What does SPF (sun protection factor) actually mean? Common misconceptions are that the number has something to do with the temperature or the proportion of UV filters in the formulation. No, an SPF 50, for example, means:

The radiation that damages my skin is reduced 50-fold.

This means that only 2% of the radiation gets through. 98% is absorbed by the protective film and rendered harmless.

Nerd fact: The function between SPF and transmission (T: radiation that is transmitted) is approximately as follows: SPF = 1/T  This also makes it clear that even an extremely high protection factor can never provide 100% protection.

What can I derive from this for myself? The time I can safely spend in the sun is extended. It is multiplied by the SPF:

Self-protection time x SPF = protected time

That's the theory. However, the calculated protection time only applies under ideal conditions, which are almost never achieved in practice. A generous amount must be applied. Studies show that most people only apply 1/4 of the amount required to achieve the SPF (1). This reduces the protection enormously. The specified SPF applies to an application amount of 2mg/cm² of skin. This corresponds to around 1g for the face and 30g for the body (depending on size). Good guidelines are, for example, 1/4 teaspoon for the face or a shot glass for the body. Or two finger lengths of sunscreen for each body region (head, arm, chest, stomach, lower leg, etc.).

In addition, swimming, sweating, rubbing, etc. cause the protective film to wear down over time. In intense sun, it is therefore recommended to reapply cream every 2 hours and after bathing.

Rule of thumb: Even as a careful user, you should only ever use about half of the theoretically calculated protection time (2). In addition, always seek shade whenever possible and wear long clothing.

Self-protection time?

First of all, how do I know how long I can stay in the sun unprotected?

In 1975, the dermatologist Fitzpatrick divided people into 6 different skin types based on skin, hair and eye color, as well as features such as freckles, etc., and assigned them an approximate self-protection time (3). We still use this classification today because it is easy to understand. Skin types I to IV are the most common in Europe and can stay in the sun unprotected for an average of 5 to 30 minutes. That's not very long! For children, a self-protection time of 5 minutes is always assumed.

Skin type I: Celtic type
People with skin type 1 have very light skin and often reddish or light blonde hair. Their eyes are typically blue, green or light grey. They get freckles easily and their skin does not usually tan. ( Self-protection time: less than 10 minutes)

Skin type II: Nordic type
People with Nordic skin types also have very light skin. Their hair is blonde or light brown, and their eyes are blue, grey or green. Their skin hardly tans or only tans very slowly. ( Self-protection time: 10 - 20 min)

Skin type III: mixed type
Most people in Central Europe belong to the mixed type. They have a light to medium skin color. Their hair color ranges from blonde to black, and their eye color varies between brown, blue, gray and green. Their skin slowly turns brown in the sun. ( Self-protection time: 20 - 30 min)

Skin type IV: Mediterranean type
The Mediterranean type naturally has a slightly tanned or olive skin tone. People with this skin type usually have dark hair and brown eyes. Their skin tans quickly. ( Self-protection time: approx. 45 min)

Skin type V: Dark type
People with this skin type always have dark skin, often with a gray undertone. They have dark eyes and black hair. Freckles do not appear on this skin type and the skin tans quickly. ( Self-protection time: approx. 60 min)

Skin type VI: Black type
People with skin type 6 have a dark brown to black skin color even without sun exposure. Their hair and eyes are dark brown to black. They have no freckles. ( Self-protection time: approx. 90 min)

Even dark skin types cannot stay in the sun unprotected indefinitely. Unfortunately, the diagnosis of skin cancer is still very unreliable, especially for dark skin types, because dermatology almost exclusively teaches on light skin (4). Sun protection is relevant for all skin types !

The natural protection times of different skin types also show that they do not differ that much and that "pre-tanning" offers hardly any protection. Even if the natural protection time increases from 10 to 20 minutes, this only corresponds to a factor of 2! Since UV-induced tanning is always a sign of skin damage, it is best not to actively cause it.

>> Click here for the blog post "5 Sun Protection Myths: A Fact Check"

Nerd fact: Doesn't the time of day you need to protect yourself also depend on the time of year, location and position of the sun? Yes, strictly speaking, the time of day you need to protect yourself applies to a UV index of 8, which corresponds to a relatively sunny summer midday. However, you shouldn't take too many variables into account when calculating, but rather just protect yourself generously and stay in the sun for a shorter time rather than too long.

Time for the sun?

Now that we know our own protection time, we can calculate our theoretical protection time based on the SPF of our sunscreen. For example, for a skin type 2 with 10 minutes of own protection time and an SPF of 50, this would be:

10 min x SPF 50 = 500 min

This corresponds to about 4 hours of protected time in the sun if we take about half the result.

Applied cream too late?

What if, for example, I am already in the sun and only apply sunscreen after a while? How long can I stay in the sun? To do this, we have to look at two things separately: the theoretical protection time that we build up with sun protection and the time actually spent in the sun . The former should always be longer than the latter! Otherwise, sunburn will occur. This means skin damage, premature skin aging and an increased risk of skin cancer.

The answer to these questions can be illustrated using an hourglass. The size of the hourglass is the theoretically calculated maximum protection time. The remaining sand is the remaining time we can stay in the sun.

When we apply sunscreen, we multiply our own protection time. If we don't apply more sunscreen, our theoretical protection time shrinks . Not because the protection "wears out", but because sunscreen is not a T-shirt and evaporates over time. Regardless of how long we were in the sun during that time.

Every morning when we go out into the sun, the sand starts to fall. The smaller our hourglass, the faster our safe time in the sun runs out.

Once the sand has run through, applying more cream is no use. The clock will theoretically get bigger again, but the sand cannot flow back up. 0 min x SPF 50 is still 0 min.

However, you should still apply sunscreen and seek shade and get out of the sun as quickly as possible.

Conclusion:

If you are on holiday and spend a lot of time in the sun, apply sunscreen generously in the morning before you go out into the sun. Reapply sunscreen regularly so that your hourglass stays nice and big and you have plenty of real sand available.

Use a high sun protection factor , because reapplying sunscreen will not bring sand back to the surface, but will only make your hourglass bigger again. It should therefore be as big as possible from the start.

If you have forgotten to put on sunscreen and your little self-protection hourglass has already run out, then make sure you stay in the shade for the rest of the day and wear long clothing . Your skin is already exhausted.

Overnight, your hourglass will be refilled with sand and you can start again.

Your Ying

Engineer, Cosmetic Scientist and Founder of Skingineered

Sources:

(1) Bibi Petersen, Hans Christian Wulf, Application of sunscreen − theory and reality, Photodermatology, Photoimmunology & Photomedicine, 2013, https://doi.org/10.1111/phpp.12099

(2) Federal Office for Radiation Protection: UV protection through sunscreen (as of March 29, 2022)

(3) Vishal Gupta, Vinod Kumar Sharma, Skin typing: Fitzpatrick grading and others, Clinics in Dermatology, 2019, https://doi.org/10.1016/j.clindermatol.2019.07.010

(4) Oma N Agbai MD et al., Skin cancer and photoprotection in people of color: A review and recommendation for physicians and the public, Journal of the American Academy of Dermatology, 2014, https://doi.org/10.1016/ j.jaad.2013.11.038