Decoding SPF: The science behind sunscreen filters

Have you ever wondered what those SPF numbers really mean, or how they're determined? From cutting-edge measurement techniques to the truth about water resistance, UV-filters, the world of sunscreen is far more interesting than you might think. Whether you're a beach enthusiast, interested in your skin`s health and beautyspan, or just curious about the science behind your daily skincare routine, this post will shed new light on the powerful protective shield between you and the sun's rays including some useful tips.

SPF
​SPF means Sun Protection Factor. The labelled SPF is not indicating the amount of time you can stay in the sun safely, like for example with SPF 50, it would be 50 minutes, however it indicates how much longer it takes for you to get a sunburn (primarily but not exclusively caused by UVB). Thus with SPF 50, it would take 50 times longer.
This is very specific for you and depends on factors like 
▌Your phototype 
▌UV index, cloudy day or not 
▌Season & climate
▌Time of day
▌Latitude & altitude
▌How much product you applied: amount
▌How well you distributed the product: coverage
▌Rubbing off: clothes or touching towelling
▌Sweating
▌Activities like swimming, sauna, etc

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SIGNIFICANT DIFFERENCES SPF
A misconception I would like to address is that the difference between an SPF 30 and SPF 50 of SPF100 is just minor and thus not worth the investment. First, the listed SPF refers predominantly to UVB rays. I will explain UVA protection. SPF 30 blocks 96.7% and SPF 50 97.8%, of UVB rays, this is about 1% difference in “blocking”, and it might seem not a big difference, however SPF50 is 33.3% more effective than SPF 30! We need to look at the % of UVB rays which are still able to damage your precious skin. This moreover translates into a significant difference in immune-suppresion, genomic stability or DNA damage (the root cause for skin cancer and major contributor to premature aging) and inflammation. For example the difference between SPF 100 and 50+ is 45% less DNA damage and 24% less inflammation and thus a significant difference. [1]
 
UVB + UVA Protection
 ▌SPF 15: Blocks approximately 93.3% of UVB rays
                 Allows about 6.7% of UVB rays to penetrate 
                 The minimum UVA protection factor should be 5
▌SPF 30: Blocks about 96.7% of UVB rays 
                 Allows about 3.3% of UVB rays to penetrate 
                 The minimum UVA protection factor should be 10
▌SPF 50: Blocks around 97.8% of UVB rays
                 Allows about 2.2% of UVB rays to penetrate 
                The minimum UVA protection factor should be approximately 16.7
▌SPF 50+ (measured SPF ≥ 60): Minimum UVA protection factor of 20
▌SPF 100 (Medical Device): Blocks approximately 99% of UVB rays
                                            Allows about 1% of UVB rays to penetrate ​
                                            The minimum UVA protection factor should be approximately 33.3

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MEASUREMENT SPF
SPF (Sun Protection Factor) measurement involves several methods, each with its own advantages and pitfalls.

In vivo method (ISO 24444)
ISO 24444 is the international standard for the in vivo determination of the Sun Protection Factor (SPF) of sunscreen products. This standard specifies a method for evaluating how well a sunscreen protects human skin against erythema, which is the reddening of the skin caused by UV radiation exposure.

▌In vivo testing: The SPF is determined by testing on human subjects. A controlled amount of sunscreen is applied to the skin, and the test involves measuring the Minimal Erythema Dose (MED) with and without sunscreen. The SPF is calculated as the ratio of these doses.
▌Procedure: The test involves exposing treated and untreated skin areas to UV radiation using a solar simulator. The MED is determined by observing the point at which slight but visible reddening occurs on the skin after exposure.
▌SPF Calculation: The SPF value is calculated as an arithmetic mean of all valid individual SPF values obtained from all test subjects.
▌Global Adoption: ISO 24444 has been widely adopted in nearly 60 countries, including those in Europe, Australia, New Zealand, Japan, and several others, ensuring a harmonized approach to SPF testing across different regions.
▌Advantages: Provides real-world data on sunscreen performance.
▌Disadvantages: Requires exposure of human subjects to UV radiation and sunburn (unethical). Can be time-consuming and expensive. Results may vary due to individual skin differences.
 
