If you understand one thing about mineral sunscreen, understand this. A single number, the size of the particles, moves the product from opaque white paste to invisible fluid, and it moves several other properties at the same time, not all of them in the direction you would want.
The relationship, stated once
When a particle is much smaller than the wavelength of the light hitting it, scattering is in the regime described by Rayleigh: it falls away with roughly the sixth power of the particle diameter and rises steeply as wavelength shortens. When the particle is comparable to the wavelength, scattering is far stronger and is described by the fuller Mie treatment. The practical upshot is that scattering efficiency for visible light peaks when particles are in the region of a few hundred nanometres, and collapses when they fall to a few tens of nanometres.
Absorption behaves differently. It is a property of the material's electronic structure and of how much material is present, not of how it is divided up. Break a particle into smaller pieces and the total absorbance across the band gap is broadly maintained, and in some respects improved, because the same mass now covers more area.
So as particle size falls, visible scattering disappears much faster than ultraviolet absorption does. That asymmetry is the entire basis of the transparent mineral sunscreen.
| Property | Coarse particles | Fine particles |
|---|---|---|
| Visible appearance | Distinctly white, opaque | Transparent or nearly so |
| Ultraviolet absorption | Present, but with poorer coverage per unit mass | Maintained, with better coverage per unit mass |
| Long wave ultraviolet A behaviour | Relatively better, since larger particles scatter longer wavelengths | Some loss at the long end for very fine grades |
| Agglomeration tendency | Lower | Much higher: surface energy rises as particles shrink |
| Dispersion difficulty | Moderate | High; coatings and dispersants become essential |
| Regulatory position | Standard entry | Nanomaterial entry, with labelling and conditions |
| Application forms permitted | Broader | Restricted where inhalation is possible |
The trade that formulators are making
Reducing particle size buys cosmetic acceptability and costs stability. Fine particles have a very high surface area to volume ratio and a strong thermodynamic drive to stick together. Agglomerated particles behave optically like large particles: they scatter visible light, so the whiteness comes back, and they cover less area per unit mass, so protection falls. A large part of the formulation work in a fine particle mineral sunscreen is preventing that.
The tools are surface coatings, dispersing agents, and often pre made dispersions supplied by the filter manufacturer rather than raw powder, precisely because getting fine particles properly separated is difficult to do well in a general purpose mixing vessel. This is why two products with identical ingredient lists can look and perform very differently, and why an ingredient list tells you much less than readers hope.
A nuance at the long wavelength end
There is a wrinkle that runs against the simple picture. Scattering contributes more to attenuation at longer wavelengths for larger particles, so very fine grades can lose some of their long wave ultraviolet A performance relative to coarser ones of the same material. Formulators sometimes deliberately blend grades, using a coarser fraction for long wave coverage and a finer fraction for transparency and ultraviolet B strength.
This is one of the reasons blanket statements about nano being better or worse do not survive contact with formulation practice. The right particle size distribution is an engineering decision that depends on what the formula needs, and it is not one number.
Non nano mineral sunscreen is safer because the particles cannot get into the body
- What would have to be true
- That particles above the nanoscale threshold cannot penetrate skin while those below it can.
- That a size threshold defined for labelling purposes corresponds to a threshold in biological behaviour.
- What is established
- The nanomaterial definition in cosmetics law is a defined size range used for regulatory and labelling purposes.
- Assessment work on both metal oxides has consistently found that penetration through intact skin is limited to the outermost dead layers and to follicular openings, for both conventional and nanoscale grades.
- What is not established
- That a labelling threshold marks a biological boundary. It was not derived from one.
- That so called non nano products contain no particles below the threshold. Powders have size distributions, and a distribution has a tail.
Powders have distributions, not sizes
This point is worth its own heading because so much marketing depends on ignoring it. A milled powder does not have a particle size. It has a distribution: a spread of sizes with a central tendency and tails at both ends. A grade described as non nano has a distribution whose bulk sits above the threshold, and it will still contain some fraction of smaller particles unless it has been very deliberately classified.
Regulatory definitions of a nanomaterial address this by referring to the proportion of particles in the defined size range, which is precisely an acknowledgement that a simple yes or no cannot be read off a powder. A product that is described in marketing as non nano is telling you about a supplier specification, not about the absence of small particles.
Then there is agglomeration in the other direction. Fine primary particles that have clumped into larger aggregates present as larger particles optically and behave as larger particles on skin, and whether an aggregate should count as a single particle is exactly the sort of question that makes the definitions complicated.
What follows for a reader
Not very much, which is itself worth saying. Particle size distribution is not declared on a pack. The presence of a nanomaterial is marked in the ingredient list, and that is the only size related information a shopper gets. A product's appearance on skin is a reasonable proxy for its optical particle size behaviour: if it disappears, the effective particle size is small or the formulation is doing something clever with refractive index matching in the surrounding phase.
The useful conclusions are negative ones. Do not treat non nano as a safety grade. Do not assume that a transparent mineral product has sacrificed protection, because the mark on the pack records the measurement. And do not assume that a visibly white product is protecting better, because whiteness is scattering of visible light and tells you nothing about ultraviolet.
Assessment work on the nanoscale metal oxides advised against applications that could result in inhalation of the particles, which is why sprayable and loose powder forms are treated differently from creams and lotions in the conditions attached to the entries. The concern is the lung, not the skin.
How particle size is measured, and why sources disagree
A reader who goes looking for numbers will find that different sources give different sizes for what sounds like the same material, and the reason is that particle size is not a single measurable quantity. It depends on the technique and on what the technique is sensitive to.
Electron microscopy images individual particles and gives a primary particle size, which is the size of the crystal itself. Dynamic light scattering measures how particles diffuse in a liquid and reports a hydrodynamic diameter, which includes any coating and any solvent dragged along, and which reports agglomerates as single large particles. Sedimentation and laser diffraction methods measure something else again. A grade with a primary particle size well inside the nanoscale can report a hydrodynamic diameter several times larger if it has agglomerated in the medium being measured.
This is why regulatory definitions have had to be careful about aggregates and agglomerates, and why a supplier specification is a statement about a particular measurement under particular conditions rather than a fact about the material in your product. It is also why claims made about particle size in marketing copy are so difficult to check: without the method, the number means very little.
Where this leads
Two articles follow directly from this one. Nano and non nano labelling sets out what the regulatory definition actually says and what the marker in an ingredient list means. The nanoparticle safety questions covers what has been assessed, what has been found and what remains genuinely open, which is a shorter list than the alarm suggests and a longer one than the reassurance does.