ISO 23675: A New Validated Standard for SPF Determination

June 2, 2025

zurko

ISO 23675: A New Validated Standard for SPF Determination

After over 10 years of testing sunscreen products, at Zurko Research we are proud to announce the incorporation of the new ISO 23675:2024 in vitro method into our technical service offering. This internationally validated in vitro approach represents a significant advancement in SPF determination and reinforces our continuous commitment to ethical, reproducible, and sustainable testing methods.

Why ISO 23675?

Prolonged exposure to ultraviolet radiation (UVR) is one of the main environmental causes of skin damage, making sunscreens essential tools in public health protection. Traditionally, SPF has been evaluated using the in vivo method described in ISO 24444, which requires exposing human volunteers to UV radiation.

ISO 23675 offers an ethically and technically sound alternative: a fully in vitro methodology that maintains a high level of correlation with the traditional in vivo method, while improving standardization, reducing costs, and shortening testing times.

Method Principle and Procedure

The method is based on UV transmittance spectrophotometry of a thin film of sunscreen product applied to synthetic plates. The measurement is performed before and after controlled UV irradiation that simulates real solar exposure conditions.

Key elements of the protocol include:

Use of two complementary types of PMMA plates (polymethyl methacrylate) to accommodate different formulations and ensure representative result

Automated product application using a robotic system (automatic spreading) that mimics manual spreading, ensuring uniformity and repeatability.

Controlled UV irradiation in compliance with ISO 24444 spectral requirements.

SPF calculation based on standardized formulas and strict quality control criteria defined by the standard.

Technical Scope and Compatible Formulations

The method is currently validated for:

Emulsions

Alcoholic one-phase formulations

Scientific Advantages of the New Method

Implementing ISO 23675 brings multiple benefits to sunscreen development:

Eliminates the need for human UV exposure, complying with replacement and reduction principles in test ethics.

Enhances inter-laboratory reproducibility by standardizing materials, application systems, and environmental conditions.

Enables year-round testing without relying on subject availability or skin condition.

Significantly reduces testing time and cost.

Fully complies with European cosmetic regulations, making it a valid alternative to ISO 24444 for SPF validation in the EU market.

Strategic Implications for Manufacturers

With the adoption of ISO 23675, Zurko Research expands its sun care study portfolio, offering:

A scientifically robust and validated alternative to the traditional in vivo SPF method.

The ability to assess products in development stages before proceeding to more costly and invasive trials.

An additional value proposition for ethical or responsible cosmetic marketing claims.

This method serves as an efficient tool for internal decision-making, pre-launch validation, filter innovation, and research on advanced sunscreen formulations.

ISO 23675: Answers to Key Questions

Is ISO 23675 accepted under European regulations?

Yes. ISO 23675 is an internationally recognized standard that meets the requirements of the European Cosmetics Regulation. It is accepted as a valid alternative to ISO 24444.

Can studies using this method already be requested?

Yes. At Zurko Research, we already have the technical infrastructure and trained staff to perform SPF studies following the ISO 23675 method. Brands can now request in vitro SPF evaluations aligned with this new methodology.

How do the results compare with the in vivo method?

Validation studies show strong correlation between the in vitro results of ISO 23675 and the in vivo results of ISO 24444, supporting their scientific equivalence.

What types of products can be tested?

Currently, emulsions and alcoholic one-phase formulations. Validation for other formats is expected in future updates.

What about sunscreens containing zinc oxide (ZnO)?

ISO 23675 has proven reliable for evaluating ZnO-based formulations. The method has been validated for this type of product, yielding consistent results with those obtained through the in vivo method.

Conclusion

ISO 23675 marks a turning point in the evaluation of sun protection: a scientifically robust method aligned with European regulatory requirements and tailored to the real-world needs of modern cosmetic development. Its adoption modernizes SPF testing while helping brands optimize resources, reduce timelines, and advance toward more ethical and reproducible innovation.

At Zurko Research, we offer ISO 23675 as a strategic tool to accelerate product development, enhance evaluation processes, and strengthen the market positioning of your sun care formulations.