In Vitro Spectrophotometric Method
▌Process: Uses a spectrophotometer to measure UV transmission through a thin film of sunscreen applied to a substrate.
▌Measurement: Calculates SPF based on the absorption spectrum.
▌Advantages: Rapid, cost-effective, and doesn't require human subjects.
▌Disadvantages: May not accurately represent real-world conditions. Results can be affected by the substrate used and application technique.  

Double Plate Method (DPM), also known as the Cosmetics Europe In vitro method
Is a technique under development as ISO 23675. The Double Plate Method offers a promising alternative for sunscreen testing by eliminating the need for human subjects and providing a more standardized approach to measuring SPF. It is expected to be officially published as an international standard in early 2025.
▌Dual plate system: Utilizes two types of PMMA plates—moulded and sandblasted—to simulate the skin's surface. The combination of these plates helps overcome limitations related to the affinity of different sunscreen formulations for a single type of plate.
▌Automated spreading: The sunscreen is applied to the plates using a robot, ensuring consistent application that mimics human application but with improved reproducibility.
▌UV exposure: The plates are exposed to UV radiation with a spectrum similar to that used in the in vivo ISO 24444 method, allowing for assessment of the sunscreen's photostability and effectiveness.
▌Measurement: Initial absorbance is measured before UV exposure, and final absorbance is measured post-exposure. These measurements are used to calculate the in vitro SPF.
▌Validation and standardization: The method is currently in the validation process by ISO experts and aims to provide accurate, repeatable, and reproducible SPF predictions.
 
Hybrid Diffuse Reflectance Spectroscopy (HDRS)
Hybrid Diffuse Reflectance Spectroscopy (HDRS) is newer technique and associated with the ISO 23698 standard. This method is being developed as a non-invasive alternative to traditional SPF measurement methods like ISO 24444, which involves in vivo testing on human skin using UV radiation to provoke an erythemal response.
▌Non-Invasive: HDRS does not require UV exposure that causes erythema (skin reddening), thus addressing ethical concerns associated with traditional SPF testing methods.
▌Hybrid approach: Combines in vivo diffuse reflectance spectroscopy on the skin with in vitro transmission measurements of sunscreen products. This allows for comprehensive assessment without causing physical harm to test subjects[5].
▌Comprehensive assessment: Provides a hybrid spectrum that evaluates both UVB and UVA protection, correlating closely with traditional in vivo SPF and in vitro UVA protection factor (UPF) test results[3].
▌Ethical and safe: Eliminates the need for UV-induced skin reactions, making it a more ethical testing method.
▌Efficient: Reduces the time required for testing compared to traditional methods.
▌Reliable: Demonstrated good correlation with established standards like ISO 24444 and ISO 24443, making it a viable alternative for sunscreen testing.
The HDRS method is currently at the Final Draft International Standard (FDIS) stage, indicating it is close to becoming an official ISO standard, expected to be published in early 2025. 
Researchers and regulatory bodies continue to work on improving these methods to ensure more accurate and reliable SPF measurements across different sunscreen formulations.
 
UVA PROTECTION
A higher SPF value generally correlate with higher UVA protection, especially in regions requiring the 1:3 UVAPF-to-SPF ratio for broad-spectrum labeling. It is called the UVA-COLIPA ratio as defined in ISO 24443 or Critical Optical Radiation Absorption (CORA). CORA is a measure used to assess the UVA protection of sunscreen products. According to European regulations, the UVA protection factor of a sunscreen must be at least one-third of its labeled SPF value. This ensures that sunscreen products provide a minimal and balanced level of protection against both UVA and UVB radiation. 
 
