SPF stands for “Sun Protection Factor” and is the abbreviation for Lichtschutzfaktor (LSF) in German, also known as the sun protection factor. The SPF describes how much longer you can theoretically stay in the sun with sunscreen without getting sunburned, compared to unprotected skin. The sun protection factor is calculated using the following formula:

SPF × skin’s natural protection time = theoretical protection time

For example, if your skin shows the first signs of sunburn after ten minutes without protection, a properly applied SPF 50 provides up to 500 minutes of protection according to this calculation. But be careful: This calculation is based on standardized laboratory conditions—in reality, many other factors come into play. Also: This calculation focuses on sunburn. Other long-term damage isn’t taken into account, even though our skin doesn’t forget it. Resulting consequences, such as premature skin aging or skin cancer, can manifest years later and then become acute. In other words: Skin damage can occur even without sunburn.

What sun protection factor do I need: SPF 50 or 30?
The sunprotection factor (SPF) indicates how well a product protects against UVB radiation. The higher the SPF, the greater the level of protection. A distinction is made between different protection categories. The following SPF table shows you how SPF values and UVA protection can be categorized.

Both the Daily Defense SPF 30 Fluid and the SPF 50 Fluid from DR. EMI ARPA SKIN fall into the “High Protection” category, meaning they provide high sun protection for the skin. They help prevent UV-related skin damage such as sunburn. A general guideline for everyday use: Sunscreen with SPF 30 or SPF 50 is always recommended; the SPF depends on the situation and your skin type. While products with SPF 100 do exist, they are not commonly used because even sunscreens with the highest SPF cannot filter out all UV radiation. According to the European Commission, claims such as “100% protection” or “sunblock” are considered misleading and may not be used, as they convey a false sense of security. Sunscreens must also carry clear warnings stating that 100 percent protection is not possible. These warnings are intended to make it clear to consumers that additional precautions—such as avoiding intense sunlight—are still necessary.

Reapplying Sunscreen = Maintaining Protection, Not Extending It

It is essential that sunscreens be reapplied regularly. The statement “Reapplying sunscreen does not extend the duration of protection” therefore sounds contradictory at first. The duration of protection is determined once and for all by the sun protection factor (SPF) multiplied by the skin’s natural protective capacity. And here’s where it gets interesting: It isn’t extended by reapplying, because this only serves to maintain the protection. Daily sunscreen is worn off or reduced by sweat, water, mechanical friction (e.g., clothing, towels), or incomplete application. Over time, the effective protective effect decreases, even if the theoretical duration of protection hasn’t yet been reached.

In the EU, sunscreen products are classified as cosmetics and are subject to Regulation (EC) No. 1223/2009. However, due to their specific protective function, additional requirements apply. For example, a standardized SPF test in accordance with ISO 24444:2019 must be conducted to determine the sun protection factor. This is an internationally recognized in vivo test in which test subjects are used to measure how long it takes for sunburn to develop on unprotected skin compared to skin treated with sunscreen. The determined SPF value indicates how much longer a person can be exposed to the sun while using sunscreen without getting sunburned.

Our skin has a natural defense mechanism against UV radiation—known as the natural protection time. It describes how long the skin can be exposed to the sun without additional sun protection before the first signs of UV-related damage, such as redness or sunburn, appear. This time period varies from person to person and depends on skin type. While very fair skin types (Type 1) have a natural protection time of only 5–10 minutes, darker skin types (Type 6) can have a natural protection time of up to 90 minutes. Skin types are classified according to the so-called Fitzpatrick scale. It helps determine the right sunscreen for each skin type—and assists, for example, in deciding whether to use sunscreen with SPF 30 or SPF 50. However, even though our skin offers a certain degree of natural protection, it is not sufficient—because UV rays penetrate much deeper and have a more intense effect than many people realize. That’s why sunscreen products are essential, regardless of skin type. Which sun protection factor is the right one? Regardless of skin phototype and natural protection time, SPF 50 is actually always a good choice. This is especially true when application methods, activity levels, or external factors reduce the protective effect in everyday life.

