Formulating with UV filters is not easy and there are numerous factors to consider when using them. There are two main types of UV filters; organic filters, also known as ‘chemical’ filters, and inorganic or ‘physical’ filters. Recently, there has been a slight move away from the use of organic filters, which absorb UV radiation, as they can degrade and can cause allergic reactions. Inorganic filters such as nano titanium  dioxide and nano zinc oxide, which reflect UV radiation, are more stable and therefore are becoming more widely used.


Avobenzone (butyl methoxydibenzoyl methane) is the most commonly used organic UVA filter, but has low stability and is susceptible to crystallisation. UVBabsorbing filters such as octocrylene and oxybenzone, or even some antioxidants, such as quercetin, can help to stabilise avobenzone. However other UVB filters contribute to its degradation. Ethylhexyl methoxycinnamate (OMC) is a widely used UVB organic filter as it provides good protection, has a light skin-feel, and is low in colour and odour. While OMC has its positives, it also destabilises avobenzone and so it is not recommended to use these together, as it can result in formulations that are not photostable. This stabilising/destabilising action comes from the fact that when avobenzone absorbs a photon of UV light, it is excited to a triplet energy state. While stabilising substances such as octocrylene help quench avobenzone to return it to its ground energy state, ethylhexyl methoxycinnamate reacts with high energy avobenzone to form products that do not function as UV filters.8 


It has also been shown that avobenzone degrades when used with untreated titanium dioxide and zinc oxide. The FDA has banned the use of these inorganic filters with avobenzone for precisely this reason.9 


source from https://www.personalcaremagazine.com



Encapsulating avobenzone can be beneficial, as it separates the filter from the environment and the skin, minimising the risk of degradation and irritation. Encapsulation in polymer microspheres effectively protects avobenzone, minimising interaction with materials such as ethylhexyl methoxycinnamate, improving photostability while also improving the skinfeel.10-13 


Formulators of natural products tend to prefer using solely inorganic filters for sun protection, and non-nano filters where possible, due to concerns surrounding the potential effects of nano materials on human and environmental safety. Non-nano filters generally provide poorer sun protection and cause more whitening than nano filters as the larger particle size means the spectrum of light they reflect is shifted away from the UVB region. 


Environmental considerations are also becoming more important in consumers’ choice of sun care products. The bleaching of the Great Barrier Reef has led to research on the impact of UV filters on coral. As a result, Hawaii has banned the common UVB organic filters, oxybenzone and ethylhexyl methoxycinnamate, and a ‘reef-safe’ claim has propagated in the sector, although this claim is poorly defined. The issue of reef safety has so far been focused on organic UV filters but there is evidence to show that untreated titanium dioxide and zinc oxide also bleach coral.14 


Many of the drawbacks of inorganic sunscreens can be countered to a degree through surface modifications. Surface treatments help to prevent agglomeration, and so reduce whitening and improve skinfeel. They also reduce interactions between the inorganic filters and other materials, improving compatibility with avobenzone and reducing the impact on coral bleaching.


Beyond UV filters, the choice of emollients in the formulation is also critical to performance. It is essential that organic UV filters remain fully solubilised as crystallisation or separation will lead to a reduction in protection. Esters are a fantastic option as they tend to be good solubilisers of organic UV filters and also have a lighter skin-feel than traditional oils. UV filters tend to have a greasy or heavy skin-feel, but this can be improved through the use of dry, light esters such as octyldodecyl neopentanoate, methylheptyl isostearate, or shea butter ethyl esters. As previously discussed, silicones are often used to formulate light W/O creams to counter the unpleasant feel of UV filters. However they tend to have poor compatibility with organic UV filters, which limits their use in some formulations. When using silicones, diphenylsiloxy phenyl trimethicone can be used to improve compatibility between silicones and organic filters due to their phenyl groups.