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Zinc oxide is an inorganic compound used in a wide variety of products, and is a common ingredient in sunblock. Micronized zinc oxide is zinc oxide that has been treated to reduce the average size of its particles, producing an ultrafine powder. The term micronized can refer to particles that are micro-sized, meaning they are a few micrometers in diameter (1 micrometer is one millionth of a meter), but can also refer to much smaller nanoparticles, meaning particles that are 1-100 nanometers in diameter (1 nanometer is one billionth of a meter). Various terms like ultrafine, nanosized, and micronized can be used to describe this kind of zinc oxide. Scientific studies have shown that both micronized zinc oxide and regular zinc oxide offer very effective sun protection, preventing harmful UVA and UVB rays in sunlight from reaching the skin.
Regular zinc oxide has been used as a sunblock ingredient for many years, and has been favored by many because it is nontoxic, usually non-irritating, and is an effective sunblock. However, regular zinc oxide can leave a white film on the skin when applied. Products containing micronized zinc oxide do not leave this film, offering a form of invisible zinc oxide protection, because the particles are too small to see with the naked eye. Both micronized and regular zinc oxide function as physical barriers against sunlight, reflecting it before it reaches the skin.
Concerns have been raised that micronized zinc oxide could enter the body through the skin, especially if it contains nanosized particles, and that this could cause health problems. Many commonly available sunblocks and sunscreens contain zinc oxide particles that are 30-200 nanometers in diameter, meaning many such products do contain nanoparticles. Most scientific studies show that while micronized zinc oxide can penetrate the outermost layer of the skin, it does not penetrate into living tissue or the blood stream, and does not pose a risk to human health.
Some studies suggest that micronized zinc oxide can be more of a problem if it is inhaled and gets into the lungs, rather than if it is applied to the skin. The small size of the micronized particles means that if they are inhaled, they could settle deep inside the lungs and cause potential health problems. Some experts advise that because of this, one should not use powder or spray sunscreens or sunblock containing ultrafine zinc oxide particles. Products like lotions and creams are considered safer, because their consistency makes inhalation unlikely.