Mineral or organic filter: what actually causes a white cast

Two families, one job

Every sunscreen on sale in Europe works by putting something between your skin and the ultraviolet. There are two ways to do it, and the difference between them is the whole of the mineral-versus-chemical argument.

Organic filters — the ones usually called chemical — are carbon-based molecules that absorb UV energy and release it again as a small amount of heat. Avobenzone, octocrylene, Tinosorb and the rest sit in the formula as dissolved molecules, which is why they leave nothing visible on the skin.

Inorganic filters — the ones called mineral — are two substances only: zinc oxide and titanium dioxide. They are solid particles suspended in the formula, and they work mostly by absorbing UV as well, with a smaller amount of scattering and reflection. The old picture of a mineral sunscreen as a mirror bouncing light off your face is wrong; absorption does most of the work in both families.

What is different is the physical state. One family is dissolved. The other is a suspension of particles sitting on top of you. That single fact produces every practical difference people notice.

Why a mineral filter can leave a cast

A particle scatters visible light most strongly when its size is close to the wavelength of that light. Visible light runs roughly 400 to 700 nanometres. A zinc oxide particle large enough to be a good UV filter can also be large enough to scatter a noticeable amount of blue and violet visible light — and scattered visible light reads to the eye as a pale, slightly grey film.

So the cast is not a flaw in the ingredient. It is the visible-light side effect of a particle that is the right size to do the UV job. Formulators reduce it by grinding the particle finer, by dispersing it well so it does not clump, and by choosing the oils and film-formers it sits in — all of which change how much visible light scatters before it reaches your eye.

Two consequences follow. A mineral sunscreen that has been well dispersed can be nearly invisible. And a badly dispersed one goes grey no matter what the front of the tube claims, because the claim does not change the physics.

What a tint is actually doing

A tint is not a cover-up. It is a colour correction of that scattered light.

The scattered fraction is weighted toward the blue end, which is why the effect reads as cool and ashy rather than simply pale. Iron oxides — declared as CI 77491, CI 77492 and CI 77499, the red, yellow and black — absorb in the blue-green range. Adding a small quantity of them removes exactly the part of the spectrum that was making the film look grey, and what is left reads as skin.

That is why a tinted mineral sunscreen can be sheer and still solve the problem, while a heavier untinted one does not. The tint is doing optics, not coverage.

There is a second reason to want iron oxides on your face, unrelated to appearance: they absorb visible light, and visible light is the part of the spectrum implicated in pigmentation on deeper skin tones. A tinted formula gives you that; a clear one does not.

The label, and what European law requires of it

Sunscreens are cosmetics under Regulation (EC) No 1223/2009, and their efficacy claims are shaped by the Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products. Six things follow from it.

1. The SPF number is not free-form

A product may only be labelled with one of a fixed set of values — 6 and 10, 15, 20 and 25, 30 and 50, and 50+ — grouped into four categories: low, medium, high and very high protection. A tube reading SPF 37 is not a more precise measurement; it is not following the scheme.

2. UVA protection has to scale with SPF

The UVA protection factor must be at least one third of the labelled SPF, and the critical wavelength at least 370 nm. This matters because SPF measures sunburn, which is mostly UVB. A product could once carry a high SPF while doing very little against UVA, which penetrates deeper and is the larger share of what reaches you through cloud and glass. The ratio rule closed that gap.

3. The UVA in a circle means the ratio is met

The letters UVA inside a circle is not decoration and not a brand mark. It is the agreed way of declaring that the one-third rule has been satisfied. Its absence on a European sunscreen is informative.

4. Some claims are prohibited outright

No sunscreen may be presented as offering total protection. “Sunblock”, “total protection” and “all-day protection” are named as claims that must not be made, because none of them is achievable and each encourages people to stay out longer than they should.

5. The instructions are part of the product

Apply generously before exposure, reapply frequently, reapply after swimming or towelling. These are required because the tested SPF assumes an application density of 2 mg per square centimetre, which is a great deal more than most people use. Half the quantity does not give you half the protection; it gives you less than that.

6. The numbers come from defined tests

SPF is determined by EN ISO 24444, UVA protection by EN ISO 24442 or EN ISO 24443. A number without a method behind it is an assertion.

Reading a mineral formula's ingredient list

Two entries decide what kind of product you have.

The filter is declared by its plain INCI name and, in a mineral sunscreen, appears at or near the top of the list because it is present in quantity. On ours the list opens with Zinc Oxide.

A colourant is declared differently: by a Colour Index number, and conventionally at the end of the list. So CI 77891 (Titanium Dioxide) further down is titanium dioxide used as a white pigment, not as a UV filter — same substance, different job, and the position in the list is what tells you which. The iron oxides doing the tinting appear the same way, as CI 77491, CI 77492, CI 77499.

This is worth knowing because titanium dioxide appears on a great many labels for reasons that have nothing to do with sun protection. Where it sits tells you why it is there. The same reading habit works on any pack — our article on how to read a supplement label applies it to a nutrition panel.

So which should you buy

Both families are effective, both are permitted, and the honest answer is that the one you will actually reapply is the one that protects you. A filter left in the drawer has an SPF of 1.

Mineral is the sensible default if your skin objects to organic filters or if you want something for a child. Tinted mineral is the version of that which does not go grey, and which adds visible-light absorption on top. Organic filters remain the lighter wear, and there is nothing wrong with them.

Whichever you choose: check for the UVA in a circle, apply more than feels necessary, and put it on again after two hours. Ours is the Tinted Mineral Sunscreen SPF 30, with the full INCI list printed on its page in order.

References: Regulation (EC) No 1223/2009 on cosmetic products; Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto (2006/647/EC); EN ISO 24444, EN ISO 24442 and EN ISO 24443.