UVA protection in sunscreens is sometimes not listed but disclosed on the product by a black circle with UVA in it, or listed and measured using different systems across various continents:
 
Europe
The UVAPF is not per se disclosed on the product.Look for the black circle with UVA written in it.
▌PPD (Persistent Pigment Darkening): Measures UVA protection directly.
▌UVAPF (UVA Protection Factor): Must be at least 1/3 of the labeled SPF value.
▌Critical Wavelength: At least 370 nm for broad-spectrum protection. 

Asia (particularly Japan and Korea)
PA System: Derived from PPD measurements.
▌PA+ (PPD 2-4)
▌PA++ (PPD 4-8)
​▌PA+++ (PPD 8-16)
▌PA++++ (PPD 16 or higher) 

United States
▌Broad Spectrum: Indicates UVA protection, but no specific rating system.
▌Critical wavelength of at least 370 nm required for broad-spectrum labeling.

Australia
▌Broad Spectrum: Similar to US, requires UVA protection to be at least 1/3 of the labeled SPF like in Europe

Measurement methods
▌In vivo PPD Test: Measures skin darkening after UVA exposure.
▌Critical Wavelength: Determines the wavelength below which 90% of UV absorption occurs.
▌In vitro PMMA Plate Method: Used for measuring UVAPF-to-SPF ratio in Europe. 

HOW SUN-FILTERS WORK
UVA FILTER 
▌absorption maximum between 320 and 400 nm 
UVB FILTER
▌absorption maximum between 290 and 320 nm 
BROADSPECTRUM FILTER 
▌absorption throughout the UV spectrum from 290 to 400 nm
 
MINERAL VS CHEMICAL
The terms "mineral" and "chemical" filters in sunscreens are often considered inaccurate because they do not accurately reflect the chemical nature of the ingredients used. Instead, the terms "organic" and "inorganic" are more precise:
 
Why the terms matter
1. Chemical nature: The term "chemical" suggests synthetic or artificial, which can be misleading since both organic and inorganic filters involve chemical processes. "Organic" refers to carbon-containing compounds, while "inorganic" refers to mineral-based compounds without carbon.
2. Mechanism of action: The terms "physical" and "chemical" imply different mechanisms of action (reflection vs. absorption), but both types of filters can absorb UV radiation.
3. Consumer perception: Using accurate terminology helps consumers make informed choices based on their preferences for natural or synthetic ingredients and their environmental impact.
 
CHEMICAL OR ORGANIC FILTERS
▌Composition: These are carbon-based compounds designed to absorb UV radiation. They include aromatic compounds with carbonyl groups, such as cinnamates and benzophenones.
▌Mechanism: Organic filters absorb UV radiation and undergo a reaction, releasing the absorbed energy as heat or light of a lower-energy longer wavelength such as infrared radiation (i.e., heat).
▌Examples: Avobenzone, octocrylene, and oxybenzone and ecamsule are common organic filters.
▌Stability: Most newer organic filters are photostable, meaning they don´t stop working after absorbing too much UV light. However, avobenzone and octinoxate are photo-unstable and are therefore often combined with other filters. Butyl Methoxydibenzoylmethane, (avobenzone), provides excellent protection across the entire UVA range, including UVA1 (340-400 nm) and UVA2 (320-340 nm). This makes it the global gold standard for UVA protection.
▌Advantages: Chemical filters have a high “staying power”, meaning they don´t clump and stay in an even layer on the skin, often have lighter pleasant textures and offer high UVA protection.
▌Act to block ultraviolet radiation, which is light with wavelengths shorter than visible light
    ▌UVA1 (300-400) also called long UVA
    ▌UVA2 (315-340) 
    ▌UVB (290-315) radiation 
    ▌UVC (100-290) nm - not relevant  

