Basics of TiO2 sunscreen

Ultraviolet (UV) Radiation

Ultraviolet radiation can be divided into three regions: UVA (320–400 nm), UVB(290–320 nm) and UVC (200–290 nm). Although UVC is the most damaging UV radiation, it is filtered out by the ozone layer in the stratosphere before reaching the earth’s surface. The radiation in the UVB region (partially filtered by ozone) can penetrate both the stratum corneum and the epidermis of human skin. It has
sufficient energy to cause damage, such as sunburn, to human skin. This is particularly true for fair-skinned individuals. The UVA radiation, which is unfiltered by ozone, has deeper penetration of human skin to the dermis; it, thereby, stimulates the formation of melanin and produces a tan, which acts as the first line of defense for the protection from sunburn. UVA radiation, therefore, is also called the “tanning region.” Although having considerably lower energy than UVB, UVA photons can cause delayed damages to the skin.

The fundamental function of a sunscreen is to serve as a filter that can prevent the penetration of ultraviolet radiation. The substances most commonly used in commercial sunscreen preparations include p-aminobenzoic acids (PABA), cinnamates, oxybenzone, salicylates, and metal oxides, such as TiO2 and ZnO. In addition to their ability to scatter sunlight, inorganic particles, such as TiO2, do
absorb strongly in the UVA and UVB regions.

Rutile and Anatase TiO2

In addition to its amorphous state, two of the most common crystalline forms of TiO2 are rutile and anatase. The two crystalline forms share many similarities, such as their physical appearance, refractive index, density, low toxicity, and high stability in the presence of strong acids and bases. As a physical blocker for sunlight, bot
crystalline forms would serve the purpose well. Their photochemical properties, however, are very different. TiO2 is a semiconductor. When a semiconductor particle absorbs light, it promotes an electron from its valence band to its conduction band, leading to a charge separation. In rutile TiO2, the charge separation is quickly diminished through a charge recombination within the particle and the energy is released as heat. This translates to a low photoactivity and an effective conversion of UV light into heat. As a result, any photoinduced reactions that can pose damage to the skin are avoided. The low photoactivity and its desirable UV absorption spectrum, which cover the entire erythemal curve make rutile TiO2 an ideal choice as UV blocker for sunscreen preparations.

For amorphous and anatase TiO2, the lifetime of the charge separation is much longer than that on a rutile particle. The electron and electron hole, therefore, have greater opportunity to undergo redox reactions on the surface; therefore, anatase has been extensively used for applications involving such electron transfer processes as
photocatalysis for environmental waste treatment and photovoltaic design for solar energy storage. Extensive reviews on these interesting topics are available. Due to their photoactivity, amorphous and anatase TiO2 are not suitable for sunscreen applications.

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How Toxic or Safe Titanium Dioxide is as a Sunscreen!

Titanium dioxide is the subject of new controversy, yet it is a substance as old as the earth itself. it is one of the top fifty chemicals produced worldwide. Titanium dioxide has a variety of uses, as it is odorless and absorbent. this mineral can be found in many products, ranging from paint to food to cosmetics. In cosmetics, it serves several purposes. it is a white pigment, an opacifier and a sunscreen. Concern has arisen from studies that have pointed to titanium dioxide as a carcinogen and photocatalyst, thus creating fear in consumers. But are these claims true? What does the research on these allegations bear out? Would we as consumers benefit from avoiding this mineral to preserve our long-term health?

