Nanoparticles of titanium dioxide (TiO2) are commonly used in sunscreens as protection against ultraviolet radiation. Partly because the particles are so small, nanoscale TiO2 does not reflect visible light and absorbs UV light, allowing for a transparent barrier that shields the skin from harmful solar rays.
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Sunscreen lotions are typically marketed as cosmetic products in most countries, including the European Union. In the United States, sunscreens are considered over-the-counter (OTC) drugs under control of the Food and Drug Administration (FDA).
The photocatalytic qualities of TiO2 nanoparticles also lend them to be used in the photodegradation of various wastes, treatment of wastewater and deterioration of tumor cells. These applications mostly depend on the capacity of TiO2 nanoparticles to create reactive oxygen species (ROS) on their exterior when triggered with UV light.
Titanium dioxide nanoparticles can be generated in various morphologies, including in spheres, tubes, and wires. Moreover, titanium dioxide can appear in multiple crystal morphologies. Different crystal structures can give distinct qualities to TiO2 nanoparticles.
Another factor that should be considered when assessing TiO2 in sunscreens is the purity of the particle. As a result of production techniques or deliberately infusing the crystal structure with other metals, the qualities of the nano-TiO2 can be altered. For instance, the company known as Oxonica-Croda uses manganese to lower the unwanted photo-activity of TiO2 nanoparticles in its product.
TiO2 nanoparticles in sunscreens tend to be covered with silica, alumina and various polymers to raise stability and lower photoactivity. The substance used for these coatings, along with the matrix where the particle resides, thickness, purity and the usage of coatings, increase the heterogeneity of TiO2 nanoparticles in sunscreen.
Together, these factors result in a tremendous diversity in TiO2 nanoparticles used in sunscreens. This heterogeneity causes more uncertainty for analysts, which forms an obstacle to the evaluation and management of these nanomaterials.
Analyzing TiO2 concentrations in sunscreens
Nanoparticles of TiO2 in commercial sunscreens can be assessed using laser scanning confocal microscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM).
One particular study of note discovered laser scanning confocal microscopy assessed primary particle aggregates and dispersions, but could not ascertain particle size due to the limited diffraction resolution of optical microscopy, which is 200 nanometers. The study team discovered that XRD could provide particle size and crystal data without a pre-treatment, a crucial benefit. When paired with energy-dispersive X-ray spectroscopy, TEM could determine particle size and compositional evaluation. XRD analysis of six commercial sunscreens labeled as containing nanoparticles showed only three samples held TiO2 particles, with average particle sizes of 15 nm, 21 nm, and 78 nm.
The study team also discovered TEM could resolve nanomaterials with one or more dimensions between 1 nm and 100 nm. In total, the combination of XRD and TEM was useful for analyzing TiO2 in commercial sunscreens.
A different notable study was capable of establishing TiO2 concentration, without sample preparation, by using a portable energy dispersive X-ray fluorescence (EDXRF) device. The EDXRF system was able to determine the physical barrier contribution of particles to the sun protection factor (SPF), which was then linked to TiO2 concentration. The EDXRF evaluation also discovered the presence of some elements, like potassium, zinc, bromine, and strontium, not listed in the formulations.
Issues with TiO2 in sunscreens
The International Agency of Research on Cancer (IARC) has categorized titanium dioxide (TiO2) as a potential human carcinogen, and the risks of using TiO2 nanoparticles in sunscreen have been actively debated. The IARC did acknowledge that TiO2 particles are used in various sizes with different properties; although the agency did not make size distinctions in its classification. The agency said it based its classification on the outcomes of animal trials and mentioned that human cohort analyses to support this classification have methodological limitations.
Researchers have also discovered that sunscreen from bathers can release considerable quantities of TiO2 into the sea, possibly harming marine life. Scientists have found that in water, the nanoparticles tend to shed their protective coating due to the impact of UV light or seawater, which exposes more-toxic TiO2 to the aquatic environs.
Despite these issues, scientists continue to stress that sunbathers use sunscreen for skin protection, adding that issues related to TiO2 could be handled awareness, public pressure, and legislation.
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