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Nanoparticle zinc oxide, ZnO, is a form of zinc oxide where the compound is formed into individual particles as small as 20 nanometers in diameter. The transparent particles, which effectively filter out ultraviolet-a (UVA) and ultraviolet-b (UVB) light, are then coated with inert silicon or aluminum oxide layers and tend to clump together into groups that are 200 to 500 nanometers in diameter. By comparison, the average diameter of a human red blood cell is 100 nanometers. The main zinc oxide uses in this form include as a blocking agent for harmful solar radiation in zinc oxide lotion and as an antibacterial and fungistatic compound added to transparent coatings like varnish for wood surfaces. Other uses for nanoparticle zinc oxide include as a protective ultraviolet (UV) light coating on plastic films for windows and eyeglasses, or in textile fabrics made from synthetic polymers.
One of the earliest and most controversial uses for nanoparticle zinc oxide is as a UV-blocking agent in sunscreen lotions. The unique size of the particles in zinc oxide sunblock and not the chemical nature of the compound itself is what poses a risk to human health. This is because, at such a small scale, nanoparticles have a much larger surface area as compared to their net mass, which can make them highly-reactive substances. The minute size of the particles allows them to pass the blood-brain barrier, where, in studies of laboratory animals, they have been shown to kill formative neural stem cells (NSCs). These cells are responsible for regenerating human neurons in the brain and their destruction could lead to eventual degenerative brain disorders.
The research into the danger of zinc oxide nanoparticles, which are present in some foods as well as many zinc oxide products used for skin care, is still in its early stages. Some research suggests that particles above 30 nanometers in size do not pose any significant increased risk to health, while other research has found that the entire range of sizes for nanoparticle zinc oxide posed dangers. The products in which they are most often used such as zinc oxide gelatin or ointments are meant to be applied to the surface of the skin. Particles above 30 nanometers cannot pass into deeper levels of tissue, though, if they are ingested, they pose risks to blood, lung tissue, and the brain. Since many industrial preparations of nanoparticle zinc oxide are in the form of a dry white powder, the greatest risk from these substances is incidental inhalation.
Commercial preparation of nanoparticle zinc oxide can be done in a manner so that no agglomeration occurs, and the individual particles remain separate from one another. This gives them both increased versatility, where they can be applied in thin layers to the surface of fabrics or other materials, and increased risks with human ingestion or inhalation. The surface coatings on the particles can also be made to be either hydrophobic or hydrophilic depending on their specific needs, and they can be consistently manufactured in a diameter of 20, 40, or 60 nanometers. Their use for a wide range of manufacturing processes where anti-corrosion, anti-bacterial, and catalytic effects are desired continues to make them a focus in industry as of 2011.