can sunscreen harm my body too?

Depending on the concentration, the chemical UV filters in particular can trigger allergic reactions and are better tolerated in low doses. Hormonal influences on the human body are also discussed. In 2019, an American study caused a sensation, which was able to prove that chemical UV filter substances from sunscreens can get into the blood via the intact skin (Environment International: Hiller et al., 2019). Animal experiments had already indicated in 2001 that some of these substances were hormone-active and interfered with the estrogen balance in experimental animals (Environmental Health Perspectives: Schlumpf et al., 2001). Carola Berking believes that sunscreens in humans can trigger a dangerous change in hormones. “What we don’t know, of course, can still come.”

And what about the environment?

The negative effect of organic UV filters on the ocean is less controversial. In several studies, researchers were able to demonstrate that light stabilizers such as oxybenzone or octocrylene accumulate in corals (Environmental Science & Technology: Tsui et al., 2017 and Bell, 2017), they bleach and contribute to their death. They were also able to detect the chemicals in marine animals such as zebra fish (Chemosphere: Zhang et al., 2016). Ten common sunscreen ingredients have been banned on the Pacific island of Palau since January 1, 2020. Other countries, including Hawaii, have announced similar measures. The ingredients are allowed in Europe. Some manufacturers have recently voluntarily renounced some of the environmentally harmful substances and advertise their new products as “coral-friendly”. However, this is not a recognized label.

Are there ecological alternatives?

Mineral sunscreens, whose light protection is based on the minerals titanium dioxide and zinc oxide, that is to say on purely “physical” filters, have long been considered a harmless ecological alternative. They were unpopular because of the white film they left on their skin. Since cosmetic manufacturers have mainly processed the ingredients as nanoparticles, i.e. in much smaller particles than before, this effect has failed to materialize. Environmental associations are, however, noisy. “Nanoparticles are still little researched and can potentially harm nature,” wrote the environmental organization WWF in a message. Because they are so small, nanoparticles can penetrate cell walls more easily. It has not yet been definitively researched whether this can have harmful consequences for humans or nature. There are nano-sized substances among both organic and inorganic filters. On sunscreen tubes they are marked with the addition “nano”.

From the consumer perspective, this cluster of warnings is confusing. Do I protect my skin, do I endanger the environment – and vice versa?

From an ecological point of view, natural cosmetics without nanoparticles are the best choice for a sunny day at the lake, the WWF wrote in its communication. Ralf Kägi, an expert in nanoparticles at the Eawag water research institute in Zurich, is not quite as strict: “We will never be able to issue a blank check that the ingredients in sun creams are 100% safe.” In any case, he considers “physical” filters to be more environmentally friendly than “chemical” ones.

The nanoparticles in the newer mineral sunscreens made him less worried. Since the small particles could not pass through intact skin, a harmful effect on the body is unlikely. A large part of the particles would also be captured by sewage treatment plants and swimming pool filters. They are sufficiently diluted in natural waters; According to current knowledge, it should not be assumed that they significantly influenced the ecosystem of natural waters.


Since many people tend to underestimate UV exposure, especially on cloudy days, the Federal Office for Radiation Protection refers to the UV index. It is a three-day forecast of the regionally expected UV exposure on a scale from one to eleven. Dermatologists recommend not only sunscreen, but also wear clothing and hats that are as long as possible, even from a UV exposure of grade three. Also: avoid the sun at noon.

Concerns about vitamin D deficiency should not lead to unreasonable sunbathing. Our skin needs relatively little UV-B radiation for vitamin synthesis. In summer, the random UV exposure in everyday life is usually sufficient. The Federal Office for Radiation Protection recommends keeping your arms, hands and face in the sun two to three times a week for about twelve minutes, depending on your skin type, a little longer or shorter. The vitamin D level does not increase proportionally if we exceed this duration. Those who suffer from vitamin D deficiency should therefore prefer to use preparations.

Incidentally, the worst argument against sun protection is vanity. Some still find themselves unprotected sunbathing – in the secret hope of a crisp tan. The application Sunface helps against this short-term seduction. It was developed by dermatologist and AI scientist Titus Brinker and gives a look at your own reflection in 25 years: with regular sun protection. And without.


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