SUN CARE
Bridging the in vivo and in vitro testing gap
Yung Chan - Solabia
Mineral sunscreens, particularly those formulated with zinc oxide (ZnO), are well recognized for their broad-spectrum ultraviolet (UV) protection and favorable safety profile. Among UV filter ingredients, ZnO and titanium dioxide (TiO2
) are the only two recognized as
generally safe and effective (GRASE) by the U.S. Food and Drug Administration (FDA). In addition to UV protection, they provide benefits such as anti-inflammatory and antimicrobial properties, making them particularly suitable for sensitive or acne-prone skin. With growing consumer demand for natural,
sensitive skin–friendly sun care products and regulatory restrictions on certain organic UV filters in some markets, formulations containing only ZnO or a combination of TiO2
and ZnO are
becoming increasingly prevalent. Compared to TiO2
-based sunscreens, ZnO formulations
provide more comprehensive UV coverage and better transparency across diverse skin tones. However, despite these advantages,
determining the sun protection factor (SPF) of ZnO-based formulations presents a significant challenge, particularly due to discrepancies between in vivo and in vitro SPF measurements. It is well-established that ZnO-based
sunscreens consistently exhibit lower SPF values in in vitro assessments compared to in vivo methods. This underestimation becomes more pronounced with increasing ZnO concentrations.1 Such inconsistency has raised concerns within the industry, regarding the accuracy and reliability of in vitro SPF testing methodologies, particularly for formulations without organic UV filters.
In vivo SPF testing: the gold standard SPF is a key metric that quantifies the effectiveness of sunscreen in preventing UVB-induced erythema. In vivo SPF testing, as outlined in ISO 24444 and FDA guidelines, involves controlled UV exposure on human subjects to measure the level of erythema reduction provided by a sunscreen product.2,3 The SPF value is determined by comparing the minimum UV dose required to cause erythema on protected skin versus unprotected skin. However, since 2019, the definition of minimal erythemal dose (MED) has changed between the FDA and ISO 24444 guidelines. According to the FDA, MED is defined as the lowest UV exposure that produces the first
www.personalcaremagazine.com
perceptible, unambiguous erythema with well- defined borders.2 In contrast, ISO 24444:2019 defines the MED as the first clearly visible redness with distinct borders appearing over more than 50% of the UV-exposed test area. The standard also provides predicted MED ranges for different skin tone categories, whereas the FDA guideline does not differentiate MED ranges in its sun protection evaluation.3 This protocol difference has increased variability in in vivo SPF results, posing challenges for global brands, especially those developing high-SPF formulations.3 Additionally, in SPF testing protocols,
standardized formulations known as reference standards are used to ensure accuracy and consistency. Each standard corresponds to a specific mean SPF value: P2 represents SPF 16, P5 corresponds to SPF 30, P6 aligns with SPF 43, and P8 equates to SPF 63. These reference standards are based
on organic UV filter oil-in-water (O/W) formulations. Since 2010, the FDA's sunscreen testing protocol has exclusively utilized the P2 standard. Similarly, ISO 24444:2019 employs P2 and has introduced additional standards—P5, P6, and P8—for SPF ≥25.2,3 Since ZnO-based formulations exhibit
significantly different film formation and UV absorption mechanisms, these organic-based SPF reference standards may not be entirely suitable for mineral sunscreens. This mismatch further contributes to SPF variability, raising
concerns about the reliability of SPF claims for ZnO-only formulations. Several additional factors influence in vivo
SPF variability, including: ■ Differences in skin type among test subjects ■ Impact of natural skin creases on sunscreen film distribution ■ Application thickness inconsistencies ■ Variability in solar simulator light output ■ Mismatch between SPF results from standardized indoor testing and real-world performance under natural sunlight6 Despite these challenges, in vivo testing
remains the gold standard for sunscreen evaluation. It is currently the most accurate method for assessing real-world sunscreen performance, making it the primary benchmark against which all alternative SPF assessment methods are compared.
In vitro SPF testing: a cost-effective alternative To address ethical, cost, and statistical challenges associated with in vivo SPF testing, in vitro methodologies have been developed. These tests assess UV transmission through a sunscreen film applied on artificial substrates, such as poly(methyl methacrylate) (PMMA) plates.
Among the established in vitro
methodologies, ISO 24443 is widely used for SPF prediction although it was originally developed for evaluating UVA protection.4
It is November 2025 PERSONAL CARE
87
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107
