ISO 24443 Cosmetics - In Vitro Determination of UVA Photoprotection of Sunscreens

The “ISO 24443 Cosmetics – In vitro determination of UVA photoprotection of sunscreens” standard, developed by the International Organization for Standardization (ISO), describes an in vitro procedure for characterizing the UVA protection of sunscreen products. This standard includes specifications to enable the reproducible determination of the spectral absorbance properties of UVA protection.

To determine the relevant UVA protection parameters, a method has been developed that provides a UV spectral absorbance curve upon which a series of calculations and evaluations can be performed.

These include the calculation of the Ultraviolet-A protection factor (UVA-PF) (persistent pigment darkening is correlated with in vivo UVA-PF obtained from the PPD test procedure), critical wavelength, and UVA absorbance proportionality. These calculations are optional and related to local sunscreen product labeling requirements. This method relies on the use of static in vivo SPF results to scale the UV absorbance curve.

This standard does not apply to powder products such as compressed powder and loose powder products.

In principle, the test is based on evaluating the UV transmittance of a thin film of sunscreen sample spread on a rough substrate before and after exposure to a controlled radiation dose from a defined UV exposure source.

Because of various variables that cannot be controlled with typical thin-film spectroscopic techniques, each set of sunscreen permeability data is mathematically adjusted so that the in vitro SPF data yield the same measured in vivo SPF value as determined by the in vivo test. Since the in vivo method may raise ethical concerns, any alternative SPF method published as an ISO method may be used.

Samples are exposed to a specific, measured dose of UV radiation to account for the photostability properties of the test product.

The obtained spectral absorbance data were shown to usefully represent both the breadth and height of the UVA protection properties of the tested sunscreen product. The mathematical modeling procedure was empirically derived to correlate with human in vivo (permanent pigment darkening) test results.

In the world of sun protection, consumers rely on labels such as SPF (sun protection factor) for UVB protection and, increasingly, UVA claims for broader-spectrum protection. While sun protection factor is determined through live (human) testing via standards such as ISO 24444, the assessment of UVA protection has traditionally involved methods such as permanent pigment darkening (PPD). However, ethical concerns and the need for repeatable, laboratory-based alternatives have led to the development of in vitro methods.

The ISO 24443 standard is the international standard for determining the UVA photoprotection of sunscreens in vitro (in a laboratory, without human subjects). This hybrid method combines spectrophotometric measurements with an adjustment based on the product's known in vivo SPF value, providing a reliable UVA protection factor (UVA-PF) that correlates well with in vivo permanent pigment darkening (PPD) results.

This standard is widely adopted, particularly in Europe and many other regions, for labeling claims such as the “UVA circle” logo (requiring at least 1/3 SPF UVA-PF). It is considered the gold standard for in vitro UVA testing and addresses photostability, a significant concern given the degradation of many UVA filters due to sun exposure.

In vitro UVA testing is important because UVA rays (320-400 nm) penetrate deeper than UVB, contributing to premature aging, wrinkles, and skin cancer, but they do not cause visible sunburn. Traditional in vivo permanent pigment darkening tests expose human skin to UVA and measure darkening, but they are time-consuming and raise ethical issues.

The ISO 24443 standard offers a repeatable and ethical alternative by measuring UV absorption through a thin film of sunscreen on a synthetic substrate before and after controlled UV exposure. This simulates real-world photostability and provides:

• UVA-PF: UVA protection level (e.g., UVA-PF 10 means 10 times more UVA exposure is needed for the same effect compared to unprotected skin).
• Critical wavelength: For broad spectrum evaluation (typically at least 370 nm is required).
• UVA absorption balance: Provides balanced protection.

This method is not applicable to powdered substances (e.g., loose or compressed powders).

This method uses specialized equipment to measure the UV transmittance of sunscreen applied to polymethyl methacrylate (PMMA) plates, a textured plastic that mimics skin texture. The test primarily follows these steps:

• Preparation and calibration: The spectrophotometer (290-400 nm range, in 1 nm steps) and UV solar simulator (xenon arc lamp with filter for realistic spectrum) are calibrated. The roughness and cleanliness of the PMMA plates are verified.
• Sunscreen application: Apply the product evenly at approximately 1,2-1,3 mg/cm2 (about 30 dots across the entire surface). Spread with a saturated finger (without gloves) using circular and linear motions for 30-60 seconds. Allow to dry in the dark at a controlled temperature (27-32 degrees) for 30-60 minutes.
• Pre-irradiation measurements: Absorbance spectrum is measured at multiple points (at least 12-16 points per plate for statistics, across 4 plates). Initial in vitro SPF and UVA protection level values ​​are calculated. The adjustment factor is determined to match the in vitro SPF with the product's in vivo SPF (ISO 24444 standard).
• UV exposure (photostability test): The plate is exposed to a calculated dose. This step is crucial because it reveals how much the protection is reduced under UV light.
• Post-irradiation measurements: The absorbance spectrum is measured again. The adjustment factor is redefined.
• Calculations: Performed according to the formula given in the standard. The final UVA-PF is the average obtained from multiple plates, along with confidence intervals. Other outputs are the UVA-PF/SPF ratio and the critical wavelength. Reference sunscreens (low protection S2 and high protection P8) are used for verification.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services compliant with the ISO 24443 standard.