SUN CARE In vitro SPF■ In vitro SPF (ISO 24443) ■ In vitro SPF (ISO 24443) ■
80 70 60 50 40 30 20 10 0
Formula 1 Figure 1: In vitro and in vivo SPF distribution for Formulation 1 and 2
when tested in the same laboratory under identical conditions. Since in vivo Although result deviations were
observed between the FDA guidelines and ISO 24444 due to differences in testing protocols, the in vivo SPF data were consolidated to support global compliance. Rather than focusing on label SPF claims, the studies prioritized analyzing SPF result distribution to confirm consistency and reliability across formulations.
Comparison of in vivo and in vitro SPF results Testing was conducted across multiple independent laboratories using both in vivo and in vitro testing utilized ISO 24443 and ISO 23675 (DPM).
Findings from in vivo testing ■ SPF values ranged between 40 and 69, with an average SPF of 53.9 for Formulation 1 and 49.6 for Formulation 2 ■ Despite differences in formulation, the in vivo SPF values consistently remained around 50 at the same ZnO concentration, as shown in Table 3. This reinforces the reproducibility of the results, even with the natural variability associated with human testing ■ SPF variability was similar across both formulations, with standard deviations of 8.64 and 8.56, respectively ■ The SPF results following the FDA protocol tend to yield higher SPF values compared to ISO 24444:2019
Findings from in vitro testing ■ ISO 24443 testing produced significantly lower SPF values (median 23.4), covering only 45.8–52.5% of in vivo SPF results. ■ DPM testing yielded even lower SPF values (median 11.9 for Formulation 1 and 16.6 for Formulation 2), covering only 23.3–37.2% of in vivo SPF results. ■ Formulation 2 showed stronger correlation between in vitro and in vivo SPF values, indicating better consistency.
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TABLE 3. ADDITIONAL IN VIVO SPF SUMMARY FOR COATED DISPERSION USED IN FORMULATION 2
Testing Lab
Testing Lab 1
In vivo Method
Formula Type
FDA 2021 Makeup stick FDA 2021 W/Si sunscreen
ISO 24444: 2019
W/Si sunscreen In vivo
SPF result 50
52.9 46 Based on Figure 1, the in vivo SPF values for
both formulas remain consistently around SPF 50, yet exhibit significant variation. This wide variability reflects the inherent inconsistencies of in vivo testing, influenced by biological responses among human subjects and variations in testing conditions between ISO 24444:2019 and FDA guidelines across multiple laboratories in different geographic locations. The relatively large box in the plot further
illustrates that, even under controlled conditions, in vivo SPF values can fluctuate significantly. Overall, the SPF distribution leans toward a positive skew, indicating that while most of the recorded in vivo SPF values are close to the median, some exhibited substantially higher protection levels. Such fluctuations are common in human testing due to variations in skin type, sunscreen application techniques, and individual biological responses to UV exposure. On the contrary, in vitro SPF results obtained using the ISO 24443 method show notably lower variation compared to in vivo testing. The narrow interquartile range (IQR) indicates high precision and reproducibility. However, the median in vitro SPF value
(23.4) is less than half of the median in vivo SPF (51.1 for Formulation 1 and 44.6 for Formula 2), accounting for only 45.8% and 52.5% of their respective in vivo SPF values. These results suggest that the ISO 24443 method may significantly underestimate the sunscreen’s actual protective ability when compared to in vivo evaluation.
Among all the methods tested, the DPM November 2025 PERSONAL CARE Formula 2
89
SPF
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