Exposure of the skin to the sun has always been a concern in all civilizations, and this from the earliest age; the related skin damages ranging from burns to cancers. For ancient Greeks and Egyptians, zinc oxide, olive oil and various plant extracts (i.e. jasmine) were used to prevent skin dryness and to reduce the risk of skin burns. Shea butter and clay are still traditionally used in Africa for the same purpose.
The very first synthetic UV filter-based sunscreen was launched in Australian in 1932 by the H. A. Milton Blake.
The chemists have developed many new synthetic UV filters since this time and the quality of the formulations of sunscreens has also evolved in terms of number of galenic formats (creams, fluid emulsions, oils, gels, pastes, sprays, foams, etc.), skin tolerance and efficacy.
Skin cancer is a growing concern
Nowadays, concern over skin cancer has been growing for many years. Skin cancer is the most common form of cancer all other the world. In the United States, more than 3.5 million cases and among them, 2.8 million of basal cell carcinoma, the most common form of skin cancer, are diagnosed every year. Non-melanoma skin cancers have also increased by 77% during the last 20 years. Skin cancer represents 25% of all malignant tumors in Brazil.
Skin cancers are induced by exposure to ultraviolet rays (UVs) which are non-visible electromagnetic radiations with a wavelength ranging from 100 to 400 nanometers (nm). They are classified as UV A (315-400 nm), UV B (280-315 nm) and UV C (100-280 nm); the UV C being unable to reach our skin as they are totally absorbed by the ozone layer (if we do not take into account the problem of ozone depletion) and the atmosphere itself.
UVs are beneficial to our metabolism: they induce the production of vitamin D (mainly cholecalciferol and ergocalciferol) by the skin; this vitamin regulates the intestinal absorption of phosphate, iron, calcium and zinc. It also stimulates our immunity.
However, UVs also induce the production of free-radicals and consecutively the formation, within the DNA, of pyrimidine dimers which are molecular lesions formed from Thymine Thymine or Cytosine-Cytosine abnormal pairs of bases. Most of these lesions are repaired by the cell mechanisms of defense such as nucleotide excision repair. When the number of pre-mutagenic lesions overtake the capacity of our cells to repair the DNA, the cells may die (burn effect) or, worst case scenario, they transmit the viable mutations to the daughter-cells. This may be the beginning of the carcinogenic process.
The use of sunscreens is probably the most efficient way to prevent the development of skin cancers. In order to reduce the quantity of UVs reaching the skin, sunscreens contain synthetic compounds (i.e. aminobenzoic acid, oxybenzone, homosalate, octyl methoxycinnamate, avobenzone, etc.) which absorb the UVs and/or mineral particles such as titanium dioxide, zinc oxide, iron oxide, talc, and kaolin, which reflect the UVs; titanium dioxide being the most commonly used inorganic filter because it is not expensive and quite effective.
Synthetic filters are usually aromatic compounds able to undergo conformational molecular changes when irradiated by UVs, or to emit radiation at higher wavelengths, or to release incident energy as heat. All these modes of action are reversible; therefore the same molecules can function repeatedly. Synthetic filters are also well tolerated by the skin in general; cases of allergy are quite rare, finally.
Mineral particles act by scattering, reflecting and/or absorbing the UV radiations. They are often associated synergistically with synthetic UV filters. However, titanium dioxide and zinc oxide in particular whiten the skin, decreasing the esthetic appeal of sunscreens, and this is something that consumers no longer accept well. Therefore, chemists have quite recently atomized titanium dioxide to a “nanostructured” format, ranging from 1 and 100 nm in size, and this leads to transparency. This process has eliminated the disliked skin whitening effect.
Nanotechnologies on the rise
Nanotechnologies have polarized the attention of the media and of the consumers during the last 5 years or so, mainly. This has not really started with the launch of nanoemulsions or nanosome-based cosmetics, many years ago, but rather with the debates and press releases generated by various trade associations and regulatory bodies; nanoparticles being suspected of having potential mutagenic or carcinogenic properties. The world production of nanomaterial in 2004 was about 2000 tons; it is estimated to reach 58000 tons in 2020.
The main problem with nanomaterials is that they have not been properly tested yet. However, some toxicological studies performed on intact human skin have demonstrated that nanoparticles are not likely to penetrate the stratum corneum. What about when they are in contact with damaged skin? What about the transdermal penetration through hair follicles and sweat glands?
It is believed that nanoparticles can be absorbed by some skin cells such as fibroblasts and melanocytes and be cytotoxic. Nano-sized titanium dioxide has also been shown to be able to damage DNA; as such it should be considered as genotoxic. Titanium dioxide particles have also shown to be carcinogenic after inhalation. Other systemic effects have not yet been fully studied; more studies are necessary to fulfil the gaps in our knowledge. This is the opinion of the Scientific Committee on Consumer Safety.
So is nano-sized titanium dioxide in sunscreen a friend or a foe?
Because of the increasing bad press surrounding nanotechnologies, many manufacturers of sunscreens have already proactively avoided the use of nano-sized materials. This is probably a very wise decision until more studies have been performed to demonstrate their eventual innocuousness.
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