A carcinogen is a substance that causes a cellular malfunction, causing the cell to become cancerous and thus potentially lethal to the surrounding tissue and ultimately the body as these rapidly growing mutated cells take over. With the surge in cancer rates among all segments of the population, many people are attempting to reduce or eliminate their exposure to carcinogens. Titanium dioxide is regarded as an inert, non-toxic substance by many regulatory bodies such as the MSDS (Material Safety Data Sheets) and others charged with the responsibility of safeguarding the health of occupational workers and public health. the MSDS states that titanium dioxide can cause some lung fibrosis at fifty times the nuisance dust, defined by the US Department of Labor as 15 mg/m cubed (OSHA) or 10 mg/m cubed (ACGIH Threshold Limit Value). the ACGIH states that titanium dioxide is not classifiable as a human carcinogen. Symptoms of chronic overexposure to titanium dioxide in an industrial setting, according to the MSDS, include a slight increase in lung tumour incidence in lab rats. it also states when titanium dioxide was fed to rats/mice in a carcinogen bioassay, it was not carcinogenic. the NIOSH declares that at 5000 mg/m cubed there was slight lung fibrosis, concluding that this substance was carcinogenic in rats.

The NIOSH declaration of carcinogenicity in rats is based on a study by Lee, Trochimowicz & Reinhardt, Pulmonary Response of Rats Exposed to Titanium Dioxide by Inhalation for two Years (1985). the authors of this study found that rats chronically exposed to excessive dust loading of 250 mg/m cubed and impaired clearance mechanisms within the rat, for six hours per day, five days per week for two years, developed slight lung tumours. they also noted that the biological relevance of this data to lung tumours in humans is negligible. it is important to note that rats are known to be an extremely sensitive species for developing tumours in the lungs when overloaded with poorly soluble, low toxicity dust particles. Rat lungs process particles very differently compared to larger mammals such as dogs, primates or humans (Warheit, 2004). this sensitivity in the lungs has not been observed in other rodent species such as mice or hamsters (Warheit, 2004), therefore using the rat model to determine carcinogenicity of titanium dioxide in humans can be misleading, as extrapolation of species-specific data to humans is erroneous.

Many organizations and businesses have perpetuated this assessment of the carcinogenicity of titanium dioxide (ewg.org). however, several studies and study reviews have been used to compile the safety disclaimers for the regulations on the permitted use of titanium dioxide. One such study review took place in Rome, 1969 between the World Health Organization and the Food & Agriculture Organization of the United Nations. Cross species analyses were performed and reviewed for possible toxicity of titanium dioxide. the conference concluded that among the following species: rats, dogs, guinea pigs, rabbits, cats and human males, ingestion of titanium dioxide at varying diet percentages and over long periods of time did not cause absorption of this mineral. Titanium dioxide particulates were not detected in the blood, liver, kidney or urine and no adverse effects were noted from its ingestion. the U.S. Food & Drug Administration (2002) allows for its ingestion, external application including the eye area, and considers it a safe substance for public health. other epidemiological studies showed that workers exposed to titanium dioxide exhibited no statistically significant relationship between such exposure with lung cancer and respiratory disease, although some cases of pulmonary fibrosis did occur. these studies were conducted in industrial settings where the increased exposure puts these individuals more at risk than the average person.

Titanium dioxide is listed as a safe pigment, with no known adverse effects. it is not listed as a carcinogen, mutagen, teratogen, comedogen, toxin or as a trigger for contact dermatitis in any other safety regulatory publications beside the NIOSH (Antczak, 2001; Physical & Theoretical Chemical Laboratory, Oxford University respectively).

One form of mineral or mineral extract, including titanium dioxide, that we should be concerned about is ultrafine or nano particles. As technology has advanced, so has its ability to take normal sized particles of minerals and reduce them to sizes never before imagined. While many are praising this new technology, others are warning of its inherent dangers to our bodies. a study by Churg et. al. at the University of British Columbia in their paper Induction of Fibrogenic Mediators by Fine and Ultrafine Titanium Dioxide in Rat Tracheal Explants (1999) found that ultrafine particles of the anatase form of titanium dioxide, which are less than 0.1 microns, are pathogenic or disease causing.

