What You Need to Know About Mineral UV Filters

"Zinc About It!" What You Need to Know About Mineral UV Filters As consumers become more aware of the harmful effects of UV radiation on skin, the market for cosmetic products containing sun protection filters continues to grow. Our skin is our first protective barrier against the environment. It is essential for our health and forms a barrier that protects us from physical and biological factors. The greatest physical threat is UV radiation from the sun, which is considered an important factor in the development of skin cancer [1]. UVB rays are absorbed by the epidermis and are mainly associated with sunburn. UVA rays that reach the skin pass through the epidermis and penetrate to a deeper layer (dermis), and are responsible for skin aging. Increasingly, more consumers are beginning to see skin aging and skin cancer as potential damage caused by UV light. They are becoming aware of the need for sun protection as part of daily habits, in addition to summer months and vacation routines. However, at the same time, they are also thinking about the reliability issues of UV filters. The growing demand for safe, environmentally friendly and effective UV filters that have no negative effects on the environment and human health is putting pressure on traditional organic UV filters. Competent authorities increasingly demand more data to prove their safety for the environment and human health. As sunscreen products containing mineral UV filters titanium dioxide (TiO2) and zinc oxide (ZnO) enter the market at an accelerated pace, more cosmetic brands are following this trend, driven by growing consumer interest in mineral sun filters. Claims such as "reef safe," "ocean friendly," and "planet friendly" are emerging on sunscreens. This trend is intensified by certain local regulations: Some countries have enacted laws banning the sale, offering, and distribution of sunscreens containing oxybenzone and octinoxate chemicals to help protect marine ecosystems and coral reefs: Palau (Western Pacific), US Virgin Islands, Bonaire, and Aruba, and Hawaii and Maui County adopted legislation (Bill 135) banning non-mineral filtered sunscreens and permitting only mineral filtered sunscreens from October 2022 onwards. Additionally, there are restrictions on innovation due to strict regulations: In the US, no new UV filter has been approved in 20 years, and although the EU has a wider range of UV filters, regulatory agencies also block the entry of new UV active components into the US. The global mineral sunscreen market is recording a CAGR of more than 5% between 2020 and 2030 [3]. In 2018, 45.1% of sunscreen launches contained mineral UV filters, this rate increased to 51.6% in 2022 (Figure 1). In the US, Google searches for "mineral sunscreen" reached +36.4% year-over-year growth [4]. The currently preferred mineral UV filter is ZnO.

What You Need to Know About Mineral UV Filters: Basic Information

Mineral UV filters such as TiO2 and ZnO are particulate in structure. They are composed of primary particles, clusters of primary particles, or agglomerates formed by clusters. Powder forms available on the market are agglomerated and typically have a particle size between 1-5 μm. Unlike organic UV filters that provide UV protection through absorption, mineral UV filters reduce UV light through both absorption and scattering. TiO2 weakens UV light primarily through absorption from 290 to 350 nm, primarily through scattering above 375 nm, and through both mechanisms between 350 and 375 nm [5,6]. ZnO weakens UV light through absorption from 290 to 380 nm and mostly through scattering from 380 to 400 nm [6]. TiO2 found in sunscreens is typically rutile with a refractive index of 2.76. ZnO has a lower refractive index of around 1.99. Due to its low refractive index, ZnO in skincare and beach products can be applied to the skin and has an acceptable transparency for most consumers (even among those with deeper and richer skin tones). This is different from TiO2, which typically leaves a whitish or bluish residue on the skin. Colored cosmetics and BB creams can more easily tolerate the use of TiO2 as a mineral UV filter, since some opacity of the formulation is generally desired. Iron oxides typically included in the formulation can reduce the bluish residue. It is crucial to understand the relationship between the particle size of a mineral UV filter in a sunscreen and UV protection efficacy. Therefore, the particle size of a mineral UV filter must be optimized for sunlight. TiO2 reduces UVB light (290-320 nm) primarily through absorption when the primary particle size is small (10-20 nm). Due to its small size, it has poor light scattering properties for UVA light, resulting in low UVA protection. TiO2 with medium particle size (35-70 nm) will effectively absorb light up to approximately 350 nm in UVA and all UVB light. This type of TiO2 will strongly scatter UV light beyond 350 nm and thus provide strong UVA protection. Therefore, it is possible to achieve broad-spectrum protection and high SPF by using only one type of this TiO2. However, the disadvantage of TiO2 of this size is the white residue. Therefore, formulations of beaches or face products made with only TiO2 are usually colored with iron oxides to serve customers with different skin tones. Dispersion technology can reduce, but not eliminate, the white residue caused by TiO2's high refractive index. As previously mentioned, ZnO absorbs UV light beyond 370 nm to 380 nm. Therefore, it is an ideal sun protection active ingredient to formulate a product that will provide broad-spectrum protection. Like TiO2, ZnO's UV protection shifts to shorter wavelengths as the primary particle size decreases. Notably, the use of ZnO with primary particle sizes ranging from 20 to 250 nm enables formulations with critical wavelengths exceeding 365 nm to 380 nm.   A Deeper Look at Particle Size A competent formulator has the ability to manipulate the particle size of mineral UV filters to achieve the desired optical properties. TiO2 and ZnO are typically coated to be hydrophobic before use. According to EU nano regulations, the types of coatings that can be used on nano TiO2 and ZnO are limited. Typical hydrophobic coatings include triethoxyoctyldimethylsilane for ZnO and dimethicone; for TiO2, dimethicone and aluminum stearate, the latter of natural origin. Hydrophobic coating increases the ability of TiO2 and ZnO powders to disperse in oils. In general, dispersed forms of coated TiO2 and ZnO are recommended for formulation, as the use of dispersions ensures that particle size remains constant from the laboratory through to industrial production and in repeated production runs. Additionally, by varying surface treatment, carrier, powder concentration, and grinding process parameters, the particle size of TiO2 or ZnO can be adjusted to obtain UV protection at the desired wavelengths. When working with powders, the formulator must have strong high-shear grinding production equipment and the ability to control the particle size obtained after dispersion. TiO2 or ZnO particle size in dispersion is typically measured using a light scattering particle size measurement instrument. Generally for ZnO, a primary size of approximately 35 nm or at least a dispersion particle size of 170 nm is effective to meet most UVA protection standards. However, when ZnO concentration in the formulation is 20% to 25%, smaller particle sizes may also be effective to achieve a critical wavelength larger than 370 nm. To achieve a very high UVA/UVB ratio or critical wavelength, larger-sized ZnO, particularly the non-nano version with particles between 250-500 nm, is extremely useful. A critical wavelength greater than 380 nm can be achieved using non-nano ZnO. ZnO is less efficient in UVB protection compared to TiO2, meaning lower SPF is achieved relative to the percentage of active weight used. The typical SPF range for ZnO and TiO2 is 1-2 and 2-3 SPF units per active weight, respectively. However, ZnO provides a higher in-vivo PA/SPF ratio that reaches its maximum when the primary particle size is in the 40-60 nm range. The PA score of 20 nm ZnO can be kept reasonably high by avoiding excessive size reduction in the grinding of its dispersion.  