The six Fitzpatrick skin phototypes were introduced in 1975 by the American dermatologist Thomas B. Fitzpatrick to create a practical classification system that would allow for a better assessment of patients’ individual sensitivity to the sun. Originally, this classification was not intended for cosmetic skin care, but rather for medical phototherapy. Fitzpatrick developed a scale based on two observations: the tendency to develop sunburn within the first 24 hours after sun exposure and the skin’s ability to develop a tan within a week.
Based on this, the classification into six skin phototypes was established:

• Type 1: Very fair skin with lots of freckles. Never tans, but reddens quickly in the sun. 
• Type 2: Fair skin with freckles. Tans very little to moderately; often gets sunburned.
• Type 3: Fair to light brown skin, rarely with freckles. Tans more quickly than Type 2, but sunburn can still occur. 
• Type 4: Light brown, olive-colored skin that tans quickly in the sun. 
• Type 5: Dark brown skin; sunburn is rare. Skin redness following UV exposure is uncommon or barely visible. 
• Type 6: Dark skin, very high natural pigmentation, sunburn is very rare, and redness is usually not visible. 
  
  Today, the Fitzpatrick classification is widely used. It serves to estimate an individual’s risk of UV-induced skin damage and skin cancer, is used to determine the dosage for light and laser treatments, and is also established in clinical studies. Nevertheless, the system is not without controversy, particularly with regard to its validity for people with dark skin.
  
  Criticism of the Fitzpatrick Scale
  The Fitzpatrick Scale was originally developed for fair-skinned patients and later expanded to include darker skin types—but without adapting the underlying methodology. A key point of criticism is that the system equates skin color with light sensitivity, even though both can vary independently of one another. Thus, it is entirely possible for darker skin to be sensitive to UV radiation as well—for example, due to a higher proportion of pheomelanin, which offers less protection. Despite these limitations, the scale has served as a practical tool for many years, enabling a rough assessment of sun sensitivity—such as determining which sunscreen with which SPF is the right choice. While people with fair skin (types 1–2) are generally advised to use sunscreen with SPF 50, for more resilient skin types (e.g., type 4) with a lower tendency to sunburn in everyday situations, even SPF 30 may be sufficient. However, it is now known that subjective assessments of skin reactions are not always reliable, and individual perceptions can influence the results. For this reason, it is increasingly recommended to consider photosensitivity and pigmentation separately and not to assess sun protection needs based solely on phototypes. Even dark skin can be damaged by UV radiation, although sunburn is less commonly visible in these cases. The risk of DNA damage from UV rays—manifesting as premature skin aging and skin cancer—is also present here, albeit at a lower level.
  Therefore, sun protection is important for all skin types, including types 5 and 6.
  
  SUN PROTECTION: PROPER APPLICATION
  For sunscreen to be fully effective, proper application is key. The so-called “3-finger rule” helps with determining the right amount: Three finger-length streaks of sunscreen are recommended for the face, neck, and décolletage. About 30 milliliters are needed for the entire body, which corresponds to a well-filled palm or, for example, a shot glass. It’s important to apply sunscreen to often-overlooked areas such as the ears, the tops of the feet, the hairline, and the scalp. The sunscreen should be applied 15–30 minutes before sun exposure and reapplied every two hours, as well as after swimming or drying off. Make sure your product offers both a sufficiently high SPF (at least SPF 30) and UVA protection.

A high SPF is important for the skin—but it’s only part of the picture. The sun protection factor (SPF) indicates how long the skin is theoretically protected from sunburn. However, reapplying sunscreen does not extend this protection time; rather, it maintains it. The skin’s natural protection time varies from person to person and depends on skin type. Still, sun protection is always advisable, regardless of skin type.
UVB rays are responsible for sunburn, while UVA rays penetrate deeper into the skin and primarily accelerate premature skin aging. A reliable sunscreen must protect against both—with SPF 30 (high protection) or SPF 50 (high protection), depending on skin type and situation. A combination of UVB, UVA, and broad-spectrum filters is therefore essential. After all, a good sunscreen is more than just a high SPF: it provides reliable protection against UVA and UVB radiation, feels good on the skin, and thus becomes a daily staple.
The SPF Fluids from DR. EMI ARPA SKIN combine modern, chemical UV filters for broad-spectrum protection against UVB and UVA radiation, helping to prevent sunburn, photoaging, and long-term skin damage such as skin cancer. What matters most is not just the SPF value, but the entire formulation—with lightweight textures, skin-nourishing ingredients, and minimal potential for irritation, making daily use a pleasure.