.PHYSICAL OR INORGANIC FILTERS
▌Composition: These are mineral-based compounds, typically metal oxides like titanium dioxide (TiO2) and zinc oxide (ZnO).
▌Mechanism: Inorganic filters primarily reflect and scatter (actually also into the skin) UV radiation but can also absorb it due to their semiconducting properties. Act to block ultraviolet radiation which is light with wavelengths shorter than visible light.
▌Advantages: They offer broad-spectrum protection, are photostable, less likely to cause irritation.
▌Disadvantages: might leave a white cast, are sometimes cosmetically less elegant (greasy and thick) or less suitable for darker phototypes, and tend to clump together on your skin, even though you might not notice this. You need quite a large amount of zinc oxide to absorb a relatively small amount of UV and the risk is rather high that you don´t use enough.
▌Dermatologists in the US were recommending mineral sunscreens, because in the US the sunfilters approved by the FDA are restricted and to reach the UVA1 protection level, had to contain either avobenzone as organic filter or zinc oxide as inorganic filter. Although zinc oxide has lower UVA-PF, it was considered to have less irritation potential and was therefore preferred. Note: Avobenzone is an excellent filter found in sunscreens suitable and tested on sensitive skin, however it is always recommended to ask for a sample and try before you buy. 
▌Experimental studies have shown that when particle sizes are very small, as in micronized sunscreens, the mechanism of action is similar to that of chemical filters. Some say that only 5-10% of the mode of action is “reflection and scattering” and the rest is comparable to chemical filters.
 
WATERRESISTANT – WATERPROOF - SWEATPROOF
In Europe and other regions, the terms "water-resistant," "waterproof," and "sweatproof" on sunscreen labels have specific meanings and regulations.
The ISO standard for measuring water resistance in sunscreens is ISO 16217. This standard outlines the procedure for evaluating water resistance by comparing the Sun Protection Factor (SPF) before and after water immersion.
 
According to the guidelines:
1. A sunscreen can be labeled as "water-resistant" if it retains at least 50% of its SPF value after 40 minutes (2 x 20 minutes) of water immersion compared to the initial SPF value before immersion.
2. For "very water-resistant" claims, the product must maintain its effectiveness after 80 minutes (4 x 20 minutes) of water immersion.
▌Measurement method: The sunscreen is applied to the skin and immersed in water according to a strict ISO-protocol for the claimed duration. Afterwards, the SPF is measured to ensure it remains effective.
▌Disadvantages: 
    ▌Variability: Differences in application thickness and skin type can affect results.
   ▌Environmental factors: Chlorine, saltwater, and physical activity can impact sunscreen effectiveness, hence the testing method does not reflect real world.
 
While there are regional differences in how water resistance is labeled and regulated, no sunscreen can be truly waterproof or sweatproof. Consumers should look for "water-resistant" labels and reapply sunscreen regularly (every 2 hours) and preferably after swimming, sweating or toweling to maintain protection.
 
Europe
Water-resistant
▌Regulations: European regulations do not allow claims of "waterproof" or "sweatproof" due to the potential for misleading consumers.
 
United States
Water-resistant
▌Definition: Similar to Europe, U.S. regulations allow sunscreens to be labeled as "water-resistant" for either 40 or 80 minutes.
▌Regulations: The FDA prohibits the use of "waterproof" and "sweatproof" on labels since 2011, requiring clear indications of how long the product remains effective in wet conditions. 

Australia
Water-resistant
▌Definition: Australian regulations are strict, allowing water-resistant claims only if the sunscreen maintains its SPF after immersion in water for up to 4 hours.
▌Measurement method: Similar testing methods are used as in Europe and the U.S., with rigorous standards set by the Therapeutic Goods Administration (TGA).
 
SAFETY CONCERNS & MYTHS

Nanoparticles: Zinc oxide and titanium must be ground into tiny particles to avoid forming a “white cast”. This can be either micro-particles (100-250o nm) or even smaller than 100 nm (nanoparticles). Even these smallest particles don´t penetrate beyond the stratum corneum and are considered safe. They might penetrate deeper and cause reactions when applied on damaged skin, for example just after an aesthetic procedure like peeling, fractional laser etc. 