How to Select the Right Mineral UV Filters for Your Formulation?

The formulator must decide between powder and dispersion. The regulatory requirements of the markets where the product will be sold should also be considered. To delve deeper, it is important to consider what the formulator wants to claim, how the regulatory and safety departments view the matter, and their opinions regarding the claims. The next consideration is the naturality of the formulation or the necessity to avoid specific ingredients. Additionally, requirements in terms of target SPF and UVA protection will determine particle size selection and whether zinc oxide should be used alone, in combination with TiO2, or with other filters. Finally, in the case of dispersion, the carrier is important. Different emollient oils with different spreading properties, different skin feel, and different volatilities can be used as carriers in dispersions. If high levels of sun protection active ingredients must be used in the formulation, which is a requirement to achieve high SPF; it is appropriate that the dispersions used are also of high active content. Additionally, comprehensive sun care is needed today, and many customers no longer accept sunscreens that leave a white mark on the skin. As previously mentioned, ZnO is much more transparent than TiO2. ZnO formulations with SPF higher than 30 can appear natural and transparent on Fitzpatrick skin types four to six. For those with the deepest and richest skin tone, the addition of transparent iron oxides will neutralize the slight milky appearance from high levels of zinc oxide. In short, mineral UV filters can be incorporated into sun care formulations that meet the needs of all skin tones.  

Regulations for Mineral UV Filters

Sunscreens are regulated as over-the-counter drugs in the US. TiO2 and ZnO are listed by the FDA as sun protection active ingredients in the category of generally recognized as safe and effective (GRASE). Both the United States and the EU permit the use of TiO2 and ZnO as sun protection active ingredients at up to 25% (Table 1). Although regulations vary between regions, sun care products must provide protection against both UVA and UVB radiation to claim broad-spectrum protection. The FDA requires 15 SPF and a critical wavelength of at least 370 nm to claim broad-spectrum protection. Critical wavelength means the wavelength equal to the area under the absorption curve representing 90% of the area between 290 and 400 nm. Over the past few years, the FDA has become increasingly concerned about the fact that excessive exposure to long-wavelength UVA can cause skin cancer in addition to skin aging and allergies. For this reason, the FDA proposed the addition of a stricter requirement: a UVA-I/UV ratio of 0.7 or higher. This ratio is the ratio of average absorbance in UVA-I (340-400 nm) to total UV (290-400 nm). European sunscreens must have balanced protection: a UVA protection factor (in-vitro or in-vivo) equal to one-third of the in-vivo SPF and a critical wavelength of at least 370 nm. Broad-spectrum UV protection requires that if your formulation's in-vivo PA (UVA) measurement is 10, the label claim for SPF may not exceed SPF 30. On the current market, there are SPF 30 and 50 sunscreen formulations claiming ZnO as the sole sun protection active ingredient (Table 2). These formulations typically contain antioxidants, anti-inflammatory agents, irritation preventers, film formers, or boosters similar to UV-absorbing emollients such as butyl octyl salicylate to provide high protection. France has mandatory annual declarations, including formula registration and notification as well as labeling of finished products, and has particularly detailed regulations throughout the EU regarding the use of nano materials. European Union regulators generally define the particle size of a nano material according to its internal structure, and consequently any degree of mineral UV filters, if determined by measuring the shortest dimension, falls within the nano size range. At least 50% of the particles have a particle size below 100 nm according to number-weighted size distribution. For products intended for the EU market, very specific criteria for nano materials must be observed, including a very restricted list of allowed surface treatments. The formulator must decide how to measure and determine nano materials, as there are two interpretations: based on primary particle size or, as interpreted by Cosmetics Europe, based on aggregate size. The use of TiO2 and ZnO also has certain restrictions due to their toxicity profiles. TiO2 is considered a carcinogen by the EU Commission and the State of California when used as airborne, respirable-sized unbound particles [7,8]. As a result, nano TiO2 cannot be used in products that could cause exposure of the end-user's lungs to inhalation. If it is airborne and respirable, no TiO2, nano or non-nano, can be used in California. As for ZnO, SCCS found it to have local effects in human lungs. Therefore, neither nano nor non-nano ZnO is permitted in a product that could cause exposure of the end-user's lungs through inhalation [9]. In practice, a respirable particle is defined as a particle with an aerodynamic particle size below 10 μm. Fortunately, most TiO2 and ZnO powders have aerodynamic particle sizes above 10 μm due to aggregation and/or agglomeration. Nevertheless, if the formulation is in powder or atomized spray form, testing of aerodynamic particle sizes is strongly recommended.  

