Creative Developments (Cosmetics) Limited

Sun Products: Chemistry In Britain 2002

John Woodruff

The first commercial sun product is believed to have been launched in the USA in 1928. It contained benzyl salicylate and benzyl cinnamate. Neither material is on the current approved list of sun filters for use in the USA or in Europe. Various products were introduced over the next thirty years but it was not until the arrival of cheap foreign travel and the post-war emigration of white Europeans to sunnier climes that they became commercially important and by the early 1960’s a number of effective UV filters were in general use to mitigate the deleterious effects of exposure to strong sunlight.

The most serious effects of sun exposure are the chronic ones of skin carcinogenesis and photo-aging. Skin carcinogenesis involves direct and indirect damage to cellular DNA that will allow malignant transformation of the cells and impairment of the skin immune system so that newly developing neoplastic cells are not recognised by the immune system and destroyed. Direct damage occurs because DNA absorbs ultraviolet light with resultant gene damage while indirect damage is caused by free radicals and singlet oxygen. The Langerhan's cells limit damage to the immune system by recognising and destroying malignant cells. Unfortunately the function of Langerhan’s cells is impaired by UV radiation and their numbers are reduced so the radiation that is the cause of carcinogenic change is also responsible for reducing the bodies natural defence mechanisms.

Solar radiation in the ultra violet region is classified in three types. Ultraviolet A (UVA) radiation of wavelength 320 – 400 nm penetrates the top layer of the skin resulting in damage to the lower layer. This causes skin to age prematurely, with effects that include roughening, blotchiness, sagging and wrinkles. UVA also contributes to the development of skin cancer but it is only during the last decade that the damaging effects of UVA have been fully appreciated. Sun-induced skin cancers occur in more than 30% of the population in Australia and similar figures are recorded in California. UVA absorbers are chemicals that absorb radiation in the 320 – 360 nm region such as benzophenones, anthranilates and dibenzoyl methanes. Microfine zinc oxide is a physical blocker and is particularly effective in this region.

Ultraviolet B (UVB) is radiation between 290 – 320 nm and causes sunburn and skin cancer. UVB radiation inhibits DNA, RNA and protein synthesis and induces erythemal responses. Pigment darkening occurs above 310 nm so when the aim was to achieve a suntan without sunburn, sun filters showing maximum absorption (λmax) between 300 – 310 nm were used. The principal UVB absorbers are para-aminobenzoic acid derivatives, salicylates, cinnamates and camphor derivatives plus microfine titanium dioxide.

Ultraviolet C (UVC) radiation from 100 – 290 is filtered out by the ozone layer and does not reach the earth’s surface, hence the current concern about ozone depletion. Exposure to UVC is damaging to skin cells and is a hazard in occupations such as welding, where such exposure is common. It is not an area of commercial interest to sun product manufacturers and the only permitted filter that is effective in this region would be microfine titanium dioxide of sufficiently small particle size. Chart I shows typical absorbance curves for sun filters in the UVA and UVB region.

Insert UV Curves

In Europe and Japan sun products are regarded as cosmetics while in Australia and the USA they are licensed as OTC drugs but throughout the world their ingredients are listed using the International Nomenclature of Cosmetic Ingredients; the INCI names. These are a simplified form of the chemical name that is intended to be easier for the consumer to understand, or at least to remember if such a person has an allergy to a particular material. The table shows the sunscreens permitted in Europe with their chemical and INCI names, their maximum permitted level and the radiation most affected. In most countries, including Europe, Australia, Japan, Brazil and the USA only sunscreens from a positive list are permitted for use. These lists are not in harmony and some materials are only allowed in certain countries and usage levels vary from market to market, which severely limits formulating for a global marketplace.

