Sunlight is composed of a continuous electromagnetic spectrum divided into UV (100 nm–390 nm), visible (400 nm–700 nm) and infrared (>700 nm). The UV section of the spectrum is subdivided into UVA (320 nm–390 nm), UVB (290 nm–320 nm) and UVC (<290 nm).1 UVC is strongly absorbed by stratospheric ozone and is practically absent at sea level, although accidental exposure still can occur. UVA comprises 90–95% of the remaining incident UV radiation.2 The interaction of UV radiation with human skin is a complex and controversial subject, although the general principles are becoming clear and well-accepted within the scientific community.
Human skin is a multilayer structure with the outermost layer, the stratum corneum (SC), serving as the protective coating. The SC forms part of the epidermis, which acts as a barrier to prevent water loss and invasion from infectious agents and other damaging substances. Beneath the epidermis lies the dermis, which consists of connective tissue, nerve endings, follicles, sweat glands and so on.3
Exposure to both UVB and UVA has important biological consequences for the skin. UVB is strongly absorbed by DNA, causing sunburn and chromosomal damage, which in some cases may lead to skin cancer. The adverse effects of UVA exposure are principally due to the formation of reactive oxygen species (ROS) after absorption of UVA light by a suitable skin chromophore. ROS, which include free radicals, oxygen ions and peroxides, are implicated in both skin aging and carcinogenesis.4,5
Skin aging is a result of chronic sun exposure, the symptoms of which include roughness, irregular pigmentation and wrinkling, among others. ROS generated during UVA exposure readily reacts with membrane lipids and amino acids and induces membrane damage resulting in the activation of various UV response genes and consequent skin damage.4
There are three types of skin cancer, the rarest but most dangerous being malignant melanoma. The incidence and mortality rates of malignant melanoma are rising in most countries throughout the world6 and the likelihood of developing the disease is a combination of inherited and environmental factors. The only environmental risk factor shown to be relevant to the development of melanoma is exposure to sunlight.7,8 It recently has become clear that not only is UVB-induced DNA damage implicated in melanoma genesis, but UVA-induced ROS also have been shown to be involved in all three stages of carcinogenesis: initiation, promotion and progression.5
Protecting the Skin
The use of sunscreen seems to be helpful in avoiding photo-damage but only to a certain extent. The International Agency for Research on Cancer (IARC) concluded in a recent review that sunscreens reduce the risk of sunburn and one type of skin cancer, but also concluded that there is inadequate evidence in humans for a cancer-preventive effect against melanoma and basal cell carcinoma.9
This could be related to variations in UVA skin exposure during sunscreen use. Sunscreens are rated according to their ability to protect against UVB-induced sunburn, the SPF factor, but it has been suggested that this alone is not an efficient gauge of the ability to protect against UVA-induced damage (see Figure 1).10
UVA protection factors now are being incorporated into sunscreen packaging. Currently, some of the most well-known are the Boots Star Rating Performance system—a scale of one to five stars—11 and persistent pigment darkening—a scale of + to +++.12
The active components incorporated into sunscreens are primarily divided into two categories: UV absorbers and antioxidants. UV-absorbing components are further split into two categories—chemical and mineral—and commonly are used as a blend of ingredients since no one compound provides suitable UV-stopping power or a broad UV protection range. Two of the most commonly used chemical absorbers are butyl methoxydibenzoylmethane (BMDM) and octyl methoxycinnamate (OMC), which offer UVA and UVB protection, respectively.
Figure 2 shows the chemical structures and absorption profiles of these materials and demonstrates how broad UV protection may be built up using a combination of materials.
Mineral UV absorbers, most prominently small particle titanium oxide (TiO2), but also zinc oxide (ZnO), offer some advantages over chemical absorbers in that they have an inherently broader UV absorption spectrum and are much less likely to penetrate the skin than chemical compounds.13 However, their UV-stopping power is less than chemical compounds and at high loadings they tend to exhibit skin whitening; consequently they tend to be used in combination with chemical sunscreens except in special cases such as sensitive skin or children’s formulations.