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Harmless natural nanoparticles show potential to replace metal-based nanoparticles in sunscreen

Quite a lot of nanotechnology research and manufacturing efforts go into synthesizing metal-based nanoparticles. Unfortunately, some of the nanoparticle manufacturing processes themselves (see: “Not so ‘green’ nanotechnology manufacturing”) as well as the final nanoparticle materials may be of potential concern for environmental regulators and for researchers attempting to address nanomaterial toxicity. As an alternative to using these potentially hazardous metal-based nanoparticles, some researchers are suggesting the use of naturally occurring nanoparticles. However, this area has not yet been well explored with regard to natural nanoparticles’ diverse properties and potential applications.
Researchers have now made the discovery that naturally occurring nanoparticles have unique optical properties. In addition, they are less toxic and biodegradable than their synthesized, metal-based counterparts. This discovery makes it likely that scientists will be able to find more biocompatible nanoparticles to replace metal-based nanoparticles, predominantly for biomedical applications.
“The concern for the biosafety and health risk for the metal-based and engineered nanoparticles in sunscreens has led to the search for alternative replacement nanoparticles,” Mingjun Zhang, an Associate Professor of Biomedical Engineering at the University of Tennessee, tells Nanowerk. “In our recent study we investigated naturally occurring ivy nanoparticles to replace titanium dioxide and zinc oxide that are currently widely used in sunscreen products. Based on experimental data, we have demonstrated that ivy nanoparticles have the potential levels of UV protection, and a much lesser level of cell toxicity than metal nanoparticles, necessary to warrant further investigation for uses in cosmetics.”
In a previous Nanowerk Spotlight, we have reported about Zhang’s work on ivy plants and nanoparticles secreted from them (see: “Ivy’s gripping nanotechnology secrete”). Now, in this new work, Zhang and his team explored the optical properties of the organic nanoparticles isolated from ivy. The researchers have reported their findings in a recent issue of Nanoscale Research Letters (“Ultraviolet Extinction and Visible Transparency by Ivy Nanoparticles”).

The morphologies of ivy nanoparticles observed by AFM. (Image: Dr. Zhang, University of Tennessee)
“Natural nanoparticles can be used as functional materials just as artificial nanoparticles” says Zhang. “Take for instance sunscreen. Here, organic and inorganic nanoparticles with ultraviolet extinction properties are widely used. The inorganic nanoparticles – such as titanium dioxide and zinc oxide – and organic nanoparticles can reflect, absorb and scatter the solar light and thus provide the sunblock effect. However, although inorganic nanoparticles have been widely used in cosmetic products, there are still concerns about the toxicity of these inorganic materials.”
According to the researchers, natural or biological nanoparticles have not been investigated in detail for their material properties. In this recent study, the University of Tennessee team investigates these type nanoparticles as the functional materials just like usual inorganic and organic nanoparticles.
Zhang notes that the ivy nanoparticles are not monodisperse in solutions. “This effect is due to the biological process of the nanoparticle formation” he says. “Nanoparticles from the original tiny ones to the mature ones all exit in the rootlets of ivy. We used size exclusion chromatography (SEC) and high-performance liquid chromatography (SEC-HPLC) to isolate the nanoparticles from the mixed solutions including molecules.”
This means that one area of future investigation needs to explore effective methods to obtain monodisperse ivy nanoparticles. The modifications of ivy nanoparticles to improve the optical properties are also expected. Zhang says that the team will also explore potential coatings for solar panel.
This makes it likely that if nature-derived harmless organic nanoparticles have strong ultraviolet absorption, they will be a potential promising alternative for sunscreen.
Quite impressively, the team’s study indicates that ivy nanoparticles can improve the extinction of ultraviolet light at least four times better than its metal counterparts.
Zhang points out that sunscreens made with ivy nanoparticles may not need to be reapplied after swimming. “That’s because the plant’s nanoparticles are a bit more adhesive so sunscreens made with them may not wash off as easily as traditional sunscreens,” he says. “And while sunscreens made with metal-based nanoparticles give the skin a white tinge, sunscreens made with ivy nanoparticles are virtually invisible when applied to the skin.”