From Consumer Desires to Personal Care Formulations

In 2021, more than half of consumers worldwide saw themselves as regular sunscreen users. However, only 22% use sunscreen daily, and 37% use sunscreen when spending time outdoors regardless of planned activities for the day. The first figure is subject to significant regional variations. In Europe, fewer than 10% of consumers protect their skin from UV radiation daily [2]. Consumer expectations and the changing attitudes of cosmetic manufacturers toward sustainable beauty products are also affecting sun protection products. The development of a new sunscreen product requires the consideration of various aspects. In addition to ongoing awareness of the health risks of UVA and UVB light, a pleasant skin feel during and after use is one of the most important factors increasing sunscreen use frequency. Consumer appreciation is very important for a successful sunscreen product. A satisfying experience encourages the consumer to use the right amount of product to obtain the desired protection, reapply the product, and ultimately likely to consider repurchasing the product. In addition to focusing on environmental dimensions and sustainability, one of the most important parameters for high consumer appreciation of a product is sensory performance or, in short, skin feel. "Skin protectors" such as multifunctional products: hybrid sunscreens with a skincare-focused approach (components, claims, formulations) may be a solution to convince consumers not yet using mineral UV protection products (Figure 2). Surprisingly, awareness of mineral filters is still low among the general population today. However, there is growing interest in natural, skin-friendly, and environmentally friendly sunscreens, which will lead to an increase in mineral sunscreen products. Existing users have a positive perception regarding protective effect, care effect, and soft ingredients and tend to rate that white spots are not bothersome when applying mineral sunscreens. Non-users are thinking of buying mineral sunscreens in the future, but the main obstacle so far appears to be product properties: difficult spreadability, whitening, and rich skin feel parameters. To convince non-users, consumers need to be better educated about mineral UV filters and formulations with perfect skin feel and minimal whitening effect. Since positive effects such as suitability for all skin types, including sensitive skin, and greater environmental compatibility outweigh negative effects, it is likely that consumers will demand mineral sunscreens more frequently in the future.     References 1) Skin Cancer Foundation, August 2021. www.skincancer.org/risk-factors/uv-radiation/ 2) Mintel report: A year of innovation in suncare, 2021 3) Future Market Insights, July, 2020. https://www.futuremarketinsights.com/reports/ mineral-sunscreen-market 4) Google Search data, average monthly search volume and year-over-year growth comparing the past 12 months ending August 2021 vs the 12 months prior (US) 5) P. Stamatakis and B. R. Palmer, J. Coating Tech., Vol. 62, No. 789, p95, October, 1990 6) M. Sakamoto, H. Okuda, H. Futamata, A. Sakai and M. Iida, J. Jpn. Soc. Mater. (Shikizai), 68 (4), P 203- 210, 1995. 7) COMMISSION REGULATION (EU) 2016/1143 of 13 July 2016 amending Annex VI to Regulation (EC) 8) No 1223/2009 https://oehha.ca.gov/proposition-65/chemicals/titanium-dioxide-airborne-unbound-particles-respirable-size 9) COMMISSION REGULATION (EU) 2016/621 of 21 April 2016, amending Annex VI to Regulation (EC) No 1223/2009 10) Mintel analysis, May 2022 where Sub-Category matches Sun - Sun/Sunbed Exposure 11) COMMISSION RECOMMENDATION of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto   Authors Yun Shao, David Schlossman; Marek Busch, Ev Suess Kobo Products Inc.; Symrise AG   Translator Özgür Çelen - Country Sales Manager Turkey Symrise AG