The industry currently tends to design formulations containing TiO2 sunscreens to reduce particle sizes to counteract whitening effects, but this also reduces the UV-stopping power or attenuation efficiency. Recent calculations based on Mie scattering theory14 have shown that the highest UV protection is obtained for a TiO2 particle of 62 nm.15
An alternative approach to skin protection is to incorporate antioxidants such as vitamins and plant extracts into formulations. Such compounds act to scavenge ROS generated in the skin. These compounds also may be delivered by an oral dose. This approach should be seen as a support activity for the skin’s natural defenses and should not substitute for protecting the skin from UV exposure.16
Protecting UV Absorbers
There is a further complication in the determination of sunscreen performance: the sunscreen itself does not remain stable during solar exposure, and both UVA and UVB protection may suffer sharp declines during topical use. The problem is particularly acute in terms of UVA protection since the user will have a reduced experience of sunburn and will be unaware of the reduction in protection.17 Figure 3 shows the photostability performance of a number of commercial sunscreen formulations containing BMDM, OMC and two TiO2 formulations over a two-hour exposure to simulated sunlight equivalent to mid-day sun in the southern Mediterranean.
It is clear that there is a significant drop off in UVA absorption performance during topical use in addition to a relatively modest initial UVA absorption, as manifested in the low Boots Star Rating. This problem has its origin in two phenomena: the inherent photostability of chemical sunscreens and photocatalytic activity of mineral components.
Organic compounds are susceptible to decomposition via sunlight-induced isomerization and consequent loss of UV protection.18 Mineral compounds are completely stable, but have a tendency to dispose of absorbed solar energy by creation of ROS, which then degrade other organics in the formulations.19
In terms of new developments in mineral UV absorbers, the principle techniques used by manufacturers to reduce the formation of ROS have been coating technologies and control of crystal form; i.e, in TiO2 the rutile crystal form is less photoactive than the anatase crystal, so ensuring that the TiO2 is entirely rutile will vastly reduce the photoactivity of the TiO2 crystal.
Typically, a layer of aluminosilicate is formed on the surface of the crystal to act as a physical barrier layer to prevent charge transfer from the mineral crystal to the formulation and hence reduce ROS generation. This technique has been shown to have limited effect, however, since uniform layers are difficult to apply during manufacture in a cost-effective manner.20,21 Both organic UV-absorbing and antioxidant compounds are then degraded by the sunscreen itself rather than providing any useful benefit to the user.
The net result of this is that sunscreen performance measurements in the UVA are only applicable at time zero and do not give the user any information regarding the duration of protection in the UVA range. SPF measurement is unaffected by photostability since it is a time related in vivo measurement, which will automatically take into account any instability during the testing.
New Developments in UV Absorbers
Developments in novel UV absorbers are an active area of interest in the scientific community, with attention being directed to the areas of broad band protection and photostability. This is particularly directed at the UVA region of the UV spectrum, as far fewer suitable UVA absorbers have been available, compared to their UVB counterparts.
Two prominent new organic materials that have been released are terephthalylidene dicamphor sulfonic acida and bis-ethylhexyloxyphenol methoxyphenyl triazineb. Both compounds claim broad-spectrum, photostable UV protection and stabilization of other organic components of the formulation.22,23
A new inorganic material based on titanium dioxidec adopts an alternative approach by modifying the TiO2 lattice with manganese ions to form a material with broad-spectrum absorbance, near zero photoactivity and free radical scavenging activity. This material has been demonstrated to enhance the photostability of other actives within the formulation such as BMDM, OMC, vitamin C and vitamin E.24
Such new materials allow high UVA protection with minimal loss of performance during topical use, as shown in Figure 4, which compares the performance of a commercial sunscreen based on the new terephthalylidene dicamphor sulfonic acida with a similar emulsion incorporating the new titanium dioxidec. New materials such as these allow enhanced sun protection benefits and a reduced exposure to UVA-induced skin pathologies.
It is becoming clear from the scientific literature that skin protection from UVA sunlight is as important as protection from UVB, the wavelengths that cause sunburn and that are traditionally measured as the SPF factor of the sunscreen. New labelling protocols are being developed to allow the consumer to judge the UVA performance of a product but these may be compromised by instability of commonly used UV-absorbing components. Many of these materials are either not photostable or are photoactive during solar exposure, resulting in a reduction in protection during use. New materials such as photostable organic terephthalylidene dicamphor sulfonic acid or photoinactive TiO2 are providing better protection against UVA exposure over extended periods of use.
The availability of new generation TiO2 and more photostable organic UV absorbers has allowed cosmetic formulators to create more sophisticated high performance formulations. It is now possible to maintain constant levels of protection during solar exposure through the incorporation of these actives, which means greatly improved UV protection for the end consumer.
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