A micron is 1 millionth of a metre, properly called a micrometre with the abbreviation µm. A nanometre is 1 billionth of a metre with the abbreviation nm. The word "nanomicrons" is not a real scientific word, and is generally used to indicate something that is a nanometre - it's used in some descriptions you'll see for mineral make-up ingredients. For those of us who are visual, here's a cool picture to give you some perspective.
Boron glow has a particle size of 30 microns, which means it is going to reflect more light and going to make your face look shinier or dewier. Something like boron nitride has a particle size of 5 to 6 microns, which means it will reflect less light, so you'll get less shine. The lower particle size means it is going to be less shinier and more transparent.
Micas are found in various micron sizes. The smaller the particle size, the less sparkly the mica will be. If you are finding larger particle sizes, these are going to be very sparkly, very pearlescent micas. Ultra fine micas are designed to be "transparent", but this is more a thing for serecite or silk micas you'll use as a filler as opposed to coloured micas (I mean, what's the point? If you want coloured micas, you kinda want them to have some colour!)
Titanium dioxide and zinc oxide are come in "regular" and "micronized forms". Normal titanium dioxide would be 1 to 2 micrometres or microns. Micronized titanium dioxide can be 0.0015 microns or 15 nanometres and micronized zinc oxide could be 0.2 to 0.31 microns (these figures from TKB trading). The lower micron sizes mean there is less whitening from these products, which means you can add more for the beneficial properties without looking like a ghost!
This is the appeal of micronized or nanoparticle ingredients in mineral make-up. And I know there's some controversy about them...
Are micronized ingredients dangerous if inhaled? So far all the reading I've done indicates that although there can be some problems when inhaling some micronized chemicals, there is little to no evidence that inhaling micronized mineral make-up ingredients can be dangerous and they are Generally Recognized As Safe right now. The general consensus is more study needs to be done in this area, but it appears there is no risk in wearing mineral make-up or using sunscreen containing nanoparticles and inhalation. If you are making a lot of mineral make-up with nanoparticles, wearing a mask might be a good idea (and if you've ever made anything with SLSa, you'll know a mask can be your best friend!)
The answer so far...no. Various studies have found that although micronized ingredients can be absorbed by the stratum corneum (the upper layer of your skin composed of dead cells that easily slough off during the day), it won't be absorbed any lower.
There can be damage to your cells if the micronized ingredients if it penetrates living cells AFTER it penetrates through the stratum corneum - but there is no evidence the micronized ingredients can penetrate past the first few layers of the stratum corneum. If the micronized ingredients do not penetrate the stratum corneum, they will not penetrate the living layers of your skin, then penetrate your into your body.
A question was raised by the EU commission looking at micronized titanium dioxide and zinc oxide: Although micronized titanium dioxide and zinc oxide might not penetrate normal skin (upon which most of the ingredients are studied), but what happens to impaired skin? There have been very few studies, but the initial conclusion appears to be nothing happens to impaired skin.
There are few studies for ingredients smaller than 10 nanometres. Since we are dealing with ingredients that are 15 nanometres or larger, these studies are more out of interest rather than applicability to what we make.
I have bolded this because I have been mired in a sea of links over the last few days of people claiming that the micronized particles can get into your organs and kill you...then they want to sell you their products containing "no chemicals" or products that are "chemical free". As usual, I have no issue with opinions, but when they are presented as fact or skewed to scare you into buying something, then I get annoyed. (And there's nothing in the world that is chemical free! If you can find something, please write to the Royal Society of Chemists to get your 1,000,000 pound prize - about $1.78 million Cdn.)
For more information on the chemistry of your skin, please click here.
For more information from the Wikipedia post on the stratum corneum, please click here.
Having said this, the beauty of making our own products is making decisions about what to include in those creations. I've never used micronized ingredients because I don't have easy access to them. If you wish to include them, as with every product we use make sure you follow the directions and the safety measures well!
What do you think? Share your thoughts and experiences!
Here are a few of my references if you're interested in reading more...
A fantastic summary of various studies by the Environmental Working Group.
The summary: They endorse the use of sunscreens containing nanoparticles. They are concerned about the spray sunscreens because of inhalation issues.
An extensive literature review by the Australian government on the safety of micronized titanium dioxide and zinc oxide
The summary: There is no evidence micronized ingredients will penetrate more than the first few layers of the stratum corneum, even through the hair follicles.
The summary: They implanted human skin onto mice who were immuno-deficient and found micronized titanium dioxide does not penetrate past an intact epidermal barrier.