There are two main concerns responsible for limiting sunscreen chemicals for topical application: their potential for causing irritation and allergy and the possibility of photochemical instability. The majority of sunscreens are aromatic compounds conjugated with a carbonyl group with an electron-releasing amine or methoxl group substituted in the ortho- or para- position of the aromatic ring. Sunscreens work by absorbing high-energy short wave radiation, which excites them to a higher energy state. The material may dissipate this energy as long wave (>380nm) radiation and return to its original ground state or isomerisation can occur and the material may fragment into non-absorbing isomers. This not only destroys the effectiveness of the product but the isomers may cause irritation and allergic reactions.

In a paper given at the European Sun Filters Conference, Paris 1998, S. Schaulder reviewed a seventeen-year history of allergic and photo-allergic reactions to sunscreens¹. Tests on eighty-five patients with contact dermatitis caused by sunscreen materials elicited 107 reactions to UVA absorbers and 79 to UVB, many patients proving positive to both types. Of the 186 reactions 94 were photo-allergic of which 59 were to UVA and 35 to UVB filters. At one time PABA and its derivatives caused the most problems but as its popularity waned benzophenones became the principal cause. Dibenzoylmethanes were a problem but after the withdrawal of isopropyldibezoylmethane from the market reactions to butyl methoxydibenzoylmethane become rare, showing that cross-sensitisation between the dibenzoylmethanes was a problem. Ethylhexyl methoxycinnamate (OMC) is the most widely used sunscreen material and allergic and photo-allergic reactions to it are of a low order. However cross-sensitisation caused by balsam of Peru, benzyl cinnamate and methylcinnamate has elicited a number of reactions, said Schauder.

Recent studies have shown that the polarity of the solvent system can have a pronounced effect on photo-stability. It has been noted that stability is greater in polar solvents and that adding small amounts of protic compounds such as butyloctyl salicylate or butyloctyl benzoate to the solvent retards instability. Another way of improving photo-stability is by combining two or more sun filters with the aim of protecting the stability of the principal active ingredient. However this has not always been successful and some filters can accelerate the photo-instability of others. Making a particular study of butyl methoxydibenzoylmethane (BMDM) C. Bonda² found that octocrylene was an effective stabiliser whereas in combination with OMC there was a significant loss of SPF on exposure to sunlight. Further studies by Herzog & Sommer³ confirmed Bonda’s findings and showed that OMC with 4-methylbenzylidene camphor was almost completely stable, even at high doses of radiation.

How high should the SPF be is a frequent query; an SPF of 15 – 20 should be adequate, however natural variations in application can result in large deviations in actual SPF and loss through abrasion and perspiration can further reduce the protection received. Australia limits the quoted value to 30 and recent changes in the USA have categorised sun protection products as SPF 2 – 11 as providing minimal protection, SPF 12 to 29 as moderate and 30+ as high. SPF values are still used up to 30, and then they can only be shown as 30+. Currently there are no restrictions on quoted values in Europe or Japan. Water-resistance claims also vary but the USA demands that if a claim for water-resistance is made the quoted SPF must be that of the product remaining on the skin after immersion under controlled conditions. Other countries allow a 50% reduction in the stated SPF after controlled immersion.

Claims for broad-spectrum or UVA protection also vary considerably. In Britain the Boots Star Rating System is the industry standard. This is the ratio of UVA radiation blocked in proportion to the UVB and is displayed as stars on the label. One star is moderate, 4 stars are maximum protection. Many people interested in sun protection believe that broad spectrum should only apply to SPF15 or higher and that products with SPF15 or more should incorporate UVA protection. This is because the higher factor products encourage users to stay in the sun longer and thus be exposed to higher doses of UVA radiation.

The levels at which sunscreen materials are allowed in products for topical application is severely limited by legislation. This was not a problem when SPF 8 was considered high protection but consumers now look for SPF15 – 25 and even higher. As can be seen from Chart II, this requires a disproportionate increase in sunscreen material. A product of SPF2 blocks 50% of UVB radiation, SPF10 blocks 90%, SPF25 blocks 96% and SPF50 blocks 98%. To achieve the higher SPF values and especially if broad-spectrum protection is required a mixture of actives is necessary and the skilled formulating chemist will look for synergy between them and SPF enhancement from the base materials. As we have seen, in part synergy may be due to stabilising the actives against photochemical degradation but some remarkable results have been obtained by using combinations of organic sunscreens with micronised oxides.

