Nanoparticles are incredibly small. Just because something is a fine powder does not in any way mean it is a nanoparticle. A nanoparticle is between 1 and 100 nanometers (nm) in size. A nanometer is equal to one billionth of a meter.
The particle sizes of the ingredients we work with are typically measured in microns or micrometers: one millionth of a meter.
The difference between one micrometer and one nanometer is a factor of 1000. That means if you are working with a powder with an average particle size of 0.5 microns/micrometers (fairly typical for non-nano titanium dioxide) that is 500 nanometers—5x the maximum size of a nanoparticle. Micronized, or nano titanium dioxide has a much smaller particle size that usually falls within the definition of a nanoparticle. None of my recipes call for micronized or nano titanium dioxide.
Your suppliers should provide the particle size of your ingredients; TKB Trading is especially good for this sort of information, and they carry a ton of different powdered ingredients. For instance, their 24k Gold Mica has an average particle size of 75 micrometers, which is 75,000–nanometers—not even close to being a nanoparticle!
Research has also not conclusively shown that nanoparticles are actually absorbed into the body. This study makes for interesting reading, but I found this section to be particularly pertinent as most of the worry around nanoparticles in this space seems to pertain to titanium dioxide:
Regarding the passive diffusion of TiO2 the EU Scientific Committee on Cosmetics and Non-Food Products (SCCNP)46 published a paper based on studies with micro- and nano-sized material. Herein they state that these particles remain on the skin surface or on the outer layer of the stratum corneum and do not penetrate into or through the living skin.47–49 Confirmation was obtained with studies on human, porcine or murine skin50–53 for particles within a size range between 10 and 100 nm. These data were recently confirmed by the outcome of an EU project (NanoDerm). Here, TiO2 are only found in the top layer of the stratum corneum and the openings of the hair follicle.54 Similar results were obtained for ZnO.50,55 Just recently two studies on ZnO56 and TiO2,57 were published; ZnO penetration was investigated in vivo with human volunteers and located the particulated materials only on the skin surface and their accumulation in skin folds and/or hair follicles.56 In vitro measurements of the penetration of TiO2 particles between 20 and 100 nm showed the nanoparticles only in the top 3–5 layers for all skin samples used (porcine skin, healthy human skin and human skin grafted on a severe combined immuno-deficient mouse model).57
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