UVA protection is now mostly measured in vitro under ISO 24443, using the absorbance spectrum of a product film spread on a PMMA plate before and after irradiation. This yields two key indicators: the UVA protection factor (UVA-PF) and the critical wavelength. Other approaches complement this baseline, including the UVA/UVB ratio, the in vivo PPD method (ISO 24442) and, more recently, the hybrid HDRS method.
Why measure UVA protection separately from SPF?
SPF mainly reflects protection against UVB, which causes sunburn. UVA (320 to 400 nm) penetrates deeper into the skin and contributes to photoageing, immunosuppression and some forms of DNA damage, without causing visible erythema. A product can therefore show a high SPF while offering poor UVA protection.
That is why most regulations require a specific measurement of UVA protection. In Europe, the UVA-PF must reach at least one third of the claimed SPF, with a critical wavelength of at least 370 nm, for a product to be considered protective.
How does the ISO 24443 method work?
ISO 24443 is the reference in vitro method for determining UVA-PF. It relies on spectrophotometric measurement of UV transmission through a product film, adjusted using the SPF obtained in vivo.
Protocol steps
- Preparation: the product is applied at the rate defined by the standard onto roughened PMMA plates that mimic skin topography, spread evenly and left to dry.
- Initial measurement: the absorbance spectrum is recorded between 290 and 400 nm at several points on each plate.
- Adjustment: the spectrum is scaled by a coefficient calculated so that the in vitro SPF matches the in vivo SPF measured under ISO 24444.
- Pre-irradiation: the plates receive a UV dose proportional to the initial UVA-PF, to account for the product’s photostability.
- Final measurement and calculation: a new spectrum is recorded and the UVA-PF is calculated using the persistent pigment darkening (PPD) action spectrum.
What the method brings
The main strength of ISO 24443 is that it builds in photostability: a filter that degrades under irradiation shows a lower UVA-PF after pre-exposure. The method also avoids exposing volunteers to UVA. Its main constraint is its reliance on the in vivo SPF for the adjustment step.
What is the critical wavelength and how is it measured?
The critical wavelength is the wavelength below which 90% of the area under the product’s absorbance curve lies, between 290 and 400 nm. The higher it is, the further protection extends into long UVA. The 370 nm threshold is used as a benchmark in both Europe and the United States to qualify protection as “broad spectrum”.
It is measured on the same principle as UVA-PF: a product film on a PMMA plate, pre-irradiation, then a spectral reading. In the United States, the FDA broad spectrum test relies on this indicator alone, with its own protocol (different application rate and pre-irradiation dose from ISO 24443). The same product may therefore need to be tested twice if it targets both markets.
Which other methods complement UVA evaluation?
ISO 24443 is not the only option. Depending on the markets and the project stage, other methods provide complementary information.
- In vivo PPD method (ISO 24442): it measures UVA-induced persistent pigmentation on volunteers’ skin (phototypes II to IV), 2 to 4 hours after exposure. It remains the historical reference in Asia, where UVA-PF underpins the PA rating.
- UVA/UVB ratio: calculated from the in vitro absorbance spectrum, it compares mean absorbance in the UVA range with that in the UVB range. It is used in particular in the UK star rating system.
- Hybrid HDRS method (ISO 23698): published in late 2024, it combines reflectance measurements on skin with in vitro spectra. It can estimate both SPF and UVA protection without relying on an in vivo SPF for adjustment.
Summary of UVA evaluation methods
| Method | Type | Indicator obtained | Strengths | Limitations | Relevant markets |
|---|---|---|---|---|---|
| ISO 24443 | In vitro (PMMA plates) | UVA-PF, critical wavelength | Accounts for photostability, no volunteers, reproducible | Requires in vivo SPF for adjustment | EU, Australia, many markets |
| FDA broad spectrum test | In vitro (PMMA plates) | Critical wavelength | Direct requirement for the US market | Specific protocol, single indicator | United States |
| ISO 24442 (PPD) | In vivo | UVA-PF, PA rating | Measured on real skin | Volunteers exposed to UVA, cost, lead times | Japan, Korea, Asia |
| UVA/UVB ratio | In vitro | Absorbance ratio, stars | Simple to calculate | Relative indicator, limited use | United Kingdom |
| ISO 23698 (HDRS) | Hybrid | SPF and UVA protection | No erythema, independent of in vivo SPF | Specific equipment, limited availability | Adoption under way |
The choice of method therefore depends first on the target markets, then on the formulation type and development timeline. To find laboratories that run in vitro, in vivo or hybrid UVA tests, you can compare providers on Skinobs.
What are the limitations of in vitro UVA methods?
In vitro methods are reproducible, but their reliability depends heavily on how well the protocol is controlled. Several parameters affect the result:
- the roughness and quality of the PMMA plates, which must meet the standard’s specifications;
- product spreading, whose uniformity drives the measurement (hence the growing interest in robotic spreading);
- the formulation type: some formats, such as powders, sticks or very oily products, are hard to spread reproducibly;
- the accuracy of the in vivo SPF used for adjustment, which carries through to the final UVA-PF.
To limit these discrepancies, ask the laboratory how many plates and measurements it uses, the specifications of its plates and the results obtained on the reference product.
Results should also be read in context. A UVA-PF/SPF ratio close to the one-third threshold leaves little margin: a slight change in formula, raw material batch or filter supplier can tip the product below the requirement. Building in a safety margin at the development stage, and re-testing after any significant formulation change, avoids having to withdraw or relabel a product later. For products marketed in several regions, it is worth planning the full test package (ISO 24444, ISO 24443, FDA broad spectrum, PPD where required) from the outset on a single batch, so that results remain consistent across markets.
FAQ
Is in vitro UVA-PF accepted to support a UVA claim in Europe?
Yes. ISO 24443 is the reference method in Europe for showing that UVA-PF reaches at least one third of the SPF. Meeting this ratio allows use of the UVA logo recommended by Cosmetics Europe.
What is the difference between UVA-PF and critical wavelength?
UVA-PF measures the level of protection against UVA. Critical wavelength shows how far that protection extends across the spectrum. The two indicators are complementary and often required together.
Is an in vivo test needed for the PA rating in Asia?
The PA rating has historically relied on the in vivo PPD method (ISO 24442). Requirements are evolving from country to country, so check the rules in force in each market before launching tests.
Does ISO 24443 account for photostability?
Yes, through the pre-irradiation step. Filter photostability will be covered in a dedicated article in this series.
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