88 SUN CARE
TABLE 1A: IN VITRO (24443) AND IN VIVO SPF SUMMARY FOR FORMULATION 1 In vivo
Testing Lab
Testing Lab 1
Testing Lab 2
Testing Lab 3
Testing Lab 4
In vivo Method FDA 2021
FDA 2021
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 FDA 2021 FDA 2021 FDA 2021
Average SPF
Standard deviation Margin of error
In vitro
SPF result 52.2
66.1
50.0 52.2 50.0
44.6 50.0 44.6 44.6 58.0 67.0 67.0 53.9 8.64 5.49
SPF result 24.4
23.4 23.4
23.4 In vitro Method
ISO 24443:2021 ISO 24443:2021 ISO 24443:2021
ISO 24443:2021
Testing Lab
Testing Lab 5
TABLE 1B: IN VITRO (24443) AND IN VIVO SPF SUMMARY FOR FORMULATION 1 In vivo
Testing Lab
Testing Lab 1
Testing Lab 2
Testing Lab 3
Testing Lab 4
23.7 Average SPF
0.50 Standard deviation 0.80 Margin of error
even approved for SPF claims in some regions, such as Thailand. However, these methods were also initially
developed for formulations containing organic UV filters. The optimization and validation on mineral-only sunscreen have not been adequately explored to date. Significant deviations between the in vivo standard indicate a gap in reliability of results if adopted.
The double plate method (DPM): a new approach to in vitro SPF testing To address the aforementioned shortcomings of current in vitro SPF tests, the ISO 23675 standard introduces the Double Plate Method (DPM). This method involves applying sunscreen between two PMMA plates with different surface textures: ■ A sandblasted plate, which replicates the rough texture of human skin better ■ A moulded plate, which mimics the natural undulations of the epidermis This dual-plate configuration aims
to improve the accuracy of in vitro SPF measurements by better simulating how sunscreen interacts with human skin. Spectrophotometric techniques are then used to measure UV radiation passing through the film.5
While early results indicate better
Testing Lab
Testing Lab 1
Testing Lab 2
Testing Lab 3
In vivo Method FDA 2021
FDA 2021
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 FDA 2021 FDA 2021 FDA 2021 FDA 2021
Average SPF
Standard deviation Margin of error
In vitro
23.4 23.4 23.4
SPF result 69.0
43.5 44.6 44.6 39.9 44.6 44.6 50.0 50.0 57.5 57.5 49.6 8.56 5.75
SPF result 24.4
In vivo Method FDA 2021
FDA 2021
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 FDA 2021 FDA 2021 FDA 2021
Average SPF
Standard deviation Margin of error
reproducibility with most organic, hybrid, and certain mineral formulations,7
variability and provides a more consistent alternative compared to older in vitro testing methods. One of the primary challenges in in vitro
SPF testing for ZnO-based sunscreens is the difference in application thickness. In vivo SPF tests require 2 mg/cm2
vitro methods (both 24443 and DPM method) typically use only 1.2–1.3 mg/cm2
of product, whereas in .1
Due to the higher density and physical
characteristics of ZnO formulations, the resulting sunscreen film on PMMA plates is often thinner compared to the actual spreadability on human skin, leading to an underestimation of SPF values.1
This key
concern will drive the industry's efforts to refine and tailor SPF measurement methods for mineral sunscreens.
Investigating ZnO-based sunscreen formulations This study examines two water-in-oil (W/O) ZnO-based sunscreen formulations, each containing 24.5% ZnO without any SPF boosters. The ZnO concentration was strategically set close to the regulatory
TABLE 2A: IN VITRO (24443) AND IN VIVO SPF SUMMARY FOR FORMULATION 2 In vivo
In vitro Method
ISO 24443:2021 ISO 24443:2021 ISO 24443:2021 ISO 24443:2021
Testing Lab
Testing Lab 4
Testing Lab
Testing Lab 1
Testing Lab 2
Testing Lab 3
23.7 Average SPF
0.43 Standard deviation 0.54 Margin of error
PERSONAL CARE November 2025
In vivo Method FDA 2021
FDA 2021
ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 ISO 24444:2019 FDA 2021 FDA 2021 FDA 2021 FDA 2021
Average SPF
Standard deviation Margin of error
the DPM method
still tends to underestimate SPF values for high- ZnO formulations.1
However, its design reduces In vitro
SPF result 52.2
66.1
50.0 52.2 50.0
44.6 50.0 44.6 44.6 58.0 67.0 67.0 53.9 8.27 5.25
SPF result 11.8
11.9 11.9
In vitro Method
ISO 23675:2024 ISO 23675:2024
Testing Lab
ISO 23675:2024 Testing Lab 6
11.9
Average SPF
0.05 Standard deviation 0.80 Margin of error
maximum, allowing a small margin for analytical deviations. ■ Formulation 1: Uncoated ZnO dispersion G-Block DZ 370 CCT ■ Formulation 2: Triethoxycaprylylsilane- coated ZnO dispersion (S-Block CocoDerm) Both ZnO dispersions are stable, anhydrous
systems with a three-year shelf life, ensuring even particle distribution to optimize UV protection efficacy. Additionally, both formulations are designed to achieve close to 100% naturality (>99.5%) in alignment with current market trends. While the emulsifier and stabilizers remain the same, some adjustments were made to ensure formula stability. The samples have successfully undergone three freeze/thaw cycles and an accelerated ageing stability test at 50°C for two months. A W/O emulsion system was selected to
enhance ZnO film formation on the skin, as ZnO disperses evenly in the continuous phase, maximizing UV protection performance. The formulations were carefully designed to provide good spreadability on the skin, which is a critical factor for in vivo SPF testing. Spreadability directly influences in vivo SPF
values, as formulations with poor spreadability lead to uneven film formation on the skin, resulting in significantly lower SPF values, even
TABLE 2B: IN VITRO (DPM) AND IN VIVO SPF SUMMARY FOR FORMULATION 2 In vivo
In vitro
SPF result 69.0
43.5 44.6 44.6 39.9 44.6 44.6 50.0 50.0 57.5 57.5 49.6 8.56 5.75
SPF result 16.2
16.6 17.2
In vitro Method
ISO 23675:2024 ISO 23675:2024 ISO 23675:2024
Testing Lab
Testing Lab 5
16.7
Average SPF
0.50 Standard deviation 1.25 Margin of error
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