It has been well known to the general public for a very long time that excessive exposure to ultraviolet light during daily activity can cause skin damage such as sunburn and skin cancer.
The primary cause of such damage is UVB light. Therefore, organic and inorganic sunscreens have been used for decades to protect people against UVB radiation. In recent years, scientists have found that UVA light can cause premature skin ageing, photo-allergies and possible skin cancer. As a result, UVA protection has gained much attention from regulatory bodies as well as from consumers. In the latest amendment of a sunscreen monograph published by the FDA in 2007, UVA testing and rating were incorporated. In September 2006, the EU Commission recommended that the PFA should be more than 1/3 of SPF value in order to provide complete UV protection. Therefore, it is essential for a future sun care product to provide adequate UVA protection while providing UVB protection in order to satisfy the market need. Inorganic UV filters, titanium dioxide (TiO2) and zinc oxide (ZnO), are widely used because they are inert and nonirritating to sensitive skin. They cannot be absorbed by the skin because they are particulates. Many grades are commercially available and studies have shown that their UV attenuation characteristics are a function of their particle size.1-3 However, practical guidance for their use to achieve high and yet balanced UVA/B protection is still not clear.
Objectives
• To investigate the attenuation behavior of TiO2 and ZnO in UVA and UVB regions.
• To develop an combination of only inorganic UV filters that can provide a SPF of 30+ and a SPF/PFA ratio of 3 or less.
Experiments
• TiO2 and ZnO (various sizes) were dispersed in cyclopetailoxane or isononyl isononanoate and milled with bead mill to achieve optimal transparency or desired particle size.
• SPF was tested according to the FDA’s method and PFA according to the JCIA method (PPD). The panel consisted of three people.
• Primary particle sizes were measured by image analysis and reported by the supplier. The particle sizes in dispersion were measured using a NiCOMp C370 Photo-correlation size analyser. Intensity-weighted mean size was used.
• UV transmittance curves were measured using a Hitachi U-3010 spectrophotometer. Samples were prepared by diluting the dispersions in chloroform to 0.001% for TiO2 and 0.005% for ZnO.
Results and discussion
Evaluation of TiO2 dispersions The UV absorption curves in Figure 1 indicate that TiO2 attenuation is very effective in UVB but not so much in UVA when size is very small. As size increases, UVB attenuation weakens but UVA attenuation increases. This is due to the fact that TiO2 is a primary UVB absorber and UVA scattering agent.1,2 It can scatter UVA effectively only when its particle size (aggregate size) is about half of wavelength of UVA light. For example, the most effective particle size for scattering a light of 360 nm is about 180 nm. But when the particle size became larger as the primary particle size increased, attenuation of visible light became strong, leading to unacceptable whitening (Fig. 2). For sunscreen lotion and skin care products, a primary particle size of 10 nm to 30 nm, preferably, 10 nm to 20 nm, should be used. Primary particle size of 30 nm or higher can be used in colour cosmetics when certain opacity is needed anyway.
In vivo study of TiO2 performance The in vivo SPF and PFA testing results in Table 1 indicate:
• Small primary particle size (10 nm to 15 nm) and aggregate size are not only required for high transparency, but also good for high SPF. In general, a smaller particle size results in higher SPF score.
• As the size becomes smaller, TiO2 becomes less effective in UVA protection. The over protection is heavily skewed to UVB and is far from the 3/1 ratio that EU Commission has recommended.
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Based on these results, it is clear that TiO2 alone cannot provide a high and yet balanced UV protection with an acceptable transparency. To achieve this, UVA sunscreen agents need to be used in combination.
Evaluation of ZnO dispersions The absorption curve of ZnO in the UV region is fairly flat (or uniform) and the shape does not change much as the size changes (Fig. 3). What is worth noting is that as the size becomes smaller, the absorption becomes stronger, but the onset of absorption shifts to a shorter wavelength, meaning the stronger absorption in a narrower range. The attenuation of UVA/B by ZnO is primary due to its absorption. The size reduction has some effects on its scattering of UVA-II light but the effect on overall UV attenuation is smaller compared with that of absorption.
In vivo study of ZnO performance ZnO has a much lower refractive index than TiO2. A wide range of particle sizes can be used without causing noticeable whitening (Fig. 4). The in vivo test results show:
• ZnO is a much less effective UVB sunscreen when compared with TiO2. However, ZnO alone can provide balanced UV protection provided that SPF is moderate (<20).
• When its size becomes very small, ZnO becomes very effective in providing SPF but its protection against UVA decreases and it will lose its balance in UVA/B protection. For high SPF (30+) sunscreen lotions, ZnO alone cannot provide balanced UV protection unless being used at a very high level (close to 25%).
Formulation for high and balanced UV protection From the in vivo studies, it is clear that neither TiO2 nor ZnO alone can provide a sunscreen lotion with high transparency, high SPF and a SPF/PFA ratio of 3 or less. To achieve this goal, the following combinations were tried: combination of TiO2 and ZnO, and combination of two grades of ZnO that have different dispersion particle sizes.
Combination of TiO2 and ZnO TiO2 dispersion with a mean particle size in the range of 110 nm to 130 nm is very effective in UVB and was chosen to provide SPF. ZnO dispersions with large mean size were used to provide adequate UVA protection. The results in Table 3 show that the right combination could result in a high and yet balanced UV protection. It was easier to achieve a SPF/PFA ratio of <3 with larger size TiO2 (35 nm), but whitening became noticeable (Fig. 5).
Combination of two grades of ZnO that have different dispersion particle sizes To provide adequate SPF, one of two grades of ZnO should have smallest size possible and be used at a high level. The UVA protection of this fine size ZnO is poor. Therefore, the second ZnO needs to have a very large size in order to make up the deficiency in PFA. The goal was achieved in formula 4A in Table 4. Although the total active content in this series is much higher than that in case with TiO2/ZnO combination, the sunscreen lotions made showed very good transparency and little ashy colour even on the dark skin. The test results for 4B ad 4C reveals that careful consideration of size and ratio needs to be made or it is easy to miss the SPF or PFA targeted score.
Conclusions
TiO2 is effective for UVB protection. It cannot provide enough UVA protection without sacrificing transparency. ZnO in general is not an efficient UVB agent and is not suitable for high SPF products. Therefore, neither can provide high transparency, SPF 30+ and a SPF/PFA ratio of 3 or less. By combining TiO2 optimised for UVB protection and ZnO optimised for UVA protection, such a goal was achieved. Combination of very fine ZnO and large ZnO with controlled size and the right ratio was also able to provide high and balanced UV protection.
References
1 Stamatakis et al. Optimum Particle Size of Titanium Dioxide and Zinc Oxide For Attenuation of Ultraviolet Radiation. J Coating Tech 62 (789), p95 (1990). 2 Sakamoto et al. J Jpn Soc Mater (Shikizai) 68 (4), 203-210 (1995). 3 Shao Y., Schlossman D. Effect of Particle Size on Performance of Physical Sunscreen Formulas, PCIA, Shanghai (2001). 4 Innes et al. Nanotechnology and the Cosmetic Chemist. Technical Bulletin, Advanced Nano Technologies Pty Ltd.
