The SPF of a cosmetic product is still determined mainly in vivo, under ISO 24444:2019, by comparing the UV dose that causes erythema on protected and unprotected skin. Since late 2024, two standardised alternatives, ISO 23675 (in vitro “double plate”) and ISO 23698 (hybrid, known as HDRS), can deliver a value without inducing sunburn. The right choice depends on the project stage, the formulation type and the target markets.
What exactly does SPF measure?
SPF (Sun Protection Factor) measures how well a product protects against actinic erythema, which is mainly caused by UVB. It is the ratio between the minimal erythemal dose (MED) on protected skin and the MED on unprotected skin. An SPF of 30 means that, under test conditions, a UV dose thirty times higher is needed to produce the same erythema.
Two points shape how this figure should be read. SPF says nothing about UVA protection, which calls for a separate measurement (UVA protection factor, critical wavelength). And the value depends heavily on the amount applied: standardised tests use 2 mg/cm², a dose consumers rarely reach. SPF is therefore a reference value, designed to compare products with one another.
How is an in vivo SPF test run under ISO 24444?
ISO 24444:2019 remains the reference method accepted by the vast majority of authorities. It induces controlled erythema on volunteers’ backs under a solar simulator, on sites protected by the product and on control sites.
Protocol steps
- Panel recruitment: fair-skinned volunteers (ITA° above 28°, i.e. phototypes I to III), with at least ten valid results required.
- Application: the product is applied at 2.0 mg/cm² on delimited sites, then left to dry for 15 to 30 minutes.
- Irradiation: a series of increasing UV doses is delivered by a xenon arc solar simulator whose spectrum is defined by the standard.
- Reading: a trained technician assesses erythema visually, 16 to 24 hours after exposure.
- Calculation: an individual SPF is calculated for each subject, then the mean. The 95% confidence interval must fall within 17% of that mean.
A reference formulation with a known SPF (standards P2, P3, P5 or P8 depending on the expected level) is tested alongside to validate each run.
Known limitations of the method
The method has well-documented weaknesses. Visual reading of erythema introduces a degree of subjectivity, and application uniformity varies from one operator to another. Significant discrepancies have been observed between laboratories testing the same product, fuelling several public controversies over the reliability of SPF claims. Add to that an ethical issue, since volunteers are deliberately sunburned, a panel restricted to fair skin, long lead times and high costs.
What alternatives to in vivo testing exist for measuring SPF?
Two standards published in December 2024 now provide validated alternatives to ISO 24444. They stem from the work of the ALT-SPF consortium and were validated through interlaboratory studies.
ISO 23675: the in vitro “double plate” method
This method is fully in vitro, with no volunteers involved. The product is spread in a standardised way, ideally by a robot, onto two types of PMMA substrates with different roughness, and UV transmission is then measured by spectrophotometry to predict SPF. It applies to emulsions and single-phase alcoholic formulations, but excludes powders and sticks. It cannot be used to assess water resistance either.
ISO 23698: the hybrid HDRS method
Hybrid Diffuse Reflectance Spectroscopy (HDRS) combines optical measurements taken directly on volunteers’ skin with in vitro spectral data. The UV doses involved are low and do not cause visible erythema. Its value lies in capturing how the film actually behaves on skin (topography, filter distribution). It does, however, require specific equipment that is not yet widely available.
In June 2025, Cosmetics Europe issued its Recommendation No. 26, which advises giving preference to these two non-invasive methods. It also states that products already tested in vivo do not need to be retested.
Comparison of SPF testing methods
| Method | Principle | Volunteers | Strengths | Limitations | Typical use |
|---|---|---|---|---|---|
| ISO 24444:2019 (in vivo) | Ratio of MEDs on protected and unprotected skin | Yes, at least 10 valid results, erythema induced | Global regulatory reference | Interlaboratory variability, ethics, cost, lead times | Final claim, markets requiring in vivo data |
| ISO 23675:2024 (in vitro double plate) | UV transmission through two PMMA substrates | No | Reproducibility, speed, cost, independent of skin type | No powders or sticks, no water resistance | Development, technical files, claims depending on the market |
| ISO 23698:2024 (hybrid HDRS) | Diffuse reflectance on skin plus in vitro spectra | Yes, no erythema | Data on real skin, less invasive | Specific equipment, limited availability | Advanced validation, alternative to in vivo |
| In vitro screening (PMMA plates, not standardised for SPF) | UV transmission on a single substrate | No | Very fast, low cost | Correlation too weak to support a claim | Early formulation screening |
Not every laboratory offers all three standardised methods yet, and HDRS capacity remains concentrated among a handful of providers. To find laboratories that run in vivo, in vitro or hybrid SPF tests and compare their expertise, you can search for a laboratory on Skinobs.
What regulatory requirements govern SPF claims?
An SPF claim must be supported by a method recognised in each target market. Requirements differ significantly from one region to another.
- European Union: Regulation (EC) No 1223/2009 and Recommendation 2006/647/EC set a minimum SPF of 6 and four labelling categories (low, medium, high and very high, the last one shown as “50+”). UVA protection must reach at least one third of the SPF, with a critical wavelength of at least 370 nm. The Recommendation is currently under review. The common criteria of Regulation (EU) No 655/2013 also apply to any claim.
- United States: SPF falls under the FDA’s OTC sunscreen framework, which requires an in vivo method and governs labelling, including the “broad spectrum” statement.
- Australia: AS/NZS 4399:2020, aligned with ISO 24444, applies under TGA oversight.
- Asia: Japan and Korea rely on ISO 24444 for SPF and on a PA grading system for UVA.
These differences will be covered in a dedicated article in this series, on broad-spectrum requirements by region.
How do you choose an SPF testing strategy?
Most manufacturers now take a stepwise approach rather than relying on a single method. In vitro screening helps shortlist formulations, ISO 23675 supports development, HDRS adds validation on skin, and in vivo testing is still used where the market requires it for the final claim.
Several factors guide the choice:
- the formulation type (sticks and powders fall outside ISO 23675);
- the target markets and how local authorities view each method;
- any other planned claims, such as water resistance, which calls for a specific in vivo protocol;
- the project timeline and budget.
Before approaching a laboratory, it helps to specify the target SPF, the markets, the expected standard, the number of subjects and the report format. A clear brief makes quotes easier to compare and reduces the need for retests.
FAQ
Can an in vitro SPF appear on a label in Europe?
The 2006 European Recommendation accepts validated, standardised methods, and Cosmetics Europe now recommends giving preference to ISO 23675 and ISO 23698. In practice, many brands keep an in vivo test for the final claim, especially when they also target markets outside the EU.
How many volunteers does an ISO 24444 SPF test require?
The standard requires at least ten valid results. More subjects are usually recruited to offset exclusions and meet the statistical criterion on the confidence interval.
Why can two laboratories report different SPF values?
Visual reading of erythema, spreading quality, the solar simulator spectrum and panel composition all affect the result. ISO 23675 and HDRS were developed precisely to reduce this variability.
Does SPF indicate UVA protection?
No. SPF mainly reflects protection against UVB. UVA protection is measured separately, most often in vitro under ISO 24443. That will be the topic of the next article in this series.
Find the right laboratory for your sun care testing
Skinobs brings together more than 9,000 users in 124 countries and lists cosmetic testing laboratories and solutions worldwide, including sun protection specialists. Explore the Skinobs platform to compare available methods and contact the providers that fit your project.
The editorial team