Endocrine disruption: Claims about hormone disruption are largely based on animal studies with unrealistically high doses. Human studies have not shown significant risks, which was confirmed after careful re-evaluation by regulatory bodies. The only filter to avoid is 4-Methylbenzylidene Camphor, also known as 4-MBC or Enzacamene, is a chemical sunscreen agent used primarily as a UVB filter. 4-MBC has been banned in the European Union due to concerns about its safety or lacking proper safety data.

Systemic absorption: While some sunscreen ingredients can be absorbed into the bloodstream, the levels are considered too low to cause harm. Larger companies and probably some smaller ones too, have serious safety departments who will make toxicology calculations taking lifetime exposure of the ingredient(s) and formula into consideration. They are in constant exchange with regulatory bodies and both exist to keep you safe. There is zero tolerance for systemic or side effect of skincare or sunscreens.

Free radical formation: Some filters in sunscreens react with UV and form free radicals, thus cause oxidative stress. Intelligent sunscreen formulations contain anti-oxidants to neutralize free radicals from UV, Blue Light and potentially UV-filters. My personal favorite is Licochalcone A, because it it is the most potent anti-oxidant to neutralize free radical activity from both UV and High Energy Visible Light. Moreover, it can work as both first line defence (extracellular) and second line defense (intracellular), backed up by science. 

DIY sunscreens: Crafting sunscreens at home can lead to uneven distribution of the filters if ingredients are not well mixed, too low concentrations of filters and thus inadequate protection. Serious sunscreen brands put their products through a long development process including SPF, UVA, microbiology, stability, safety and tolerability testing, product in use studies with hundreds of volunteers and clinical studies under supervision of a dermatologist. The potential skin damage from insufficient SPF far outweighs any cost savings, for both aesthetic and health reasons. 

Sunscreens cause skin cancer: They don´t and there is ample scientific evidence to support this. I do want to re-emphasise to apply sunscreen in the recommended amount and ensure adequate coverage to be well protected. Reapply every 2 hours, especially after swimming, perspiring or towelling.

Sunlight is inherently healthy: While some sun exposure is absolutely beneficial, excessive exposure is a known carcinogen and will make your skin age faster or cause hyperpigmentation. Do I need to remind you of famous pictures of a woman with leather-like looking very tanned wrinkled skin and the truck driver with severe solar elastosis on the side of his face exposed to sunlight? Sun is fun, however please be safe. Read more.

I must apply sunscreen every day: In case of skin cancer prevention I would consider Australia a reliable benchmark. The Cancer Council Australia and the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) recommend using sunscreen on days when the UV Index forecast is 3 or higher. However, if you want to be safe and significantly decrease the risk of skin cancer, prevent premature aging and/or hyperpigmentation, daily use of sunscreen in face (or other unexposed areas) is highly recommended even with a lower UV Index, especially when using medication, skincare or undergo treatments making your skin more prone to sun-damage. Too much or too often is almost not possible when it comes to sunscreen use. 

TikTok trends and celebrity recommendations: Use common sense and what works well for them, might not work for you. "Scary sunscreen stories" seem to go viral at the moment and I wish the same people with huge following like Dr. Andrew David Huberman (associate professor of neurobiology and ophthalmology at Stanford University School of Medicine), or Gary Brecka (human biologist and biohacker) would instead of creating and spreading sunscreen myths focus on proper evidence based education on sunscreen use and skin cancer prevention. 