More than fifty years ago powdered titanium dioxide or zinc oxide dispersed in a base of petrolatum were used as sunblocks but with few exceptions the general public did not want a white paste smeared on its skin. In the last decade these two materials have emerged as the preferred ingredients for most high SPF products. It was found that if the particle size was small enough it could block UV radiation but remain transparent to visible light. Titanium dioxide crystals of approximately 60nm in diameter block radiation according to Chart 1. Zinc oxide is not so effective against UVB but can be used as the basis for broad-spectrum protection, as shown on the chart. Several papers at the 1998 European Sun Filters Conference described different aspects of these materials and improved production techniques coupled with a better understanding of their properties has substantially contributed to their success. At first they were introduced as alternatives to organic filters but are now increasingly seen as having a synergistic action when properly formulated in conjunction with traditional filters. A paper by Julian Hewitt⁴ underlined the benefits that accrue from such an approach. Hewitt described the use of a dispersion of titanium dioxide in conjunction with OMC. While the SPF to be expected from 4% coated titanium dioxide is about 9 – 10 and that for 1% OMC about 2 – 3, when used in conjunction the results were >15. Further experiment showed that there was an improvement in the photostability of the OMC. While titanium dioxide is used for obtaining high SPFs zinc oxide is useful for providing UVA protection. The two oxides are often used in combination to provide broad-spectrum protection but there can be problems with high solids content in the formulation. Hewitt showed that the use of zinc oxide in combination with OMC boosted the expected SPF from about 15 to >22 while the presence of zinc oxide increased the UVA/UVB ratio from 0.17 to 0.42.

It is well known that the base can affect the sunscreen efficacy. W. Johncock⁵ has described some of the problems that range from OMC turning products bright yellow if packed in clear bottles to the formation of long needle-like crystals if the pH of a product containing phenylbenzimidazole sulfonic acid falls below 7. Octyl triazone, methylbenzylidene camphor, butyl methoxydibenzoylmethane and benzophenone-3 crystallise if they are insufficiently solubilised. The solubility parameter of most liquid UV filters is between 9 and 10.3, which is similar to that for the polymers used in many packaging materials. This can result in the filter migrating into the plastic with a consequent degradation of the pack and loss of active in the formulation.

Not all reactions between base and active are negative and in many instances the SPF of a given combination of sunscreens can be significantly enhanced. This may be achieved by controlling the rheology of the product and improving the spreading effects and the nature of the film deposited on the skin or by improvements in solubilising the sunscreen. Enhancing performance through improved base formulation is one of the most active areas of product development. Improving product durability and water-resistance is also an active area of research.

In summary; sun protection products have past through a transition phase of enabling the user to achieve a deep tan without blistering sunburn on the annual holiday to products that provide essential protection for daily use and which provide such protection effectively and safely. For the chemist they present a multitude of challenges ranging from preventing photo-degradation of the active material to controlling product stability and rheology, all within the severe restraints placed on the materials that may be used.

1. Schauder S., Hellmut I; Contact & photocontact sensitivity to sunscreens, European Sun Filters Conference, Paris 1998

2. Bonda C; The photochemistry of sunscreen photostability; 2^nd European Sun Filters Conference, Paris 1999

3. Sommer K, Herzog B; Investigations on photostability of UV-Absorbers for cosmetic sunscreens. Xxist International IFSCC Congress, Berlin 2000.

4. Hewitt J; Novel formulation strategies for high SPF and broad-spectrum sunscreen products, European UV Sunfilters Conference 1998; Paris, France

Glossary of terms

Minimal Erythemal Dose (MED) is the minimum dose of UV radiation required to show the first signs of burning or reddening of the skin.

Sun Protection Factor (SPF) = MED on Protected Skin

MED on Unprotected Skin