Ocean safe and sustainable formulas/products: The term "reef-safe" has become a buzzword in the sunscreen industry. Ocean or reef safe formula´s are usually formulated without microplastics (UNEP definition), with biodegradable polymers and improved filter-systems complying with regulations like the ones in Hawaii and Palau, are more sustainable formula´s in preferably in ditto packaging. Sustainability is extremely complicated, involving the whole supply-chain from ingredient sourcing, production, packaging (primary and secondary), transportation to recyclability and even marketing materials. I consider every step towards preserving our marine life and environment in general a significant one. 

TIPS
 
1. Select the right sunscreen: It's crucial to choose a sunscreen that suits your skin type, purpose and one you enjoy using. Opt for a higher SPF than you think you need, as you often apply less than the recommended amount to reach the labelled SPF on the product. The findings of this study suggest that at the start of the workday proper application of 2 mg/cm2 of SPF50+ (which is 60 or higher) sunscreen will degrade to an SPF level of less than 30 at 4 hours after application. Read more Take this into consideration when buying your sunscreen, you don’t reapply before your lunch break and go outside in the sun for a walk. Big disclaimer is that matters might be worse than reflected, as in some areas your sunscreen will have worn off completely and coverage is important for protection. A useful tip is to apply sunscreen twice; studies show that double application helps achieve the labeled SPF more reliably. Of course you can double up with a daycare containing SPF and a sunscreen.

2. Apply sunscreen properly: The most important of all tips. Take the time to apply sunscreen thoroughly about 15-20 minutes prior to going outside. Coverage and even distribution of the correct amount are key. The majority of sunscreens can be used after your daily moisturiser or serum and before (gently applied) make-up. Not all ingredients might go well together. Tinted products containing iron oxides offer additional protection against UV and High Energy Visible Light, however make-up with SFP is not sufficient as you will probably not apply enough of it to reach the listed SPF without looking cakey. 

3. Be aware of Blue Light: Although not mentioned in this post, blue light from sunlight can harm your skin. It's important to be informed about its effects, particularly darker phototypes. Read more.

4. Rethink tanning: There is no such thing as a healthy tan (except maybe a spray tan). A tan indicates skin damage. It's essential to recognize this and take protective measures. Read more.

5. Consider DNA damage: DNA damage from UV exposure is serious, though the skin can repair itself to an extent, there are ways to prevent damage (sunscreen) and support this repair process. Read more.

6. Prioritise SPF: Using (expensive) rejuvenating serums or creams is futile without daily sunscreen protection. Sunscreen is the foundation of any effective skincare routine. Moisturisers with a high SPF will offer the same UV protection as sunscreen, because SPF is regulated. The same amount as sunscreen is recommended to be applied and reapplied: 2mg per cm2. Calculate about 1 gram for face, 1 gram for the neck, 1 gram for décolletage, 1 gram for the back of 1 hand, 2 grams for your scalp and 2 grams per forearm. The precise amount depends on your skin surface.

7. Eye safety: Some filters may cause irritation when they migrate into the eye area. This is very annoying. You can avoid migration of the product by applying a little bit of translucent powder, a trick used by make-up artists to “set” foundation and concealer, however this works well for sunscreen too. Wear sunglasses for extra protection of the delicate eye area. Although some might recommend the use of mineral filters in the eye area, I am hesitant to make such a recommendation as mineral filters are more prone to migrate and clump than chemical filters. 
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8. Shiny greasy skin: Some sunscreens might make your skin look greasy or shiny. Moreover, skin´s sebum production is increased during daytime: circadian rhythms. There are special sunscreens for oily skin types with mattifying pigments and even sebum regulating technology. For example L-Carnitine has shown to reduce sebum production by 48%. Careful blotting, the use of a translucent or even better a powder with iron oxide containing colour pigments also help to mattify. 
 
Always consult a qualified healthcare professional to determine what the most suitable approach is for your skin health and beauty.
Sun is fun! Take care.

Anne-Marie
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Reference
 [1] van Bodegraven et. al. Redefine photoprotection: Sun protection beyond sunburn. Experimental Dermatology, 2024 ​​

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